extensions/ARB/ARB_clear_texture.txt

Name

    ARB_clear_texture

Name Strings

    GL_ARB_clear_texture

Contact

    Daniel Koch, NVIDIA Corporation (dkoch 'at' nvidia.com)

Contributors

    Graham Sellers, AMD
    Jonathan Putsman, Imagination
    Ian Rominick, Intel
    Jeff Bolz, NVIDIA
    Pat Brown, NVIDIA
    Piers Daniel, NVIDIA
    James Helferty, NVIDIA
    Members of the ARB working group

Notice

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

Specification Update Policy

    Khronos-approved extension specifications are updated in response to
    issues and bugs prioritized by the Khronos OpenGL Working Group. For
    extensions which have been promoted to a core Specification, fixes will
    first appear in the latest version of that core Specification, and will
    eventually be backported to the extension document. This policy is
    described in more detail at
        https://www.khronos.org/registry/OpenGL/docs/update_policy.php

Status

    Complete. Approved by the ARB on June 3, 2013.
    Ratified by the Khronos Board of Promoters on July 19, 2013.

Version

    Last Modified Date: August 12, 2013
    Revision: 16

Number

    ARB Extension #145

Dependencies

    OpenGL 1.3 is required.

    This extension is written against the OpenGL 4.3 (Core Profile)
    Specification (October 18, 2012).

    This extension interacts with EXT_texture_integer.

    This extension interacts with ARB_texture_buffer_object.

    This extension interacts with ARB_texture_multisample.

    This extension interacts with ARB_depth_texture.

    This extension interacts with EXT_packed_depth_stencil and
    ARB_framebuffer_object.

    This extension interacts with ARB_texture_stencil8.

    This extension interacts with ARB_shader_image_load_store.

    This extension interacts with ARB_internalformat_query2.

Overview

    Texture objects are fundamental to the operation of OpenGL. They are
    used as a source for texture sampling and destination for rendering
    as well as being accessed in shaders for image load/store operations
    It is also possible to invalidate the contents of a texture. It is
    currently only possible to set texture image data to known values by
    uploading some or all of a image array from application memory or by
    attaching it to a framebuffer object and using the Clear or ClearBuffer
    commands.

    Both uploading initial texture data and clearing by attaching to a
    framebuffer have potential disadvantages when one simply wants to
    initialize texture data to a known value. Uploading initial data
    requires the application to allocate a (potentially large) chunk
    of memory and transferring that to the GL.  This can be a costly
    operation both in terms of memory bandwidth and power usage.
    Alternatively, attaching a texture level to a framebuffer to clear it
    may not be possible if the texture format isn't supported for
    rendering, or even if it is, attaching the image to a framebuffer object
    may cause the texture to be allocated in certain types of memory, which
    it may otherwise not need to be placed in.

    This extension solves these problems by providing a mechanism whereby
    the contents of a texture image array can be set to known values by
    using the ClearTexImage or ClearTexSubImage commands.  These commands
    can also be useful for initializing an image that will be used for
    atomic shader operations.

IP Status

    No known IP claims.

New Procedures and Functions

    void ClearTexImage(uint texture, int level,
                       enum format, enum type,
                       const void * data);

    void ClearTexSubImage(uint texture, int level,
                          int xoffset, int yoffset, int zoffset,
                          sizei width, sizei height, sizei depth,
                          enum format, enum type,
                          const void * data);

New Types

    None

New Tokens

    Accepted by the <pname> parameter for GetInternalformativ and
    GetInternalformati64v:

        CLEAR_TEXTURE           0x9365

Additions to Chapter 7 of the OpenGL 4.3 (Core Profile) Specification

    In section 7.12.2 (Shader Memory Access Synchronization) edit the
    description of the TEXTURE_UPDATE_BARRIER_BIT to add ClearTexImage
    and ClearTexSubImage to the list of commands that can write to
    texture images.

Additions to Chapter 8 of the OpenGL 4.3 (Core Profile) Specification
(Textures and Samplers)

    Add a new Section 8.x (Clearing Texture Image Data) after
    Section 8.20 (Invalidating Texture Image Data):

    "All or part of a texture image may be filled with a constant value
    by calling the command

      void ClearTexSubImage(uint texture, int level,
                            int xoffset, int yoffset, int zoffset,
                            sizei width, sizei height, sizei depth,
                            enum format, enum type,
                            const void * data);

    with <texture> and <level> indicating which texture array image is being
    cleared.  It is an error if <texture> is zero or not the name of a texture
    object, if <texture> is a buffer texture, or if the texture image has
    a compressed internal format.

