Name

    ARB_texture_compression

Name Strings

    GL_ARB_texture_compression

Contact

    Pat Brown, Intel Corporation (patrick.r.brown 'at' intel.com)

Notice

    Copyright (c) 2000-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Complete. Approved by ARB on March 16, 2000

Version

    Last Modified Date: May 23, 2000
    Author Revision: 1.03

Number

    ARB Extension #12

Dependencies

    OpenGL 1.1 is required.

    This extension is written against the OpenGL 1.2.1 Specification.

    This extension is written against the GLX Extensions for OpenGL
    Specification (Version 1.3).

    Depends on GL_ARB_texture_cube_map, as cube maps may be stored in
    compressed form.

Overview

    Compressing texture images can reduce texture memory utilization and
    improve performance when rendering textured primitives.  This extension
    allows OpenGL applications to use compressed texture images by providing:

        (1) A framework upon which extensions providing specific compressed
            image formats can be built.

        (2) A set of generic compressed internal formats that allow
            applications to specify that texture images should be stored in
            compressed form without needing to code for specific compression
            formats.

    An application can define compressed texture images by providing a texture
    image stored in a specific compressed image format.  This extension does
    not define any specific compressed image formats, but it does provide the
    mechanisms necessary to enable other extensions that do.

    An application can also define compressed texture images by providing an
    uncompressed texture image but specifying a compressed internal format.
    In this case, the GL will automatically compress the texture image using
    the appropriate image format.  Compressed internal formats can either be
    specific (as above) or generic.  Generic compressed internal formats are
    not actual image formats, but are instead mapped into one of the specific
    compressed formats provided by the GL (or to an uncompressed base internal
    format if no appropriate compressed format is available).  Generic
    compressed internal formats allow applications to use texture compression
    without needing to code to any particular compression algorithm.  Generic
    compressed formats allow the use of texture compression across a wide
    range of platforms with differing compression algorithms and also allow
    future GL implementations to substitute improved compression methods
    transparently.

    Compressed texture images can be obtained from the GL in uncompressed form
    by calling GetTexImage and in compressed form by calling
    GetCompressedTexImageARB.  Queried compressed images can be saved and
    later reused by calling CompressedTexImage[123]DARB.  Pre-compressed
    texture images do not need to be processed by the GL and should
    significantly improve texture loading performance relative to uncompressed
    images.

    This extension does not define specific compressed image formats (e.g.,
    S3TC, FXT1), nor does it provide means to encode or decode such images.
    To support images in a specific compressed format, a hardware vendor
    would:

      (1) Provide a new extension defininig specific compressed
          <internalformat> and <format> tokens for TexImage[123]D,
          TexSubImage[123]D, CopyTexImage[12]D, CompressedTexImage[123]DARB,
          CompressedTexSubImage[123]DARB, and GetCompressedTexImageARB calls.

      (2) Specify the encoding of compressed images of that specific format.

      (3) Specify a method for deriving the size of compressed images of that
          specific format, using the <internalformat>, <width>, <height>,
          <depth> parameters, and (if necessary) the compressed image itself.

IP Status

    No known intellectual property issues on this general extension.

    Specific compression algorithms used to implement this extension (and any
    other specific texture compression extensions) may be protected and
    require licensing agreements.

Issues

    (1) Should we define additional internal formats that strongly tie an
    underlying compression algorithm to the format?

      RESOLVED:  Not here.  Explicit compressed formats will be provided by
      other extensions built on top of this one.

    (2) Should we provide additional compression state that gives more control
    on the level/quality of compression?  If so, how?

      RESOLVED:  Yes, as a hint.  Could have also been implemented as a [0.0,
      1.0] floating-point TexParameter "quality" state variable (such as the
      JPEG quality scale found in many apps).  This control will affect only
      the speed (and quality) with which a driver compresses incoming images,
      but will not affect the compressed image format selected by the driver.

      As the spec is currently formulated, the requirement that quality
      control not affect compression format selection could have been relaxed
      by loosening the invariance requirements (so that the quality control
      can affect the choice of internal format).  The risk was the potential
      for subtle mipmap consistency issues if the hint changes.

    (3) Most current compression algorithms handle primarily RGB and RGBA
    images.  Does it make sense having generic compressed formats for alpha,
    intensity, luminance, and luminance-alpha?

      RESOLVED:  Yes.  It is conceivable that some or all of these formats may
      be compressed.  Implementations not having compression algorithms for
      these formats can simply choose not to compress and use the appropriate
      base internal format instead.

    (4) Full GetTexImage support requires that the renderer decompress the
    whole image.  Should this extra implementation burden be imposed on the
    renderer?

      RESOLVED:  Yes, returning the uncompressed image is a useful feature for
      evaluating the quality of the compressed image.  A decompression engine
      may also be required for a number of other areas, including software
      rasterization.

    (5) Full TexSubImage support may require that the renderer decompress
    portions of the image (or perhaps the whole image), do a merge, and then
    recompress.  Even if this were done, portions of the image outside the
    "modified" area may also be modified due to lossy compression. Should this
    extra implementation burden be imposed on the renderer?

      RESOLVED:  No.  To avoid the complications involved with modifying a
      compressed texture image, only the lower-left corner may be modified by
      TexSubImage.  In addition, after calling TexSubImage, the "unmodified"
      portion of the image is left undefined. An INVALID_OPERATION error
      results from any other TexSubImage calls.

      This behavior allows for the use of compressed images whose dimensions
      are not powers of two, which TexImage will not accept.  The recommended
      sequence of calls for defining such images is to first call TexImage
      with a NULL <data> pointer and the image size parameters padded out to
      the next power of two, and then call CompressedTexSubImageARB or
      TexSubImage with <xoffset>, <yoffset>, and <zoffset> parameters of zero
      and the compressed data pointed to by <data>.  This behavior also allows
      TexSubImage to be used as a light-weight replacement of TexImage, where
      only the image contents are modified.

      Certain compressed formats may allow a wider variety of edits -- their
      specifications will document the restrictions under which these edits
      are permitted.  it is impossible to document such restrictions for
      unknown generic formats.  It is desirable to keep the behavior of
      generic formats and the specific formats they map to as consistent as
      possible.

    (6) What do the return values of the component sizes (RED_BITS,
    GREEN_BITS, ...) give for compressed textures?  Compressed proxy textures?

