| Name |
| |
| NV_texture_compression_s3tc |
| |
| Name Strings |
| |
| GL_NV_texture_compression_s3tc |
| |
| Contributors |
| |
| Contributors to the OpenGL EXT_texture_compression_s3tc extension |
| Greg Roth, NVIDIA |
| |
| Contact |
| |
| Ian Stewart, NVIDIA Corporation (istewart 'at' nvidia.com) |
| |
| Status |
| |
| Complete. |
| |
| Version |
| |
| Last Modifed Date: Sep 27, 2012 |
| NVIDIA Revision: 1 |
| |
| Number |
| |
| OpenGL ES Extension #128 |
| |
| Dependencies |
| |
| This extension is written against the OpenGL ES 2.0.25 |
| Specification. |
| |
| This extension interacts with NV_texture_array |
| |
| Overview |
| |
| This extension adds support for S3's S3TC texture compression |
| formats, including DXT1, DXT3, and DXT5. For the DXT1 format, |
| this specification supports an RGB-only mode and a special RGBA |
| mode with single-bit alpha. |
| |
| IP Status |
| |
| Contact S3 Incorporated (http://www.s3.com) regarding any |
| intellectual property issues associated with implementing this |
| extension. |
| |
| WARNING: Vendors able to support S3TC texture compression in |
| Direct3D drivers do not necessarily have the right to use the same |
| functionality in OpenGL ES. |
| |
| New Tokens |
| |
| Accepted by the <internalformat> parameter of CompressedTexImage2D |
| and CompressedTexSubImage2D: |
| |
| COMPRESSED_RGB_S3TC_DXT1_NV 0x83F0 |
| COMPRESSED_RGBA_S3TC_DXT1_NV 0x83F1 |
| COMPRESSED_RGBA_S3TC_DXT3_NV 0x83F2 |
| COMPRESSED_RGBA_S3TC_DXT5_NV 0x83F3 |
| |
| Additions to Chapter 3 of the OpenGL ES 2.0.25 Specification |
| |
| Modify Section 3.7.3, "Compressed Texture Images" |
| |
| (Replace first two sentences with) |
| |
| Texture images may also be specified or modified using image data |
| already stored in a known compressed image format. The GL defines |
| some specific compressed formats, and others may be defined by GL |
| extensions. |
| |
| (Insert after section describing CompressedTexImage2D) |
| |
| The specific compressed texture formats supported by |
| CompressedTexImage2D, and the corresponding base internal format |
| for each specific format, are defined in table 3.X. |
| |
| Table 3.X: "Specific compressed texture formats" |
| |
| Compressed Internal Formats Base Internal Format |
| =========================== ==================== |
| COMPRESSED_RGB_S3TC_DXT1_NV RGB |
| COMPRESSED_RGBA_S3TC_DXT1_NV RGBA |
| COMPRESSED_RGBA_S3TC_DXT3_NV RGBA |
| COMPRESSED_RGBA_S3TC_DXT5_NV RGBA |
| |
| (Replace last paragraph with) |
| |
| If the internal format is one of COMPRESSED_RGB_S3TC_DXT1_NV, |
| COMPRESSED_RGBA_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT3_NV, or |
| COMPRESSED_RGBA_S3TC_DXT5_NV, the compressed texture is stored |
| using one of several S3TC compressed texture image formats and is |
| easily edited along 4x4 texel boundaries. In this case, |
| CompressedTexImage2D will result in an INVALID_OPERATION error |
| if one of the following conditions occurs: |
| |
| * <width> is not a multiple of four, and <width> plus |
| <xoffset> is not equal to TEXTURE_WIDTH; |
| |
| * <height> is not a multiple of four, and <height> plus |
| <yoffset> is not equal to TEXTURE_HEIGHT; or |
| |
| * <xoffset> or <yoffset> is not a multiple of four. |
| |
| For any other formats, calling CompressedTexSubImage2D will result |
| in an INVALID_OPERATION error if <xoffset> or <yoffset> is not |
| equal to zero, or if <width> and <height> do not match the width |
| and height of the texture, respectively. The contents of any texel |
| outside the region modified by the call are undefined. These |
| restrictions may be relaxed for other specific compressed internal |
| formats whose images are easily modified. |
| |
| Interactions with NV_texture_array |
| |
| If NV_texture_array is supported, the S3TC compressed formats may |
| also be used as the internal formats given to |
| CompressedTexImage3DNV and CompressedTexSubImage3DNV. The |
| restrictions for the <width>, <height>, <xoffset>, and <yoffset> |
| parameters of the CompressedTexSubImage2D function when used with |
| S3TC compressed texture formats, described in this extension, also |