    Arguments <xoffset>, <yoffset>, and <zoffset> specify the lower left
    texel coordinates of a <width>-wide by <height>-high by <depth>-deep
    rectangular subregion of the texel array and are interpreted as they
    are in TexSubImage3D as described in section 8.6 (Alternate Texture
    Image Specification Commands).

    For 1D array textures, <yoffset> is interpreted as the first layer to be
    cleared and <height> is the number of layers to clear. For 2D array
    textures, <zoffset> is interpreted as the first layer to be cleared
    and <depth> is the number of layers to clear. Cube map textures are
    treated as an array of six slices in the z-dimension, where the value
    of <zoffset> is interpreted as specifying the cube map face for the
    corresponding <layer> in table 9.3 (Layer numbers for cube map texture
    faces) and <depth> is the number of faces to clear. For cube map array
    textures, <zoffset> is the first layer-face to clear, and <depth> is the
    number of layer-faces to clear. Each layer-face is translated into an
    array layer and a cube map face as described for layer-face numbers in
    section 8.5.3.

    Negative values of <xoffset>, <yoffset>, and <zoffset> correspond
    to the coordinates of border texels, addressed as in Figure 8.3.
    Taking <w_s>, <h_s>, <d_s>, <w_b>, <h_b>, and <d_b> to be the specified
    width, height, depth, and the border width, border height, and border
    depth of the texel array and taking <x>, <y>, <z>, <w>, <h>, and <d> to
    be the <xoffset>, <yoffset>, <zoffset>, <width>, <height>, and <depth>
    argument values, any of the following relationships generate an error:
              <x> < - <w_b>
        <x> + <w> > <w_s> - <w_b>
              <y> < - <h_b>
        <y> + <h> > <h_s> - <h_b>
              <z> < - <d_b>
        <z> + <d> > <d_s> - <d_b>
    For texture types that do not have certain dimensions, this
    command treats those dimensions as having a size of 1.  For example,
    to clear a portion of a two-dimensional texture, the application would
    use <zoffset> equal to zero and <depth> equal to one.

    <format> and <type> specify the format and type of the source
    data and are interpreted as they are for TexImage3D, as described in
    section 8.4.4 (Transfer of Pixel Rectangles). Textures with a base
    internal format of DEPTH_COMPONENT, STENCIL_INDEX, or DEPTH_STENCIL
    require depth component, stencil, or depth/stencil component data
    respectively. Textures with other base internal formats require RGBA
    formats. Textures with integer internal formats (see table 8.12) require
    integer data.

    <data> is a pointer to an array of between one and four components of
    texel data that will be used as the source for the constant fill value.
    The elements of <data> are converted by the GL into the
    <internalformat> of the texture image (that was specified when the level
    was defined by any of the TexImage, TexStorage or CopyTexImage
    commands) in the manner described in section 8.4.4 (Transfer of Pixel
    Rectangles), and then used to fill the specified range of the
    destination texture level.  If <data> is NULL, then the pointer is
    ignored and the sub-range of the texture image is filled with zeros.
    If <texture> is a multisample texture, all the samples in a texel
    are cleared to the same value, that which was specified by <data>.

    Errors

    An INVALID_OPERATION error is generated if <texture> is zero or not the
    name of a texture object.
    An INVALID_OPERATION error is generated if <texture> is a buffer texture.
    An INVALID_OPERATION error is generated if <texture> has a compressed
    internal format.
    An INVALID_OPERATION error is generated if the base internal format is
    DEPTH_COMPONENT and <format> is not DEPTH_COMPONENT.
    An INVALID_OPERATION error is generated if the base internal format is
    DEPTH_STENCIL and <format> is not DEPTH_STENCIL.
    An INVALID_OPERATION error is generated if the base internal format is
    STENCIL_INDEX and <format> is not STENCIL_INDEX.
    An INVALID_OPERATION error is generated if the base internal format is
    RGBA and the <format> is DEPTH_COMPONENT, STENCIL_INDEX, or DEPTH_STENCIL.
    An INVALID_OPERATION error is generated if the internal format is integer
    and <format> does not specify integer data.
    An INVALID_OPERATION error is generated if the internal format is
    not integer and <format> does specify integer data.
    An INVALID_OPERATION error is generated if the <xoffset>, <yoffset>,
    <zoffset>, <width>, <height>, and <depth> parameters (or combinations
    thereof) fall outside the defined texture image array (including border,
    if any).