      RESOLVED:  Some behavior has to be defined. For both normal and proxy
      textures, we return the bit depths of an uncompressed sized image that
      would most closely match the quality of the compression algorithm for an
      "average" texture image.  Since compressed image quality is highly data
      dependent, the actual compressed image quality may be better or worse
      than the renderer's best guess at the best matching sized internal
      format.  To implement this feature in a driver, it is expected that an
      error analysis would be done on a set of representative images, and the
      resultant "equivalent bit depths" would be hardwired constants.

    (7) What should GetTexLevelParameter with TEXTURE_COMPRESSED_
    IMAGE_SIZE_ARB return for existing uncompressed formats?  For proxy
    textures?

      RESOLVED: For both, an INVALID_OPERATION error results.  The actual
      image to be compressed is not available for proxies, so actually
      compressing the specified image is not an option.

      For uncompressed internal formats, we could return the actual amount of
      memory taken by the texture image.  Such a mechanism might be useful as
      a metric of "how much space does this texture image take".  It's not
      particularly useful for an application based texture management scheme,
      since there is no information available indicating the amount of
      available memory.  In addition, because of implementation-dependent
      hardware constraints, the amount of texture memory consumed by a texture
      object is not necessarily equal to the sum of the memory consumed by
      each of its mipmaps.  The OpenGL ARB decided against adopting this
      behavior when this specification was approved.

    (8) What about texture borders?

      RESOLVED:  Not a problem for generic compressed formats since a base
      internal format can be used if borders are not supported in the
      compressed image format.  Borders may pose problems for specific
      compression extensions, and compressed textures with borders might well
      be disallowed by those extensions.

    (9) Should certain pixel operations be disallowed for compressed texture
    internal formats (e.g., PixelStorage, PixelTransfer)?  What about byte
    swapping?

      RESOLVED:  For uncompressed source images, all pixel storage and pixel
      transfer modes will be applied prior to compression.  For compressed
      source images, all pixel storage and transfer modes will be ignored.
      The encoding of compressed images should be specified as a byte stream
      that matches the disk file format defined for the corresponding image
      type.

    (10) Should functionality be provided to allow applications to save
    compressed images to disk and reuse them in subsequent runs without
    programming to specific formats?  If so, how?

      RESOLVED:  Yes.  This can be done without knowledge of specific
      compression formats in the following manner:

        * Call TexImage with an uncompressed image and a generic compressed
          internal format.  The texture image will be compressed by the GL, if
          possible.

        * Call GetTexLevelParameteriv with a <value> of TEXTURE_COMPRESSED_ARB
          to determine if the GL was able to store the image in compressed
          form.

        * Call GetTexLevelParameteriv with a <value> of
          TEXTURE_INTERNAL_FORMAT to determine the specific compressed image
          format in which the image is stored.

        * Call GetTexLevelParameteriv with a <value> of
          TEXTURE_COMPRESSED_IMAGE_SIZE_ARB to determine the size (in ubytes)
          of the compressed image that will be returned by the GL.  Allocate a
          buffer of at least this size.

        * Call GetCompressedTexImageARB.  The GL will write the compressed
          texture image into the allocated buffer.

        * Save the returned compressed image to disk, along with the
          associated width, height, depth, border parameters and the returned
          values of TEXTURE_COMPRESSED_IMAGE_SIZE_ARB and
          TEXTURE_INTERNAL_FORMAT.

        * Load the compressed image and its parameters, and call
          CompressedTexImage_[123]DARB to use the compressed image.  The value
          of TEXTURE_INTERNAL_FORMAT should be used as <internalFormat> and
          the value of TEXTURE_COMPRESSED_IMAGE_SIZE_ARB should be used as
          <imageSize>.

      The saved images will be valid as long as they are used on a device
      supporting the returned <internalFormat> parameter.  If the saved images
      are used on a device that does not support the compressed internal
      format, an INVALID_ENUM error would be generated by the call to
      CompressedTexImage_[123]D because of the unknown format.

      Note also that to reliably determine if the GL will compress an image
      without actually compressing it, an application need only define a proxy
      texture image and query TEXTURE_COMPRESSED_ARB as above.

    (11) Without knowing of the compressed image format, there is no
    convenient way for the client-side GLX library or tracing tools to
    ascertain the size of a compressed texture image when sending a
    TexImage1D, TexImage2D, or TexImage3D packet or interpret pixel storage
    modes.  To complicate matters further, it is possible to create both
    indirect (that might not understand an image format) and direct rendering
    contexts (that might understand an image format) on the same renderer.
    How should this be solved?

      RESOLVED:  A separate set of CompressedTexImage and
      CompressedTexSubImage calls has been created that allows libraries to
      pass compressed images along to the renderer without needing to
      understand their specific image formats or how to interpret pixel
      storage modes.

    (12) Are the CompressedTexImage[123]DARB entry points really needed?

      RESOLVED:  Yes.  To robustly support images of unknown format, specific
      compressed entry points are required.  While the extension does not
      support images in a completely unspecified format (early drafts did),
      having a separate call means that GLX and tools such as GLS (stream
      encoder) do not need intimate knowledge of every compressed image
      format.  Having separate calls also cleanly solves the problem where
      pixel storage and pixel transfer operations apply if and only if the
      source image is uncompressed.

    (13) Is variable-ratio compression supported?

      RESOLVED:  Yes.  Fixed-ratio compression is currently the predominant
      texture compression format, but this spec should not preclude the use of
      other compression schemes.

    (14) Should the <imageSize> parameter be validated on CompressedTexImage
    calls?

      RESOLVED: Yes.  Enforcement overhead is generally trivial.  Without
      enforcement, an application could specify incorrect image sizes but
      notice them only when run on an indirect renderer, causing portability
      problems.  There is also a reliability issue with respect to the GLX
      environment -- if the compressed image size provided by the user is less
      than the required image size, the GLX server may run off the end of the
      image and access invalid memory.  A size check may thus be desirable to
      prevent server crashes (even though that could be considered an
      "undefined" result).

      While enforcing correct <imageSize> parameters is trivial for current
      compressed internal formats, it might not be reasonable on others
      (particular variable-ratio compression formats).  For such formats, this
      restriction should be overridden in the spec defining the formats.  The
      <imageSize> check was made mandatory only in the final draft approved at
      the March 2000 OpenGL ARB meeting.

