Name ARB_texture_compression_rgtc Name Strings GL_ARB_texture_compression_rgtc Contributors Mark J. Kilgard, NVIDIA Pat Brown, NVIDIA Yanjun Zhang, S3 Attila Barsi, Holografika Contact Mark J. Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com) Notice Copyright (c) 2008-2013 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 Approved by the ARB on July 11, 2008 Version Last Modified Date: January 27, 2015 Revision: 1.7 Number ARB Extension #52 Dependencies OpenGL 1.3 or ARB_texture_compression required This extension is written against the OpenGL 2.0 (September 7, 2004) specification. This extension interacts with OpenGL 2.0 and ARB_texture_non_power_of_two. Overview This extension introduces four new block-based texture compression formats suited for unsigned and signed red and red-green textures (hence the name "rgtc" for Red-Green Texture Compression). These formats are designed to reduce the storage requirements and memory bandwidth required for red and red-green textures by a factor of 2-to-1 over conventional uncompressed luminance and luminance-alpha textures with 8-bit components (GL_LUMINANCE8 and GL_LUMINANCE8_ALPHA8). The compressed signed red-green format is reasonably suited for storing compressed normal maps. This extension uses the same compression format as the EXT_texture_compression_latc extension except the color data is stored in the red and green components rather than luminance and alpha. Representing compressed red and green components is consistent with the BC4 and BC5 compressed formats supported by DirectX 10. New Procedures and Functions None. New Tokens Accepted by the parameter of TexImage2D, CopyTexImage2D, and CompressedTexImage2D and the parameter of CompressedTexSubImage2D: COMPRESSED_RED_RGTC1 0x8DBB COMPRESSED_SIGNED_RED_RGTC1 0x8DBC COMPRESSED_RG_RGTC2 0x8DBD COMPRESSED_SIGNED_RG_RGTC2 0x8DBE Additions to Chapter 2 of the OpenGL 2.0 Specification (OpenGL Operation) None. Additions to Chapter 3 of the OpenGL 2.0 Specification (Rasterization) -- Section 3.8.1, Texture Image Specification Add to Table 3.17 (page 155): Specific compressed internal formats Compressed Internal Format Base Internal Format --------------------------------- -------------------- COMPRESSED_RED_RGTC1 RED COMPRESSED_SIGNED_RED_RGTC1 RED COMPRESSED_RG_RGTC2 RG COMPRESSED_SIGNED_RG_RGTC2 RG -- Section 3.8.2, Alternative Texture Image Specification Commands Add to the end of the section (page 163): "If the internal format of the texture image being modified is COMPRESSED_RED_RGTC1, COMPRESSED_SIGNED_RED_RGTC1, COMPRESSED_RG_RGTC2, or COMPRESSED_SIGNED_RG_RGTC2, the texture is stored using one of the two RGTC compressed texture image encodings (see appendix). Such images are easily edited along 4x4 texel boundaries, so the limitations on TexSubImage2D or CopyTexSubImage2D parameters are relaxed. TexSubImage2D and CopyTexSubImage2D will result in an INVALID_OPERATION error only if one of the following conditions occurs: * is not a multiple of four, plus is not equal to TEXTURE_WIDTH, and either or is non-zero; * is not a multiple of four, plus is not equal to TEXTURE_HEIGHT, and either or is non-zero; or * or is not a multiple of four. The contents of any 4x4 block of texels of an RGTC compressed texture image that does not intersect the area being modified are preserved during valid TexSubImage2D and CopyTexSubImage2D calls." -- Section 3.8.3, Compressed Texture Images Add after the 4th paragraph (page 164) at the end of the CompressedTexImage discussion: "If is COMPRESSED_RED_RGTC1, COMPRESSED_SIGNED_RED_RGTC1, COMPRESSED_RG_RGTC2, or COMPRESSED_SIGNED_RG_RGTC2, the compressed texture is stored using one of several RGTC compressed texture image formats. The RGTC texture compression algorithm supports only 2D images without borders. CompressedTexImage1D and CompressedTexImage3D produce an INVALID_ENUM error if is an RGTC