    The command

      void ClearTexImage(uint texture, int level,
                         enum format, enum type,
                         const void * data);

    is equivalent to calling ClearTexSubImage with <xoffset>, <yoffset>,
    and <zoffset> equal to -<b> and <width>, <height>, and <depth> equal
    to the dimensions of the texture image plus 2x<b> (or zero and one
    for dimensions the texture doesn't have).

    Errors

    An INVALID_OPERATION error is generated if the image array identified by
    <level> has not previously been defined by a TexImage* or TexStorage*
    command."

Additions to Chapter 22 of the OpenGL 4.3 (Core Profile) Specification

    In section 22.3 add the following to the list of <pname> values accepted
    by GetInternalformativ and GetInternalformati64v:

        CLEAR_TEXTURE: The support for using the resource with the
        ClearTex*Image commands is returned in <params>.  Possible values
        returned are FULL_SUPPORT, CAVEAT_SUPPORT, or NONE.  If the resource
        or operation is not supported, NONE is returned.

Additions to the AGL/EGL/GLX/WGL Specifications

    None

Dependencies on EXT_texture_integer

    If EXT_texture_integer or equivalent functionality is not supported,
    ignore all references to integer textures.

Dependencies on ARB_texture_buffer_object

    If ARB_texture_buffer_object of equivalent functionality is not supported,
    ignore all references to buffer textures.

Dependencies on ARB_texture_multisample

    If ARB_texture_multisample or equivalent functionality is not supported,
    ignore all references to multisample textures.

Dependencies on ARB_depth_texture

    If ARB_depth_texture or equivalent functionality is not supported,
    ignore all references to depth textures and depth component formats.

Dependencies on EXT_packed_depth_stencil and ARB_framebuffer_object

    If EXT_packed_depth_stencil, ARB_framebuffer_object, or equivalent
    functionality is not supported, ignore all refereces to depth/stencil
    textures and depth/stencil components.

Dependencies on ARB_texture_stencil8

    If ARB_texture_stencil8 or equivalent functionality is not supported,
    ignore all references to stencil textures and stencil index formats.

Dependencies on ARB_shader_image_load_store

    If ARB_shader_image_load_store or equivalent functionality is not
    supported, ignore the references to TEXTURE_UPDATE_BARRIER_BIT and
    edits to section 7.12.2.

Dependencies on ARB_internalformat_query2

    If ARB_internalformat_query2 or equivalent functionality is not
    supported, ignore the references GetInternalformat* and the CLEAR_TEXTURE
    enum is not added.

Errors

    An INVALID_OPERATION error is generated by ClearTexImage or
    ClearTexSubImage if <texture> is zero or not the name of a texture object.

    An INVALID_OPERATION error is generated by ClearTexImage or
    ClearTexSubImage if <texture> is a buffer texture.

    An INVALID_OPERATION error is generated by ClearTexImage or
    ClearTexSubImage if <texture> has a compressed internal format.

    An INVALID_OPERATION error is generated by ClearTexImage or
    ClearTexSubImage if the base internal format is DEPTH_COMPONENT and
    <format> is not DEPTH_COMPONENT.

    An INVALID_OPERATION error is generated by ClearTexImage or
    ClearTexSubImage if the base internal format is STENCIL_INDEX and
    <format> is not STENCIL_INDEX.

    An INVALID_OPERATION error is generated by ClearTexImage or
    ClearTexSubImage if the base internal format is DEPTH_STENCIL and
    <format> is not DEPTH_STENCIL.

    An INVALID_OPERATION error is generated by ClearTexImage or
    ClearTexSubImage if the base internal format is RGBA and the <format>
    is DEPTH_COMPONENT, STENCIL_INDEX, or DEPTH_STENCIL.

    An INVALID_OPERATION error is generated by ClearTexImage or
    ClearTexSubImage if the internal format is integer and <format> does not
    specify integer data.