    (15) Should TexImage calls fall back to uncompressed image formats when
    <internalformat> is a specific compressed format but its use in
    combination with other parameter values passed is not supported by the
    renderer?

      RESOLVED:  Yes.  Advantages:  Works in exactly the same way as generic
      formats, meaning no extra code/error checking.  Inherent limitations of
      TexImage on specific formats should be documented in their specs and
      observed by their users.  One simple query can detect fallback cases.
      Disadvantages: Silent fallback to a format not requested by the user.

    (16) Should the texture format invariance requirements disallow scanning
    of the image data to select a compression method?  What about for a base
    (uncompressed) internal format?

      RESOLVED:  The primary issue is mipmap consistency.  The 1.2.1 spec
      defines a set of mipmaps as consistent if all are specified using the
      same internal format.  However, it doesn't require that all mipmaps are
      allocated using the same format -- the renderer is responsible for
      ensuring mipmap consistency if it selects different formats for
      different images.  There is no reason to disallow scanning for base
      internal formats; the renderer is responsible for doing the right thing.

      The selection of a specific compressed internal format is different.  It
      must be independent of the the image data because the GL treats the
      texture image as though it were specified using the specific compressed
      internal format chosen by the renderer.

    (17) Should functionality be provided to enumerate the specific compressed
    formats supported by the renderer?  If so, how and what will it accomplish?

      RESOLVED:  Yes.  A glGet* query is added to return the number of
      compressed internal formats supported by the renderer and the
      <internalformat> tokens for each.  These tokens can subsequently be used
      as <internalformat> parameters for normal TexImage calls and the new
      CompressedTexImage calls.

      Providing an internal format enumeration allows applications to weigh
      the suitability of the various compression methods provided to it by the
      renderer without needing specific knowledge of the formats.
      Applications can query the component sizes (see issue 6) to determine
      the base format and approximate precision.  Applications can directly
      evaluate image compression quality by having the renderer generate
      compressed texture images (using the returned <internalformat> values)
      and return them in uncompressed form using GetTexImage.  Applications
      should also be aware that the use of the internal formats returned by
      this query is subject to the restrictions imposed by the specification
      defining them.  The use of proxy textures allows the application to
      determine if a specific set of TexImage parameters is supported for a
      given internal format.

      The renderer should enumerate all supported compression formats EXCEPT
      those that operate fundamentally differently from a normal uncompressed
      format.  For example, the DirectX DXT1 compression format is
      fundamentally an RGB format, but it has a "transparent" encoding where
      the red, green, and blue component values are forced to zero, regardless
      of their original (uncompressed) values.  Since such formats may have
      caveats that must be understood before being used, they should not be
      enumerated by this query.

      This allows for forward compatibility -- an application can exploit
      compression techniques provided by future renderers.

    (18) Should the separate GetCompressedTexImageARB function exist, or is
         GetTexImage with special <format> and/or <type> parameters
         sufficient?

      RESOLVED:  Provide a separate GetCompressedTexImageARB function.  The
      primary rationale is for GLX indirect rendering.  The client GetTexImage
      would require information to determine if an image is uncompressed (and
      should be decoded using pixel storage state) or compressed (pixel
      storage ignored).  In addition, if the image is compressed, the actual
      image size would be required, but the only image size that could be
      inferred from the GLX protocol is padded out to a multiple of four
      bytes.  A separate call is the cleanest solution to both issues.

New Procedures and Functions

    void CompressedTexImage3DARB(enum target, int level,
                                 enum internalformat, sizei width,
                                 sizei height, sizei depth,
                                 int border, sizei imageSize,
                                 const void *data);
    void CompressedTexImage2DARB(enum target, int level,
                                 enum internalformat, sizei width,
                                 sizei height, int border, 
                                 sizei imageSize, const void *data);
    void CompressedTexImage1DARB(enum target, int level,
                                 enum internalformat, sizei width,
                                 int border, sizei imageSize,
                                 const void *data);
    void CompressedTexSubImage3DARB(enum target, int level, 
                                    int xoffset, int yoffset,
                                    int zoffset, sizei width,
                                    sizei height, sizei depth,
                                    enum format, sizei imageSize,
                                    const void *data);
    void CompressedTexSubImage2DARB(enum target, int level, 
                                    int xoffset, int yoffset,
                                    sizei width, sizei height,
                                    enum format, sizei imageSize,
                                    const void *data);
    void CompressedTexSubImage1DARB(enum target, int level, 
                                    int xoffset, sizei width,
                                    enum format, sizei imageSize,
                                    const void *data);
    void GetCompressedTexImageARB(enum target, int lod,
                                  void *img);

New Tokens

    Accepted by the <internalformat> parameter of TexImage1D, TexImage2D,
    TexImage3D, CopyTexImage1D, and CopyTexImage2D:

        COMPRESSED_ALPHA_ARB                            0x84E9
        COMPRESSED_LUMINANCE_ARB                        0x84EA
        COMPRESSED_LUMINANCE_ALPHA_ARB                  0x84EB
        COMPRESSED_INTENSITY_ARB                        0x84EC
        COMPRESSED_RGB_ARB                              0x84ED
        COMPRESSED_RGBA_ARB                             0x84EE

    Accepted by the <target> parameter of Hint and the <value> parameter of
    GetIntegerv, GetBooleanv, GetFloatv, and GetDoublev:

        TEXTURE_COMPRESSION_HINT_ARB                    0x84EF

    Accepted by the <value> parameter of GetTexLevelParameter:

        TEXTURE_COMPRESSED_IMAGE_SIZE_ARB               0x86A0
        TEXTURE_COMPRESSED_ARB                          0x86A1

    Accepted by the <value> parameter of GetIntegerv, GetBooleanv, GetFloatv,
    and GetDoublev:

        NUM_COMPRESSED_TEXTURE_FORMATS_ARB              0x86A2
        COMPRESSED_TEXTURE_FORMATS_ARB                  0x86A3

Additions to Chapter 2 of the OpenGL 1.2.1 Specification (OpenGL Operation)

    None.