format. CompressedTexImage2D will produce an INVALID_OPERATION error if is non-zero. Add to the end of the section (page 166) at the end of the CompressedTexSubImage discussion: "If the internal format of the texture image being modified is COMPRESSED_RED_RGTC1, COMPRESSED_SIGNED_RED_RGTC1, COMPRESSED_RG_RGTC2, or COMPRESSED_SIGNED_RG_RGTC2, the texture is stored using one of the several RGTC compressed texture image formats. Since the RGTC texture compression algorithm supports only 2D images, CompressedTexSubImage1D and CompressedTexSubImage3D produce an INVALID_ENUM error if is an RGTC format. Since RGTC images are easily edited along 4x4 texel boundaries, the limitations on CompressedTexSubImage2D are relaxed. CompressedTexSubImage2D will result in an INVALID_OPERATION error only if one of the following conditions occurs: * is not a multiple of four, and plus is not equal to TEXTURE_WIDTH; * is not a multiple of four, and plus is not equal to TEXTURE_HEIGHT; or * or is not a multiple of four. The contents of any 4x4 block of texels of an RGTC compressed texture image that does not intersect the area being modified are preserved during valid TexSubImage2D and CopyTexSubImage2D calls." -- Section 3.8.8, Texture Minification Add a sentence to the last paragraph (page 174) just prior to the "Mipmapping" subheading: "If the texture's internal format lacks components that exist in the texture's base internal format, such components are considered zero when the texture border color is sampled. (So despite the RGB base internal format of the COMPRESSED_RED_RGTC1 and COMPRESSED_SIGNED_RED_RGTC1 formats, the green and blue components of the texture border color are always considered zero. Likewise for the COMPRESSED_RG_RGTC2, and COMPRESSED_SIGNED_RG_RGTC2 formats, the blue component is always considered zero.)" Additions to Chapter 4 of the OpenGL 2.0 Specification (Per-Fragment Operations and the Frame Buffer) None. Additions to Chapter 5 of the OpenGL 2.0 Specification (Special Functions) None. Additions to Chapter 6 of the OpenGL 2.0 Specification (State and State Requests) None. Additions to Appendix A of the OpenGL 2.0 Specification (Invariance) None. Additions to the AGL/GLX/WGL Specifications None. GLX Protocol None. Dependencies on ARB_texture_compression If ARB_texture_compression is supported, all the errors and accepted tokens for CompressedTexImage1D, CompressedTexImage2D, CompressedTexImage3D, CompressedTexSubImage1D, CompressedTexSubImage2D, and CompressedTexSubImage3D also apply respectively to the ARB-suffixed CompressedTexImage1DARB, CompressedTexImage2DARB, CompressedTexImage3DARB, CompressedTexSubImage1DARB, CompressedTexSubImage2DARB, and CompressedTexSubImage3DARB. Dependencies on OpenGL 2.0 or ARB_texture_non_power_of_two If OpenGL 2.0 or ARB_texture_non_power_of_two is supported, compressed texture images can have sizes that are neither multiples of four nor small values like one or two. The original version of this specification didn't allow TexSubImage2D and CompressedTexSubImage2D to update only a portion of such images. The spec has been updated to allow such edits in the spirit of the resolution of issue (3) of the EXT_texture_compression_s3tc specification. See the "Implementation Note" section for more details. Errors INVALID_ENUM is generated by CompressedTexImage1D or CompressedTexImage3D if is COMPRESSED_RED_RGTC1, COMPRESSED_SIGNED_RED_RGTC1, COMPRESSED_RG_RGTC2, or COMPRESSED_SIGNED_RG_RGTC2. INVALID_OPERATION is generated by CompressedTexImage2D if is COMPRESSED_RED_RGTC1, COMPRESSED_SIGNED_RED_RGTC1, COMPRESSED_RG_RGTC2, or COMPRESSED_SIGNED_RG_RGTC2 and is not equal to zero. INVALID_ENUM is generated by CompressedTexSubImage1D or CompressedTexSubImage3D if is COMPRESSED_RED_RGTC1, COMPRESSED_SIGNED_RED_RGTC1, COMPRESSED_RG_RGTC2, or COMPRESSED_SIGNED_RG_RGTC2. INVALID_OPERATION is generated by TexSubImage2D