| apply to the identically named parameters of |
| CompressedTexSubImage3DNV. |
| |
| Errors |
| |
| INVALID_OPERATION is generated by CopyTexSubImage2D if the texture |
| image <level> bound to <target> has internal format |
| COMPRESSED_RGB_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT1_NV, |
| COMPRESSED_RGBA_S3TC_DXT3_NV, or COMPRESSED_RGBA_S3TC_DXT5_NV. |
| |
| INVALID_OPERATION is generated by CompressedTexSubImage2D if |
| <internalformat> is COMPRESSED_RGB_S3TC_DXT1_NV, |
| COMPRESSED_RGBA_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT3_NV, or |
| COMPRESSED_RGBA_S3TC_DXT5_NV and any of the following apply: |
| |
| * <width> is not a multiple of four, and <width> plus |
| <xoffset> is not equal to the texture width; |
| |
| * <height> is not a multiple of four, and <height> plus |
| <yoffset> is not equal to the texture height; or |
| |
| * <xoffset> or <yoffset> is not a multiple of four. |
| |
| New State |
| |
| The queries for NUM_COMPRESSED_TEXTURE_FORMATS and |
| COMPRESSED_TEXTURE_FORMATS include COMPRESSED_RGB_S3TC_DXT1_NV, |
| COMPRESSED_RGBA_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT3_NV, and |
| COMPRESSED_RGBA_S3TC_DXT5_NV. |
| |
| Appendix |
| |
| S3TC Compressed Texture Image Formats |
| |
| Compressed texture images stored using the S3TC compressed image |
| formats are represented as a collection of 4x4 texel blocks, where |
| each block contains 64 or 128 bits of texel data. The image is |
| encoded as a normal 2D raster image in which each 4x4 block is |
| treated as a single pixel. If an S3TC image has a width or height |
| that is not a multiple of four, the data corresponding to texels |
| outside the image are irrelevant and undefined. |
| |
| When an S3TC image with a width of <w>, height of <h>, and block |
| size of <blocksize> (8 or 16 bytes) is decoded, the corresponding |
| image size (in bytes) is: |
| |
| ceil(<w>/4) * ceil(<h>/4) * blocksize. |
| |
| When decoding an S3TC image, the block containing the texel at |
| offset (<x>, <y>) begins at an offset (in bytes) relative to the |
| base of the image of: |
| |
| blocksize * (ceil(<w>/4) * floor(<y>/4) + floor(<x>/4)). |
| |
| The data corresponding to a specific texel (<x>, <y>) are |
| extracted from a 4x4 texel block using a relative (x,y) value of |
| |
| (<x> modulo 4, <y> modulo 4). |
| |
| There are four distinct S3TC image formats: |
| |
| COMPRESSED_RGB_S3TC_DXT1_NV: Each 4x4 block of texels consists of |
| 64 bits of RGB image data. |
| |
| Each RGB image data block is encoded as a sequence of 8 bytes, |
| called (in order of increasing address): |
| |
| c0_lo, c0_hi, c1_lo, c1_hi, bits_0, bits_1, bits_2, bits_3 |
| |
| The 8 bytes of the block are decoded into three quantities: |
| |
| color0 = c0_lo + c0_hi * 256 |
| color1 = c1_lo + c1_hi * 256 |
| bits = bits_0 + 256 * (bits_1 + 256 * (bits_2 + 256 * bits_3)) |
| |
| color0 and color1 are 16-bit unsigned integers that are |
| unpacked to RGB colors RGB0 and RGB1 as though they were |
| 16-bit packed pixels with a <format> of RGB and a type of |
| UNSIGNED_SHORT_5_6_5. |
| |
| bits is a 32-bit unsigned integer, from which a two-bit |
| control code is extracted for a texel at location (x,y) in the |
| block using: |
| |
| code(x,y) = bits[2*(4*y+x)+1 .. 2*(4*y+x)+0] |
| |
| where bit 31 is the most significant and bit 0 is the least |
| significant bit. |
| |
| The RGB color for a texel at location (x,y) in the block is |
| given by: |
| |
| RGB0, if color0 > color1 and code(x,y) == 0 |
| RGB1, if color0 > color1 and code(x,y) == 1 |
| (2*RGB0+RGB1)/3, if color0 > color1 and code(x,y) == 2 |
| (RGB0+2*RGB1)/3, if color0 > color1 and code(x,y) == 3 |
| |
| RGB0, if color0 <= color1 and code(x,y) == 0 |
| RGB1, if color0 <= color1 and code(x,y) == 1 |
| (RGB0+RGB1)/2, if color0 <= color1 and code(x,y) == 2 |
| BLACK, if color0 <= color1 and code(x,y) == 3 |
| |
| Arithmetic operations are done per component, and BLACK refers |
| to an RGB color where red, green, and blue are all zero. |
| |
| Since this image has an RGB format, there is no alpha |