Additions to Chapter 3 of the OpenGL 1.2.1 Specification (Rasterization)

    Modify Section 3.8.1, Texture Image Specification (p.113)

    (p.113, modify 3rd paragraph) <internalformat> may be specified as one of
    the six base internal format symbolic constants listed in table 3.15, as
    one of the sized internal format symbolic constants listed in table 3.16,
    as one of the specific compressed internal format symbolic constants
    listed in table 3.16.1, or as one of the six generic compressed internal
    format symbolic constants listed in table 3.16.2.

    (p.113, add after 3rd paragraph)

    The ARB_texture_compression specification provides no specific compressed
    internal formats but does provide a mechanism to obtain the enums for such
    formats provided by other specifications.  If the ARB_texture_compression
    extension is supported, the number of specific compressed internal format
    symbolic constants supported by the renderer can be obtained by querying
    the value of NUM_COMPRESSED_TEXTURE_FORMATS_ARB.  The set of specific
    compressed internal format symbolic constants supported by the renderer
    can be obtained by querying the value of COMPRESSED_TEXTURE_FORMATS_ARB.
    The only symbolic constants returned by this query are those suitable for
    general-purpose usage.  The renderer will not enumerate formats with
    restrictions that need to be specifically understood prior to use.

    Generic compressed internal formats are never used directly as the
    internal formats of texture images.  If <internalformat> is one of the six
    generic compressed internal formats, its value is replaced by the symbolic
    constant for a specific compressed internal format of the GL's choosing
    with the same base internal format.  If no specific compressed format is
    available, <internalformat> is instead replaced by the corresponding base
    internal format.  If <internalformat> is given as or mapped to a specific
    compressed internal format, but the GL can not support images compressed
    in the chosen internal format for any reason (e.g., the compression format
    might not support 3D textures or borders), <internalformat> is replaced by
    the corresponding base internal format and the texture image will not be
    compressed by the GL.

    (p.113, modify 4th paragraph) ... If a compressed internal format is
    specified, the mapping of the R, G, B, and A values to texture components
    is equivalent to the mapping of the corresponding base internal format's
    components, as specified in table 3.15.  The specified image is compressed
    using a (possibly lossy) compression algorithm chosen by the GL.

    (p.113, 5th paragraph) A GL implementation may vary its allocation of
    internal component resolution or compressed internal format based on any
    TexImage3D, TexImage2D, or TexImage1D (see below) parameter (except
    <target>, but the allocation and chosen compressed image format must not
    be a function of any other state and cannot be changed once they are
    established.  In addition, the choice of a compressed image format may not
    be affected by the <data> parameter.  Allocations must be invariant; the
    same allocation and compressed image format must be chosen each time a
    texture image is specified with the same parameter values.  These
    allocation rules also apply to proxy textures, which are described in
    section 3.8.7.

    Add Table 3.16.1:  Specific Compressed Internal Formats

        Compressed Internal Format         Base Internal Format
        ==========================         ====================
        none provided here -- defined by dependent extensions


    Add Table 3.16.2:  Generic Compressed Internal Formats

        Generic Compressed Internal
        Format                             Base Internal Format
        ==========================         ====================
        COMPRESSED_ALPHA_ARB               ALPHA
        COMPRESSED_LUMINANCE_ARB           LUMINANCE
        COMPRESSED_LUMINANCE_ALPHA_ARB     LUMINANCE_ALPHA
        COMPRESSED_INTENSITY_ARB           INTENSITY
        COMPRESSED_RGB_ARB                 RGB
        COMPRESSED_RGBA_ARB                RGBA


    Modify Section 3.8.2, Alternate Image Specification

    (add to end of TexSubImage discussion, p.123)

    Texture images with compressed internal formats may be stored in such a
    way that it is not possible to edit an image with subimage commands
    without having to decompress and recompress the texture image being
    edited.  Even if the image were edited in this manner, it may not be
    possible to preserve the contents of some of the texels outside the region
    being modified.  To avoid these complications, the GL does not support
    arbitrary edits to texture images with compressed internal formats.
    Calling TexSubImage3D, CopyTexSubImage3D, TexSubImage2D,
    CopyTexSubImage2D, TexSubImage1D, or CopyTexSubImage1D will result in an
    INVALID_OPERATION error if <xoffset>, <yoffset>, or <zoffset> is not equal
    to -b_s (border).  In addition, the contents of any texel outside the
    region modified by such a call are undefined.  These restrictions may be
    relaxed for specific compressed internal formats whose images are easily
    edited.

    (add new subsection at end of section, p.123)

    Compressed Texture Images

    Texture images may also be specified or modified using image data already
    stored in a known compressed image format.  The ARB_texture_compression
    extension defines no such formats, but provides the mechanisms for other
    extensions that do.

    The commands

      void CompressedTexImage1DARB(enum target, int level,
                                   enum internalformat, sizei width,
                                   int border, sizei imageSize,
                                   const void *data);
      void CompressedTexImage2DARB(enum target, int level,
                                   enum internalformat, sizei width,
                                   sizei height, int border, 
                                   sizei imageSize, const void *data);
      void CompressedTexImage3DARB(enum target, int level,
                                   enum internalformat, sizei width,
                                   sizei height, sizei depth,
                                   int border, sizei imageSize,
                                   const void *data);

    define one-, two-, and three-dimensional texture images, respectively,
    with incoming data stored in a specific compressed image format.  The
    <target>, <level>, <internalformat>, <width>, <height>, <depth>, and
    <border> parameters have the same meaning as in TexImage1D, TexImage2D,
    and TexImage3D.  <data> points to compressed image data stored in the
    compressed image format corresponding to <internalformat>.  Since this
    extension provides no specific image formats, using any of the six generic
    compressed internal formats as <internalformat> will result in an
    INVALID_ENUM error.

    For all other compressed internal formats, the compressed image will be
    decoded according to the specification defining the <internalformat>
    token.  Compressed texture images are treated as an array of <imageSize>
    ubytes beginning at address <data>.  All pixel storage and pixel transfer
    modes are ignored when decoding a compressed texture image.  If the
    <imageSize> parameter is not consistent with the format, dimensions, and
    contents of the compressed image, an INVALID_VALUE error results.  If the
    compressed image is not encoded according to the defined image format, the
    results of the call are undefined.

    Specific compressed internal formats may impose format-specific
    restrictions on the use of the compressed image specification calls or
    parameters.  For example, the compressed image format might be supported
    only for 2D textures or may not allow non-zero <border> values.  Any such
    restrictions will be documented in the specification defining the
    compressed internal format; violating these restrictions will result in an
    INVALID_OPERATION error.