or CopyTexSubImage2D if TEXTURE_INTERNAL_FORMAT is COMPRESSED_RED_RGTC1, COMPRESSED_SIGNED_RED_RGTC1, COMPRESSED_RG_RGTC2, or COMPRESSED_SIGNED_RG_RGTC2 and any of the following apply: * is not a multiple of four, plus is not equal to TEXTURE_WIDTH, and either or is non-zero; * is not a multiple of four, plus is not equal to TEXTURE_HEIGHT, and either or is non-zero; or * or is not a multiple of four. INVALID_OPERATION is generated by CompressedTexSubImage2D if TEXTURE_INTERNAL_FORMAT is COMPRESSED_RED_RGTC1, COMPRESSED_SIGNED_RED_RGTC1, COMPRESSED_RG_RGTC2, or COMPRESSED_SIGNED_RG_RGTC2 and any of the following apply: * is not a multiple of four, and plus is not equal to TEXTURE_WIDTH; * is not a multiple of four, and plus is not equal to TEXTURE_HEIGHT; or * or is not a multiple of four. The following restrictions from the ARB_texture_compression specification do not apply to RGTC texture formats, since subimage modification is straightforward as long as the subimage is properly aligned. DELETE: INVALID_OPERATION is generated by TexSubImage1D, TexSubImage2D, DELETE: TexSubImage3D, CopyTexSubImage1D, CopyTexSubImage2D, or DELETE: CopyTexSubImage3D if the internal format of the texture image is DELETE: compressed and , , or does not equal DELETE: -b, where b is value of TEXTURE_BORDER. DELETE: INVALID_VALUE is generated by CompressedTexSubImage1D, DELETE: CompressedTexSubImage2D, or CompressedTexSubImage3D if the DELETE: entire texture image is not being edited: if , DELETE: , or is greater than -b, + is DELETE: less than w+b, + is less than h+b, or DELETE: + is less than d+b, where b is the value of DELETE: TEXTURE_BORDER, w is the value of TEXTURE_WIDTH, h is the value of DELETE: TEXTURE_HEIGHT, and d is the value of TEXTURE_DEPTH. See also errors in the GL_ARB_texture_compression specification. New State 4 new state values are added for the per-texture object GL_TEXTURE_INTERNAL_FORMAT state. In the "Textures" state table( page 278), increment the TEXTURE_INTERNAL_FORMAT subscript for Z by 4 in the "Type" row. [NOTE: The OpenGL 2.0 specification actually should read "n x Z48*" because of the 6 generic compressed internal formats in table 3.18.] New Implementation Dependent State None Appendix RGTC Compressed Texture Image Formats Compressed texture images stored using the RGTC compressed image encodings 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 RGTC 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 RGTC image with a width of , height of , and block size of (8 or 16 bytes) is decoded, the corresponding image size (in bytes) is: ceil(/4) * ceil(/4) * blocksize. When decoding an RGTC image, the block containing the texel at offset (, ) begins at an offset (in bytes) relative to the base of the image of: blocksize * (ceil(/4) * floor(/4) + floor(/4)). The data corresponding to a specific texel (, ) are extracted from a 4x4 texel block using a relative (x,y) value of ( modulo 4, modulo 4). There are four distinct RGTC image formats: COMPRESSED_RED_RGTC1: Each 4x4 block of texels consists of 64 bits of unsigned red image data. Each red image data block is encoded as a sequence of 8 bytes, called (in order of increasing address): red0, red1, bits_0, bits_1, bits_2, bits_3, bits_4, bits_5 The 6 "bits_*" bytes of the block are decoded into a 48-bit bit vector: bits = bits_0 + 256 * (bits_1 + 256 * (bits_2 + 256 * (bits_3 + 256 * (bits_4 + 256 * bits_5)))) red0 and red1 are 8-bit unsigned integers that are unpacked to red values RED0 and RED1 as though they were pixels with a of LUMINANCE and a type of UNSIGNED_BTYE. 