| component and the image is considered fully opaque. |
| |
| |
| COMPRESSED_RGBA_S3TC_DXT1_NV: Each 4x4 block of texels consists |
| of 64 bits of RGB image data and minimal alpha information. The |
| RGB components of a texel are extracted in the same way as |
| COMPRESSED_RGB_S3TC_DXT1_NV. |
| |
| The alpha component for a texel at location (x,y) in the block |
| is given by: |
| |
| 0.0, if color0 <= color1 and code(x,y) == 3 |
| 1.0, otherwise |
| |
| |
| COMPRESSED_RGBA_S3TC_DXT3_NV: Each 4x4 block of texels consists |
| of 64 bits of uncompressed alpha image data followed by 64 bits of |
| RGB image data. |
| |
| Each RGB image data block is encoded according to the |
| COMPRESSED_RGB_S3TC_DXT1_NV format, with the exception that |
| the two code bits always use the non-transparent encodings. |
| In other words, they are treated as though color0 > color1, |
| regardless of the actual values of color0 and color1. |
| |
| Each alpha image data block is encoded as a sequence of 8 |
| bytes, called (in order of increasing address): |
| |
| a0, a1, a2, a3, a4, a5, a6, a7 |
| |
| The 8 bytes of the block are decoded into one 64-bit integer: |
| |
| alpha = a0 + 256 * (a1 + 256 * (a2 + 256 * (a3 + 256 * (a4 + |
| 256 * (a5 + 256 * (a6 + 256 * a7)))))) |
| |
| alpha is a 64-bit unsigned integer, from which a four-bit |
| alpha value is extracted for a texel at location (x,y) in the |
| block using: |
| |
| alpha(x,y) = bits[4*(4*y+x)+3 .. 4*(4*y+x)+0] |
| |
| where bit 63 is the most significant and bit 0 is the least |
| significant bit. |
| |
| The alpha component for a texel at location (x,y) in the block |
| is given by alpha(x,y) / 15. |
| |
| |
| COMPRESSED_RGBA_S3TC_DXT5_NV: Each 4x4 block of texels consists |
| of 64 bits of compressed alpha image data followed by 64 bits of |
| RGB image data. |
| |
| Each RGB image data block is encoded according to the |
| COMPRESSED_RGB_S3TC_DXT1_NV format, with the exception that |
| the two code bits always use the non-transparent encodings. |
| In other words, they are treated as though color0 > color1, |
| regardless of the actual values of color0 and color1. |
| |
| Each alpha image data block is encoded as a sequence of 8 |
| bytes, called (in order of increasing address): |
| |
| alpha0, alpha1, bits_0, bits_1, bits_2, bits_3, bits_4, bits_5 |
| |
| The alpha0 and alpha1 are 8-bit unsigned bytes converted to |
| alpha components by multiplying by 1/255. |
| |
| The 6 "bits" bytes of the block are decoded into one 48-bit |
| integer: |
| |
| bits = bits_0 + 256 * (bits_1 + 256 * (bits_2 + 256 * (bits_3 + |
| 256 * (bits_4 + 256 * bits_5)))) |
| |
| bits is a 48-bit unsigned integer, from which a three-bit |
| control code is extracted for a texel at location (x,y) in the |
| block using: |
| |
| code(x,y) = bits[3*(4*y+x)+2 .. 3*(4*y+x)+0] |
| |
| where bit 47 is the most significant and bit 0 is the least |
| significant bit. |
| |
| The alpha component for a texel at location (x,y) in the block |
| is given by: |
| |
| alpha0, code(x,y) == 0 |
| alpha1, code(x,y) == 1 |
| |
| (6*alpha0 + 1*alpha1)/7, alpha0 > alpha1 and code(x,y) == 2 |
| (5*alpha0 + 2*alpha1)/7, alpha0 > alpha1 and code(x,y) == 3 |
| (4*alpha0 + 3*alpha1)/7, alpha0 > alpha1 and code(x,y) == 4 |
| (3*alpha0 + 4*alpha1)/7, alpha0 > alpha1 and code(x,y) == 5 |
| (2*alpha0 + 5*alpha1)/7, alpha0 > alpha1 and code(x,y) == 6 |
| (1*alpha0 + 6*alpha1)/7, alpha0 > alpha1 and code(x,y) == 7 |
| |
| (4*alpha0 + 1*alpha1)/5, alpha0 <= alpha1 and code(x,y) == 2 |
| (3*alpha0 + 2*alpha1)/5, alpha0 <= alpha1 and code(x,y) == 3 |
| (2*alpha0 + 3*alpha1)/5, alpha0 <= alpha1 and code(x,y) == 4 |
| (1*alpha0 + 4*alpha1)/5, alpha0 <= alpha1 and code(x,y) == 5 |
| 0.0, alpha0 <= alpha1 and code(x,y) == 6 |
| 1.0, alpha0 <= alpha1 and code(x,y) == 7 |
| |
| |
| Issues |
| |
| None |
| |
| Revision History |
| |
| Rev. Date Author Changes |
| ---- -------- --------- ------------------------------------- |
| 1 09/27/2012 istewart First revision, based of the |
| EXT_texture_compression_s3tc |
| specification. |
| |
| # vim:ai:ts=4:sts=4:expandtab:textwidth=70 |