    Any restrictions imposed by specific compressed internal formats will be
    invariant, meaning that if the GL accepts and stores a texture image in
    compressed form, providing the same image to CompressedTexImage1DARB,
    CompressedTexImage2DARB, CompressedTexImage3DARB will not result in an
    INVALID_OPERATION error if the following restrictions are satisfied:

      * <data> points to a compressed texture image returned by
        GetCompressedTexImageARB (Section 6.1.4).

      * <target>, <level>, and <internalformat> match the <target>, <level>
        and <format> parameters provided to the GetCompressedTexImageARB call
        returning <data>.

      * <width>, <height>, <depth>, <border>, <internalformat>, and
        <imageSize> match the values of TEXTURE_WIDTH, TEXTURE_HEIGHT,
        TEXTURE_DEPTH, TEXTURE_BORDER, TEXTURE_INTERNAL_FORMAT, and
        TEXTURE_COMPRESSED_IMAGE_SIZE_ARB for image level <level> in effect at
        the time of the GetCompressedTexImageARB call returning <data>.

    This guarantee applies not just to images returned by
    GetCompressedTexImageARB, but also to any other properly encoded
    compressed texture image of the same size and format.


    The commands

      void CompressedTexSubImage1DARB(enum target, int level, 
                                      int xoffset, sizei width,
                                      enum format, sizei imageSize,
                                      const void *data);
      void CompressedTexSubImage2DARB(enum target, int level, 
                                      int xoffset, int yoffset,
                                      sizei width, sizei height,
                                      enum format, sizei imageSize,
                                      const void *data);
      void CompressedTexSubImage3DARB(enum target, int level, 
                                      int xoffset, int yoffset,
                                      int zoffset, sizei width,
                                      sizei height, sizei depth,
                                      enum format, sizei imageSize,
                                      const void *data);


    respecify only a rectangular region of an existing texture array, with
    incoming data stored in a known compressed image format.  The <target>,
    <level>, <xoffset>, <yoffset>, <zoffset>, <width>, <height>, and <depth>
    parameters have the same meaning as in TexSubImage1D, TexSubImage2D, and
    TexSubImage3D.  <data> points to compressed image data stored in the
    compressed image format corresponding to <format>.  Since this extension
    provides no specific image formats, using any of these six generic
    compressed internal formats as <format> will result in an INVALID_ENUM
    error.

    The image pointed to by <data> and the <imageSize> parameter are
    interpreted as though they were provided to CompressedTexImage1DARB,
    CompressedTexImage2DARB, and CompressedTexImage3DARB.  These commands do
    not provide for image format conversion, so an INVALID_OPERATION error
    results if <format> does not match the internal format of the texture
    image being modified.  If the <imageSize> parameter is not consistent with
    the format, dimensions, and contents of the compressed image (too little
    or too much data), an INVALID_VALUE error results.

    As with CompressedTexImage calls, compressed internal formats may have
    additional restrictions on the use of the compressed image specification
    calls or parameters.  Any such restrictions will be documented in the
    specification defining the compressed internal format; violating these
    restrictions will result in an INVALID_OPERATION error.

    Any restrictions imposed by specific compressed internal formats will be
    invariant, meaning that if the GL accepts and stores a texture image in
    compressed form, providing the same image to CompressedTexSubImage1DARB,
    CompressedTexSubImage2DARB, CompressedTexSubImage3DARB will not result in
    an INVALID_OPERATION error if the following restrictions are satisfied:

      * <data> points to a compressed texture image returned by
        GetCompressedTexImageARB (Section 6.1.4).

      * <target>, <level>, and <format> match the <target>, <level> and
        <format> parameters provided to the GetCompressedTexImageARB call
        returning <data>.

      * <width>, <height>, <depth>, <format>, and <imageSize> match the values
        of TEXTURE_WIDTH, TEXTURE_HEIGHT, TEXTURE_DEPTH,
        TEXTURE_INTERNAL_FORMAT, and TEXTURE_COMPRESSED_IMAGE_SIZE_ARB for
        image level <level> in effect at the time of the
        GetCompressedTexImageARB call returning <data>.

      * <width>, <height>, <depth>, <format> match the values of
        TEXTURE_WIDTH, TEXTURE_HEIGHT, TEXTURE_DEPTH, and
        TEXTURE_INTERNAL_FORMAT currently in effect for image level <level>.

      * <xoffset>, <yoffset>, and <zoffset> are all "-<b>", where <b> is the
        value of TEXTURE_BORDER currently in effect for image level <level>.

    This guarantee applies not just to images returned by
    GetCompressedTexImageARB, but also to any other properly encoded
    compressed texture image of the same size.

    Calling CompressedTexSubImage3D, CompressedTexSubImage2D, or
    CompressedTexSubImage1D will result in an INVALID_OPERATION error if
    <xoffset>, <yoffset>, or <zoffset> is not equal to -b_s (border), or if
    <width>, <height>, and <depth> do not match the values of TEXTURE_WIDTH,
    TEXTURE_HEIGHT, or TEXTURE_DEPTH, respectively.  The contents of any texel
    outside the region modified by the call are undefined.  These restrictions
    may be relaxed for specific compressed internal formats whose images are
    easily edited.

Additions to Chapter 4 of the OpenGL 1.2.1 Specification (Per-Fragment
Operations and the Frame Buffer)

    None.

Additions to Chapter 5 of the OpenGL 1.2.1 Specification (Special Functions)

    Modify Section 5.6, Hints (p.180)

    (p.180, modify first paragraph)

    ...; FOG_HINT, indicating whether fog calculations are done per pixel or
    per vertex; and TEXTURE_COMPRESSION_HINT_ARB, indicating the desired
    quality and performance of compressing texture images.

    For the texture compression hint, a <hint> of FASTEST indicates that
    texture images should be compressed as quickly as possible, while NICEST
    indicates that the texture images be compressed with as little image
    degradation as possible.  FASTEST should be used for one-time texture
    compression, and NICEST should be used if the compression results are to
    be retrieved by GetCompressedTexImageARB (Section 6.1.4) for reuse.