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 red value R for a texel at location (x,y) in the block is given by: RED0, if red0 > red1 and code(x,y) == 0 RED1, if red0 > red1 and code(x,y) == 1 (6*RED0+ RED1)/7, if red0 > red1 and code(x,y) == 2 (5*RED0+2*RED1)/7, if red0 > red1 and code(x,y) == 3 (4*RED0+3*RED1)/7, if red0 > red1 and code(x,y) == 4 (3*RED0+4*RED1)/7, if red0 > red1 and code(x,y) == 5 (2*RED0+5*RED1)/7, if red0 > red1 and code(x,y) == 6 ( RED0+6*RED1)/7, if red0 > red1 and code(x,y) == 7 RED0, if red0 <= red1 and code(x,y) == 0 RED1, if red0 <= red1 and code(x,y) == 1 (4*RED0+ RED1)/5, if red0 <= red1 and code(x,y) == 2 (3*RED0+2*RED1)/5, if red0 <= red1 and code(x,y) == 3 (2*RED0+3*RED1)/5, if red0 <= red1 and code(x,y) == 4 ( RED0+4*RED1)/5, if red0 <= red1 and code(x,y) == 5 MINRED, if red0 <= red1 and code(x,y) == 6 MAXRED, if red0 <= red1 and code(x,y) == 7 MINRED and MAXRED are 0.0 and 1.0 respectively. Since the decoded texel has a red format, the resulting RGBA value for the texel is (R,0,0,1). COMPRESSED_SIGNED_RED_RGTC1: Each 4x4 block of texels consists of 64 bits of signed red image data. The red values of a texel are extracted in the same way as COMPRESSED_RED_RGTC1 except red0, red1, RED0, RED1, MINRED, and MAXRED are signed values defined as follows: red0 and red1 are 8-bit signed (two's complement) integers. { red0 / 127.0, red0 > -128 RED0 = { { -1.0, red0 == -128 { red1 / 127.0, red1 > -128 RED1 = { { -1.0, red1 == -128 MINRED = -1.0 MAXRED = 1.0 CAVEAT for signed red0 and red1 values: the expressions "red0 > red1" and "red0 <= red1" above are considered undefined (read: may vary by implementation) when red0 equals -127 and red1 equals -128, This is because if red0 were remapped to -127 prior to the comparison to reduce the latency of a hardware decompressor, the expressions would reverse their logic. Encoders for the signed LA formats should avoid encoding blocks where red0 equals -127 and red1 equals -128. COMPRESSED_RG_RGTC2: Each 4x4 block of texels consists of 64 bits of compressed unsigned red image data followed by 64 bits of compressed unsigned green image data. The first 64 bits of compressed red are decoded exactly like COMPRESSED_RED_RGTC1 above. The second 64 bits of compressed green are decoded exactly like COMPRESSED_RED_RGTC1 above except the decoded value R for this second block is considered the resulting green value G. Since the decoded texel has a red-green format, the resulting RGBA value for the texel is (R,G,0,1). COMPRESSED_SIGNED_RG_RGTC2: Each 4x4 block of texels consists of 64 bits of compressed signed red image data followed by 64 bits of compressed signed green image data. The first 64 bits of compressed red are decoded exactly like COMPRESSED_SIGNED_RED_RGTC1 above. The second 64 bits of compressed green are decoded exactly like COMPRESSED_SIGNED_RED_RGTC1 above except the decoded value R for this second block is considered the resulting green value G. Since this image has a red-green format, the resulting RGBA value is (R,G,0,1). Issues 1) What should these new formats be called? RESOLVED: "rgtc" for Red-Green Texture Compression. 2) How should the uncompressed and filtered texels be returned by texture fetches? RESOLVED: Red values show up as (R,0,0,1) where R is the red value, green and blue are forced to 0, and alpha is forced to 1. Likewise, red-green values show up as (R,G,0,1) where G is the green value. Prior extensions such as NV_float_buffer and NV_texture_shader have introduced formats such as GL_FLOAT_R_NV and GL_DSDT_NV where one- and two-component texture formats show up as (X,0,0,1) or (X,Y,0,1) RGBA texels. The RGTC formats mimic these two-component formats. The (X,Y,0,1) convention, particularly with signed components, is nice for normal maps because a normalized vector can be formed by a shader program by computing sqrt(abs(1-X*X-Y*Y)) for the Z component. While GL_RED is a valid external format, core OpenGL provides no GL_RG external format. Applications can either use GL_RGB or GL_RGBA and pad out the blue and alpha components, or use the two-component GL_LUMINANCE_ALPHA color format and use the color matrix functionality to swizzle the luminance and alpha values into red and green respectively. 