    An INVALID_OPERATION error is generated by ClearTexImage or
    ClearTexSubImage if the internal format is not integer and <format>
    does specify integer data.

    An INVALID_OPERATION error is generated by ClearTexSubImage if the
    <xoffset>, <yoffset>, <zoffset>, <width>, <height>, and <depth>
    parameters (or combinations thereof) fall outside the defined texture
    image array (including border, if any).

    An INVALID_OPERATION error is generated by ClearTexImage if the
    image array identified by <level> has not previously been defined.

New State

    None.

New Implementation Dependent State

    None.

Sample Code

    TBD

Conformance Tests

    Needed.

Issues

    1. Should these commands use <target> or <name> to specify the object?

    DISCUSSION: TexImage/TexSubImage pass in a target and operate on the
    object that is currently bound to that target. However the
    InvalidateTexImage/InvalidateTexSubImage commands operate on a named
    object. A target would be necessary if ClearTex*Image supported
    defining new textures. It does not support this. It could also be
    useful for clearing only one face of a cube map - if this is useful
    this can be accomplished by creating a 2D texture which is view of one
    face of a cube map.

    RESOLVED. We will use named objects and be consistent with
    the InvalidateTexImage entry points.

    2. Should scissor and viewport settings be respected?

    DISCUSSION: The scissor and viewport settings really have no meaning
    with respect to a texture that is not attached to a framebuffer object.
    D3D11's ClearRenderTargetView also does not apply scissor or viewport.

    RESOLVED. Scissor and viewport are not relevant to this command.

    3. Should we support clearing only a portion of a texture?

    RESOLVED. Yes, it seems like it would be useful functionality.

    4. How many command variants do we need? We may want to provide
    integer, unsigned integer, or floating-point data. Is a void
    parameter sufficient, or do we need to use different commands
    similar to ClearBuffer?

    RESOLVED. Similar to the initial texure data specification via
    TexSubImage, a single command is provided. The format and type
    parameters specify how the data should be interpreted, and enough
    data needs to provided to completely define one texel. This operates
    similarly to the ClearBufferData commands.

    5. Should depth/stencil textures be supported? If so, are the depth
    and stencil values provided separately?

    RESOLVED. Depth/stencil textures are supported in the same manner
    they are for TexSubImage.  The format and type describe how the
    data is interpreted.

    6. Should compressed textures be supported? If so, does a whole
    block of data need to be specified?

    DISCUSSION. For many compressed formats it may be fairly straight
    forward for a driver to construct a block based on a single value.
    It is unclear if this applies to all formats or not. In any case,
    if compressed textures are supported, the sub region that is cleared
    is required to be aligned on with block boundaries.

    RESOLUTION: No. Compressed textures are not support. Not all formats
    support runtime compression so providing uncompressed data would not
    be viable. Providing a single block of compressed data may be possible
    but then it would have to be aligned to block boundaries and this becomes
    more of a sub-image command than a clear operation. This does not seem
    to have a compelling use-case so it is not being added at this time. It
    can always be added in a layered extension if desired.

    7. Should we add an entry point for renderbuffers?

    RESOLVED. No. At this point there is really no use case that
    renderbuffers support that cannot be supported by multisample textures.
    Thus in order to avoid aggrandizing renderbuffers, this will not be
    provided.  Additionally, renderbuffers are by definition renderable,
    and thus they can be cleared by the standard mechanisms without affecting
    memory allocation patterns.

    8. Should these commands support support loading <data> from the pixel
    unpack buffer (ie PBO) or only client memory?

    RESOLVED. ClearTex*Image only supports <data> in client memory, like the
    comparable ClearBuffer*Data commands. There seems to be little value in
    supporting buffer-backed sources when there is only one texels' worth of
    data to upload. If the worst case (RGBA32F) it is only 16 bytes of data.

    9. How does clearing multisample textures work?

    RESOLVED: As with regular textures, the dimensions are specified in texels.
    All the samples in a texel are cleared to the same values, ie the one set
    of values which are provided in <data>.

    10. Does conditional rendering interact with the ClearTex*Image commands?

    RESOLVED: No. This is treated in a similar manner to the TexSubImage
    commands rather than the other image clearing commands and is thus not
    affected by conditional rendering.  This is similar to the clear buffer
    data commands.