Additions to Chapter 6 of the OpenGL 1.2.1 Specification (State and
State Requests)

    Modify Section 6.1.3, Enumerated Queries (p.183)

    (p.183, modify next-to-last paragraph)

    For texture images with uncompressed internal formats, queries of
    TEXTURE_RED_SIZE, TEXTURE_GREEN_SIZE, TEXTURE_BLUE_SIZE,
    TEXTURE_ALPHA_SIZE, TEXTURE_LUMINANCE_SIZE, and TEXTURE_INTENSITY_SIZE
    return the actual resolutions of the stored image array components, not
    the resolutions specified when the image array was defined.  For texture
    images with a compressed internal format, the resolutions returned specify
    the component resolution of an uncompressed internal format that produces
    an image of roughly the same quality as the compressed image in question.
    Since the quality of the implementation's compression algorithm is likely
    data-dependent, the returned component sizes should be treated only as
    rough approximations.  ...

    (p.183, add to end of next-to-last paragraph)

    TEXTURE_COMPRESSED_IMAGE_SIZE_ARB returns the size (in ubytes) of the
    compressed texture image that would be returned by
    GetCompressedTexImageARB (Section 6.1.4).  Querying
    TEXTURE_COMPRESSED_IMAGE_SIZE_ARB is not allowed on texture images with an
    uncompressed internal format or on proxy targets and will result in an
    INVALID_OPERATION error if attempted.

    Modify Section 6.1.4, Texture Queries (p.184)

    (add immediately after the GetTexImage section and before the IsTexture
    section)

    The command

      void GetCompressedTexImageARB(enum target, int lod,
                                    void *img);

    is used to obtain texture images stored in compressed form.  The
    parameters <target>, <lod>, and <img> are interpreted in the same manner
    as in GetTexImage.  When called, GetCompressedTexImageARB writes
    TEXTURE_COMPRESSED_IMAGE_SIZE_ARB ubytes of compressed image data to the
    memory pointed to by <img>.  The compressed image data is formatted
    according to the specification defining INTERNAL_FORMAT.  All pixel
    storage and pixel transfer modes are ignored when returning a compressed
    texture image.

    Calling GetCompressedTexImageARB with an <lod> value less than zero or
    greater than the maximum allowable causes an INVALID_VALUE error.  Calling
    GetCompressedTexImageARB with a texture image stored with an uncompressed
    internal format causes an INVALID_OPERATION error.

Additions to Appendix A of the OpenGL 1.2.1 Specification (Invariance)

    None.


Additions to the AGL/GLX/WGL Specifications

    None.

GLX Protocol

    (Add after GetTexImage to Section 2.2.2 of the GLX 1.3 encoding spec,
     p.74)

    GetCompressedTexImageARB

        1       CARD8               opcode (X assigned)
        1       160                 GLX opcode
        2       4                   request length
        4       GLX_CONTEXT_TAG     context tag
        4       ENUM                target
        4       INT32               level

     -->

        1       1                   Reply
        1       1                   unused
        2       CARD16              sequence number
        4       n                   reply length
        8                           unused
        4       INT32               compressed image size (in bytes) --
                                      should be between 4n-3 and 4n
        12                          unused
        4*n     LISTofBYTE          teximage

    Note that n may be zero, indicating that a GL error occurred.

    Since pixel storage modes do not apply to compressed texture images,
    teximage is simply an array of bytes.  The client library will ignore
    pixel storage modes and should copy only <compressed image size> bytes,
    regardless of the value of <reply length>.

    (Add to end of Section 2.3 of the GLX 1.3 encoding spec, p.147)

    CompressedTexImage1DARB

        2       32+n+p          rendering command length
        2       214             rendering command opcode
        4       ENUM            target
        4       INT32           level
        4       ENUM            internalformat
        4       INT32           width
        4                       unused
        4       INT32           border
        n       LISTofBYTE      image
        4       INT32           imageSize
        p                       unused, p=pad(n)

    If the command is encoded in a glXRenderLarge request, the command
    opcode and command length fields are expanded to 4 bytes each.

        4       36+n+p          rendering command length
        4       214             rendering command opcode

    CompressedTexImage2DARB

        2       32+n+p          rendering command length
        2       215             rendering command opcode
        4       ENUM            target
        4       INT32           level
        4       ENUM            internalformat
        4       INT32           width
        4       INT32           height
        4       INT32           border
        4       INT32           imageSize
        n       LISTofBYTE      image
        p                       unused, p=pad(n)

    If the command is encoded in a glXRenderLarge request, the command
    opcode and command length fields are expanded to 4 bytes each.

        4       36+n+p          rendering command length
        4       215             rendering command opcode

    CompressedTexImage3DARB

        2       36+n+p          rendering command length
        2       216             rendering command opcode
        4       ENUM            target
        4       INT32           level
        4       INT32           internalformat
        4       INT32           width
        4       INT32           height
        4       INT32           depth
        4       INT32           border
        4       INT32           imageSize
        n       LISTofBYTE      image
        p                       unused, p=pad(n)

    If the command is encoded in a glXRenderLarge request, the command
    opcode and command length fields are expanded to 4 bytes each.

        4       36+n+p          rendering command length
        4       216             rendering command opcode

    CompressedTexSubImage1DARB

        2       36+n+p          rendering command length
        2       217             rendering command opcode
        4       ENUM            target
        4       INT32           level
        4       INT32           xoffset
        4                       unused
        4       INT32           width
        4                       unused
        4       ENUM            format
        4       INT32           imageSize
        n       LISTofBYTE      image
        p                       unused, p=pad(n)

    If the command is encoded in a glXRenderLarge request, the command
    opcode and command length fields are expanded to 4 bytes each.

        4       40+n+p          rendering command length
        4       217             rendering command opcode

    CompressedTexSubImage2DARB

        2       36+n+p          rendering command length
        2       218             rendering command opcode
        4       ENUM            target
        4       INT32           level
        4       INT32           xoffset
        4       INT32           yoffset
        4       INT32           width
        4       INT32           height
        4       ENUM            format
        4       INT32           imageSize
        n       LISTofBYTE      image
        p                       unused, p=pad(n)

    If the command is encoded in a glXRenderLarge request, the command
    opcode and command length fields are expanded to 4 bytes each.