3) Should red and red-green compression formats with signed components be introduced when the core specification lacked uncompressed red and red-green texture formats? RESOLVED: Yes, signed red and red-green compression formats should be added. Signed red-green formats are suited for compressed normal maps. Compressed normal maps may well be the dominant use of this extension. Unsigned red-green formats require an extra "expand normal" operation to convert [0,1] to [-1,+1]. Direct support for signed red-green formats avoids this step in a shader program. 4) Should there be a mix of signed red and unsigned green or vice versa? RESOLVED: No. NV_texture_shader provided an internal format (GL_SIGNED_RGB_UNSIGNED_ALPHA_NV) with mixed signed and unsigned components. The format saw little usage. There's no reason to think a GL_SIGNED_RED_UNSIGNED_GREEN format would be any more useful or popular. 5) How are signed integer values mapped to floating-point values? RESOLVED: A signed 8-bit two's complement value X is computed to a floating-point value Xf with the formula: { X / 127.0, X > -128 Xf = { { -1.0, X == -128 This conversion means -1, 0, and +1 are all exactly representable, however -128 and -127 both map to -1.0. Mapping -128 to -1.0 avoids the numerical awkwardness of have a representable value slightly more negative than -1.0. This conversion is intentionally NOT the "byte" conversion listed in Table 2.9 for component conversions. That conversion says: Xf = (2*X + 1) / 255.0 The Table 2.9 conversion is incapable of exactly representing zero. 6) How will signed components resulting from GL_COMPRESSED_SIGNED_RED_RGTC1 and GL_COMPRESSED_SIGNED_RG_RGTC2 texture fetches interact with fragment coloring? RESOLVED: The specification language for this extension is silent about clamping behavior leaving this to the core specification and other extensions. The clamping or lack of clamping is left to the core specification and other extensions. For assembly program extensions supporting texture fetches (ARB_fragment_program, NV_fragment_program, NV_vertex_program3, etc.) or the OpenGL Shading Language, these signed formats will appear as expected with unclamped signed components as a result of a texture fetch instruction. If ARB_color_buffer_float is supported, its clamping controls will apply. NV_texture_shader extension, if supported, adds support for fixed-point textures with signed components and relaxed the fixed-function texture environment clamping appropriately. If the NV_texture_shader extension is supported, its specified behavior for the texture environment applies where intermediate values are clamped to [-1,1] unless stated otherwise as in the case of explicitly clamped to [0,1] for GL_COMBINE. or clamping the linear interpolation weight to [0,1] for GL_DECAL and GL_BLEND. Otherwise, the conventional core texture environment clamps incoming, intermediate, and output color components to [0,1]. This implies that the conventional texture environment functionality of unextended OpenGL 1.5 or OpenGL 2.0 without using GLSL (and with none of the extensions referred to above) is unable to make proper use of the signed texture formats added by this extension because the conventional texture environment requires texture source colors to be clamped to [0,1]. Texture filtering of these signed formats would be still signed, but negative values generated post-filtering would be clamped to zero by the core texture environment functionality. The expectation is clearly that this extension would be co-implemented with one of the previously referred to extensions or used with GLSL for the new signed formats to be useful. 