    11. Would there be any benefit to providing some form of colormask style
    behavior?

    RESOLVED: No. This is intended to be a simple operation like memset.
    If more complicated behaviour is required, one of the other mechanisms
    for specifying texture image data should be used.

    12. Can ClearTexImage be used on buffer textures?

    RESOLVED: No. In the same way that TexSubImage cannot be used for
    buffer textures, neither can ClearTexImage.  The ClearBuffer{Sub}Data
    commands can be used for this purpose.

    13. What happens if the subregion to be cleared is larger than the defined
    texture level's size?

    RESOLVED: This is an error similar to TexSubImage3D.

    14. What about texture zero?

    RESOLVED: Which one? There are many zero textures (one of each target type).
    Since these commands don't distinguish based on target, this is an error.
    Also we really don't want to aggrandize texture zero.

    15. What happens for undefined levels?

    RESOLVED: This depends on the command.  An undefined level will have
    width, height, and depth of zero, and thus a ClearTexSubImage command with
    a non-zero sized clear region will fail the bounds check and result in
    an INVALID_OPERATION. A zero-sized clear region with no-offset is allowed,
    but is a no-op. ClearTexImage though, is defined in terms of
    ClearTexSubImage called with appropriate dimensions and offsets. Since
    it's unclear how many dimensions an undefined level would have, we've
    defined this to be an error.

    16. What happens for incomplete textures?

    RESOLVED: Incomplete textures do not affect the behaviour of these commands
    since they operate on individual levels.  However see Issue 15 for
    behaviour on undefined levels.

    17. How does ClearTexture interact with the (compatibility) GENERATE_MIPMAP
    texture state?

    RESOLVED: It doesn't (i.e. it doesn't provoke automatic mipmap generation).
    In general the policy is that new functionality doesn't need to interact with
    legacy functionality. This also falls out of the way the spec is written
    since section 8.14.5 on automatic mipmap generation says it applies to the
    image specification operations defined in sections 8.5 to 8.7. ClearTexture
    is defined in section 8.21.

Revision History

    Revision 16, 2013/08/12, Jon Leech
        - Clarify issue 8 (public bug 934).
    Revision 15, 2013/06/25, dgkoch
        - assign value for GL_CLEAR_TEXTURE
    Revision 14, 2013/06/21, dgkoch
        - add CLEAR_TEXTURE for GetInternalformat* query.
    Revision 13, 2013/06/04, dgkoch
      - add issue 17.
    Revision 12, 2013/05/30, Jon Leech
      - Move new error to per-command Errors section.
    Revision 11, 2013/05/30, dgkoch
      - Add Issues 15, 16. Add error for ClearTexImage on undefined levels.
      - fix naming of ARB_texture_stencil8 dependency.
    Revision 10, 2013/05/30, Jon Leech
      - Remove (some) redundant error descriptions from inline spec language
        and simplify description of layer-faces.
    Revision 9, 2013/05/29, dgkoch
      - change error back to INVALID_OP
    Revision 8, 2013/05/29, dgkoch
      - added general error conditions to command description
      - change error for non-texture names to be INVALID_VALUE
      - minor grammatical and formatting fixes.
    Revision 7, 2013/05/29, dgkoch
      - add interaction with ARB_shader_image_load_store / MemoryBarrier
    Revision 6, 2013/05/02, dgkoch
      - add interaction with stencil textures
      - fix typos and formatting
    Revision 5, 2013/04/23, dgkoch
      - fix more typos, confirm out of bounds error
    Revision 4, 2013/04/22, dgkoch
      - resolved issue 13, and update language appropriately
      - add error for texture zero, remove error for undefined levels
      - fix typos
    Revision 3, 2013/04/21, dgkoch
      - various formatting fixes
      - add dependencies
      - flesh out a lot of spec language instead of referring to other sections
      - add error conditions
      - allow larger dimensions than the texture image array.
      - Resolved issues 6,7,10,11
      - Added issues 12,13.
    Revision 2, 2013/03/13, dgkoch
      - Added issues 9,10,11
      - Resolved issues 1,2,3,4,5,8,9
      - update contact & contributors
    Revision 1, 2013/01/15, dgkoch
      - Initial revision