        4       40+n+p          rendering command length
        4       218             rendering command opcode

    CompressedTexSubImage3DARB

        2       44+n+p          rendering command length
        2       219             rendering command opcode
        4       ENUM            target
        4       INT32           level
        4       INT32           xoffset
        4       INT32           yoffset
        4       INT32           zoffset
        4       INT32           width
        4       INT32           height
        4       INT32           depth
        4       ENUM            format
        4       INT32           imageSize
        n       LISTofBYTE      image
        p                       unused, p=pad(n)

    If the command is encoded in a glXRenderLarge request, the command
    opcode and command length fields are expanded to 4 bytes each.

        4       48+n+p          rendering command length
        4       219             rendering command opcode


Errors

    Errors for compressed TexImage and TexSubImage calls specific to
    compression:

    INVALID_OPERATION is generated by TexSubImage1D, TexSubImage2D,
    TexSubImage3D, CopyTexSubImage1D, CopyTexSubImage2D, or CopyTexSubImage3D
    if the internal format of the texture image is compressed and <xoffset>,
    <yoffset>, or <zoffset> does not equal -b, where b is value of
    TEXTURE_BORDER.

    INVALID_VALUE is generated by CompressedTexSubImage1DARB,
    CompressedTexSubImage2DARB, or CompressedTexSubImage3DARB if the entire
    texture image is not being edited:  if <xoffset>, <yoffset>, or <zoffset>
    is greater than -b, <xoffset> + <width> is less than w+b, <yoffset> +
    <height> is less than h+b, or <zoffset> + <depth> is less than d+b, where
    b is the value of TEXTURE_BORDER, w is the value of TEXTURE_WIDTH, h is
    the value of TEXTURE_HEIGHT, and d is the value of TEXTURE_DEPTH.

    INVALID_ENUM is generated by CompressedTexImage1DARB,
    CompressedTexImage2DARB, or CompressedTexImage3DARB,
    CompressedTexSubImage1DARB, CompressedTexSubImage2DARB, or
    CompressedTexSubImage3DARB, if <internalformat> is any of the six generic
    compressed internal formats (e.g., COMPRESSED_RGBA_ARB)

    INVALID_OPERATION is generated by CompressedTexImage1DARB,
    CompressedTexImage2DARB, CompressedTexImage3DARB,
    CompressedTexSubImage1DARB, CompressedTexSubImage2DARB, or
    CompressedTexSubImage3DARB, if any parameter combinations are not
    supported by the specific compressed internal format.  Such invalid
    combinations are documented in the specification defining the internal
    format.

    INVALID_VALUE is generated by CompressedTexImage1DARB,
    CompressedTexImage2DARB, or CompressedTexImage3DARB,
    CompressedTexSubImage1DARB, CompressedTexSubImage2DARB, or
    CompressedTexSubImage3DARB, if <imageSize> is not consistent with the
    format, dimensions, and contents of the specified image.  The appropriate
    value for the <imageSize> parameter is documented in the specification
    defining the compressed internal format.

    Undefined results (including abnormal program termination) are generated
    by CompressedTexImage1DARB, CompressedTexImage2DARB, or
    CompressedTexImage3DARB, CompressedTexSubImage1DARB,
    CompressedTexSubImage2DARB, or CompressedTexSubImage3DARB, is not encoded
    in a manner consistent with the specification defining the internal
    format.

    INVALID_OPERATION is generated by CompressedTexSubImage1DARB,
    CompressedTexSubImage2DARB, or CompressedTexSubImage3DARB if <format> does
    not match the internal format of the texture image being modified.

    INVALID_OPERATION is generated by GetTexLevelParameter[if]v if <target> is
    PROXY_TEXTURE_1D, PROXY_TEXTURE_2D, or PROXY_TEXTURE_3D and <value> is
    TEXTURE_COMPRESSED_IMAGE_SIZE_ARB.

    INVALID_OPERATION is generated by GetTexLevelParameter[if]v if the
    internal format of the queried texture image is not compressed and <value>
    is TEXTURE_COMPRESSED_IMAGE_SIZE_ARB.

    INVALID_OPERATION is generated by GetCompressedTexImageARB if the internal
    format of the queried texture image is not compressed.


    Errors for compressed TexImage and TexSubImage calls not specific to
    compression:

    INVALID_ENUM is generated by CompressedTexImage3DARB or
    CompressedTexSubImage3DARB if <target> is not TEXTURE_3D.

    INVALID_ENUM is generated by CompressedTexImage2DARB or
    CompressedTexSubImage2DARB if <target> is not TEXTURE_2D,
    TEXTURE_CUBE_MAP_POSITIVE_X_ARB, TEXTURE_CUBE_MAP_NEGATIVE_X_ARB,
    TEXTURE_CUBE_MAP_POSITIVE_Y_ARB, TEXTURE_CUBE_MAP_NEGATIVE_Y_ARB,
    TEXTURE_CUBE_MAP_POSITIVE_Z_ARB, or TEXTURE_CUBE_MAP_NEGATIVE_Z_ARB.

    INVALID_ENUM is generated by CompressedTexImage1DARB or
    CompressedTexSubImage1DARB if <target> is not TEXTURE_1D.

    INVALID_VALUE is generated by CompressedTexImage1DARB,
    CompressedTexImage2DARB, CompressedTexImage3DARB,
    CompressedTexSubImage1DARB, CompressedTexSubImage1DARB, or
    CompressedTexSubImage3DARB if <level> is negative.

    INVALID_VALUE is generated by CompressedTexImage1DARB,
    CompressedTexImage2DARB, CompressedTexImage3DARB,
    CompressedTexSubImage1DARB, CompressedTexSubImage2DARB, or
    CompressedTexSubImage3DARB, if <width>, <height>, or <depth> is negative.

    INVALID_VALUE is generated by CompressedTexImage1DARB,
    CompressedTexImage2DARB, or CompressedTexImage3DARB if <width>, <height>,
    or <depth> can not be represented as 2^k+2 for some integer value k.

    INVALID_VALUE is generated by CompressedTexImage1DARB,
    CompressedTexImage2DARB, or CompressedTexImage3DARB if <border> is not
    zero or one.

    INVALID_VALUE is generated by CompressedTexImage1DARB,
    CompressedTexImage2DARB, CompressedTexImage3DARB,
    CompressedTexSubImage1DARB, CompressedTexSubImage1DARB, or
    CompressedTexSubImage3DARB if the call is made between a call to Begin and
    the corresponding call to End.