7) Should a specific normal map compression format be added? RESOLVED: No. It's probably short-sighted to design a format just for normal maps. Indeed, NV_texture_shader added a GL_SIGNED_HILO_NV format with exactly the kind of "hemisphere remap" useful for normal maps and the format went basically unused. Instead, this extension provides the mechanism for compressed normal maps based on the more conventional red-green format. The GL_COMPRESSED_RG_RGTC2 and GL_COMPRESSED_SIGNED_RG_RGTC2 formats are sufficient for normal maps with additional shader instructions used to generate the 3rd component. 8) Should uncompressed signed red and red-green formats be added by this extension? RESOLVED: No, this extension is focused on just adding compressed texture formats. The NV_texture_shader extension adds such uncompressed signed texture formats. A distinct multi-vendor extension for signed fixed-point texture formats could provide all or a subset of the signed fixed-point uncompressed texture formats introduced by NV_texture_shader. 9) What compression ratios does this extension provide? The RGTC1 formats are 8 bytes (64 bits) per 4x4 pixel block. A 4x4 block of GL_LUMINANCE8 data requires 16 bytes (1 byte per texel). This is a 2-to-1 compression ratio. The RGTC2 formats are 16 bytes (128 bits) per 4x4 pixel block. A 4x4 block of GL_LUMINANCE8_ALPHA8 data requires 32 bytes (2 bytes per texel). This is again a 2-to-1 compression ratio. In contrast, the comparable compression ratio for the S3TC formats is 4-to-1. Arguably, the lower compression ratio allows better compression quality particularly because the RGTC formats compress each component separately. 10) How do these new formats compare with the existing GL_LUMINANCE4, GL_LUMINANCE4_ALPHA4, and GL_LUMINANCE6_ALPHA2 internal formats? RESOLVED: The existing GL_LUMINANCE4, GL_LUMINANCE4_ALPHA4, and GL_LUMINANCE6_ALPHA2 internal formats provide a similar 2-to-1 compression ratio but mandate a uniform quantization for all components. In contrast, this extension provides a compression format with 3-bit quantization over a specifiable min/max range that can vary per 4x4 texel tile. Additionally, many OpenGL implementations do not natively support the GL_LUMINANCE4, GL_LUMINANCE4_ALPHA4, and GL_LUMINANCE6_ALPHA2 internal formats but rather silently promote these formats to store 8 bits per component, thereby eliminating any storage/bandwidth advantage for these formats. 11) Does this extension require EXT_texture_compression_s3tc? RESOLVED: No. As written, this specification does not rely on wording of the EXT_texture_compression_s3tc extension. For example, certain discussion added to Sections 3.8.2 and 3.8.3 is quite similar to corresponding EXT_texture_compression_s3tc language. 12) Should anything be said about the precision of texture filtering for these new formats? RESOLVED: No precision requirements are part of the specification language since OpenGL extensions typically leave precision details to the implementation. Realistically, at least 8-bit filtering precision can be expected from implementations (and probably more). 13) Should these formats be allowed to specify 3D texture images when NV_texture_compression_vtc is supported? RESOLVED: The NV_texture_compression_vtc stacks 4x4 blocks into 4x4x4 bricks. It may be more desirable to represent compressed 3D textures as simply slices of 4x4 blocks. However the NV_texture_compression_vtc extension expects data passed to the glCompressedTexImage commands to be "bricked" rather than blocked slices. 