    INVALID_VALUE is generated by CompressedTexSubImage1DARB,
    CompressedTexSubImage2DARB, or CompressedTexSubImage3DARB if <xoffset>,
    <yoffset>, or <zoffset> is less than -b, <xoffset> + <width> is greater
    than w+b, <yoffset> + <height> is greater than h+b, or <zoffset> + <depth>
    is greater than d+b, where b is the value of TEXTURE_BORDER, w is the
    value of TEXTURE_WIDTH, h is the value of TEXTURE_HEIGHT, and d is the
    value of TEXTURE_DEPTH.

    INVALID_VALUE is generated by GetCompressedTexImageARB if <lod> is
    negative or greater than the maximum allowable level.

New State

    (table 6.12, p.202)
                                                                Initial
    Get Value                           Type    Get Command     Value   Description Sec.    Attribute
    ---------                           ----    -----------     ------- ----------- ----    ---------
    TEXTURE_COMPRESSED_IMAGE_SIZE_ARB   n x Z+  GetTexLevel-    0       size (in    3.8     -
                                                Parameter               ubytes)
                                                                        of xD compressed
                                                                        texture image i.
    TEXTURE_COMPRESSED_ARB              n x B   GetTexLevel-    FALSE   True if xD  3.8     -
                                                Parameter               image i has
                                                                        a compressed
                                                                        internal format

    (table 6.23, p.213)
                                                                Initial
    Get Value                           Type    Get Command     Value   Description Sec.    Attribute
    ---------                           ----    -----------     ------- ----------- ----    ---------
    TEXTURE_COMPRESSION_HINT_ARB        Z_3     GetIntegerv     DONT_   Texture     5.6     hint
                                                                CARE    compression
                                                                        quality hint

    (table 6.25, p. 215)
                                                                Minimum
    Get Value                           Type    Get Command     Value   Description Sec.    Attribute
    ---------                           ----    -----------     ------- ----------- ----    ---------
    NUM_COMPRESSED_TEXTURE_FORMATS_ARB  Z       GetIntegerv     0       Number of   3.8     -
                                                                        enumerated
                                                                        compressed
                                                                        texture
                                                                        formats

    COMPRESSED_TEXTURE_FORMATS_ARB      0* x Z  GetIntegerv     -       Enumerated  3.8     -
                                                                        compressed
                                                                        texture
                                                                        formats

Revision History

    1.03, 05/23/00 prbrown1: Removed stray "None." paragraph in modifications
                             to Chapter 5.

    1.02, 05/08/00 prbrown1: Fixed prototype of GetCompressedTexImageARB (no
                             "const" qualifiers) in "New Procedures and
                             Functions" section.  Changed <internalformat>
                             parameter of CompressedTexImage functions to be
                             an "enum" instead of an "int".  "int" was carried
                             over only on TexImage calls as a 1.0 legacy --
                             the newer CopyTexImage call takes an "enum".

    1.01, 04/11/00 prbrown1: Minor bug fixes to the first published version.
                             Fixed prototypes to match extension spec
                             standards (no "GL" type prefixes).  Fixed a
                             couple erroneous function names.  Added "const"
                             qualifier to prototypes involving image data not
                             modified by the GL.  Added text to indicate that
                             compressed formats apply to texture maps
                             supported by GL_ARB_texture_cube_map.

    1.0,  03/24/00 prbrown1: Applied changes approved as part of the extension
                             at the March 2000 ARB meeting, as follows:

                             * CompressedTexSubImage:  Only allowed if the
                               entire image is replaced.  Document that this
                               restriction can be relaxed for specific
                               compression extensions.
                             * Renamed TEXTURE_IMAGE_SIZE_ARB to
                               TEXTURE_COMPRESSED_IMAGE_SIZE_ARB.
                             * Querying image size on uncompressed images is
                               now an INVALID_OPERATION error.
                             * INVALID_VALUE error is generated if <imageSize>
                               is inconsistent with the image data.  This
                               restriction may be overridden by specific
                               extensions only if requiring an image size
                               check is unreasonable.
                             * Added documentaion of undefined behavior for
                               CompressedTexImage/SubImage if the image data
                               is encoded in a manner inconsistent with the
                               spec defining the compressed image format.
                             * Fixed issue (16).  Text was truncated.
                             * Modified invariance section.  <data> can not
                               affect the choice of compressed internal
                               format, but can theoretically affect regular
                               component resolution.
                             * Add new function GetCompressedTexImage to deal
                               with subtle GLX issues.
                             * GLX protocol for CompressedTexImage/SubImage
                               and GetCompressedTexImage holds both a padded
                               image size (for GLX data transfer) and actual
                               image size (for packing in user buffers).

                             Minor wording clean-ups.

                             Added enum and GLX opcode values allocated from
                             OpenGL Extensions and GLX registries.

    0.81, 03/07/00 prbrown1: Fixed error documentation for TexSubImage calls
                             of arbitrary alignment (did not document that the
                             internal format had to be compressed).  Removed
                             references to CopyTexImage3D, which doesn't
                             actually exist.

                             Per Kurt Akeley suggestions: (1) Renamed
                             TexImageCompressed to CompressedTexImage to
                             conform with naming conventions, (2) clarified
                             that the main feature distinguishing
                             CompressedTex[Sub]Image calls from normal
                             Tex[Sub]Image calls is compressed input data, (3)
                             added query to explicitly determine whether the
                             internal format of a texture is compressed.

    0.8,  02/23/00 prbrown1: Marked previously unresolved issues as resolved
                             per the ARB working group.  Added docs for errors
                             not specific to compression for the new
                             CompressedTexImage and CompressedTexSubImage
                             calls.  Added queries to enumerate specific
                             compressed texture formats.
    0.76, 02/16/00 prbrown1: Removed "gl" and "GL_" prefixes.
    0.75, 02/07/00 prbrown1: Incorporated feedback from 12/99 ARB meeting
                             and a number of other revisions.
    0.7,  12/03/99 prbrown1: Incorporated comments from public review of 0.2
                             document.
    0.2,  10/28/99 prbrown1: Renamed to ARB_texture_compression.  Significant
                             functional changes.
    0.11, 10/21/99 prbrown1: Edits suggested by 3dfx.
    0.1,  10/19/99 prbrown1: Initial revision.