14) How is the texture border color handled for the blue component of an RGTC2 texture and the green and blue components of an RGTC1 texture? RESOLVED: The base texture format is RGB for the RGTC1 and RGTC2 texture formats. This would mean table 3.15 would be used to determine how the texture border color is interpreted and which components are considered. However since only red or red/green components exist for the RGTC1 and RGTC2 formats, it makes little sense to require the blue component be supplied by the texture border color and hence be involved (meaningfully only when the border is sampled) in texture filtering. For this reason, a statement is added to section 3.8.8 says that if a texture's internal format lacks components that exist in the texture's base internal format, such components contain zero (ignoring the texture's corresponding texture border color component value) when the texture border color is sampled. So the green and blue components of the filtered result of a RGTC1 texture are always zero, even when the border is sampled. Similarly the blue component of the filtered result of a RGTC2 texture is always zero, even when the border is sampled. 15) What should glGetTexLevelParameter return for GL_TEXTURE_GREEN_SIZE and GL_TEXTURE_BLUE_SIZE for the RGTC1 formats? What should glGetTexLevelParameter return for GL_TEXTURE_BLUE_SIZE for the RGTC2 formats? RESOLVED: Zero bits. These formats always return 0.0 for these respective components and have no bits devoted to these components. Returning 8 bits for red size of RGTC1 and the red and green sizes of RGTC2 makes sense because that's the maximum potential precision for the uncompressed texels. 16) Should the token names contain R and RG or RED and RED_GREEN? RESOLVED: RED and RG. See issue #3 in the ARB_texture_rg specification. 17) Can you use the GL_RED external format with glTexImage2D and other such commands to load textures with the GL_COMPRESSED_RED_RGTC1 or GL_COMPRESSED_SIGNED_RED_RGTC1 internal formats? RESOLVED: Yes. GL_RED has been a valid external format parameter to glTexImage and similar commands since OpenGL 1.0. 18) Should any of the generic compression GL_COMPRESSED_* tokens in OpenGL 2.1 map to RGTC formats? RESOLVED: No. The RGTC formats are missing color components so are not adequate implementations for any of the generic compression formats. 19) Should the GL_NUM_COMPRESSED_TEXTURE_FORMATS and GL_COMPRESSED_TEXTURE_FORMATS queries return the RGTC formats? RESOLVED: No. The OpenGL 2.1 specification says "The only values returned by this query [GL_COMPRESSED_TEXTURE_FORMATS"] are those corresponding to formats suitable for general-purpose usage. The renderer will not enumerate formats with restrictions that need to be specifically understood prior to use." Compressed textures with just red or red-green components are not general-purpose so should not be returned by these queries because they have restrictions. Applications that seek to use the RGTC formats should do so by looking for this extension's name in the string returned by glGetString(GL_EXTENSIONS) rather than what GL_NUM_COMPRESSED_TEXTURE_FORMATS and GL_COMPRESSED_TEXTURE_FORMATS return. 20) Why don't the new tokens and entry points in this extension have "ARB" suffixes like other ARB extensions? RESOLVED: Unlike most ARB extensions, this is a strict subset of functionality already approved in OpenGL 3.0. This extension exists only to support that functionality on older hardware that cannot implement a full OpenGL 3.0 driver. Since there are no possible behavior changes between the ARB extension and core features, source code compatibility is improved by not using suffixes on the extension. Implementation Note This extension allows TexSubImage2D and CompressedTexSubImage2D to perform partial updates to compressed images, but generally requires that the updated area be aligned to 4x4 block boundaries. If the width or height is not a multiple of four, there will be 4x4 blocks at the edge of the image that contain "extra" texels that are not part of the image. This spec has an exception allowing edits that partially cover such blocks as long as the edit covers all texels in the block belonging to the image. For example, in a 2D texture of size 70x50, it is legal to update the single partial block covering the four texels from (68,48) to (69,49) by setting (, ) to (68,48) and and to 2. This specification derived some of its language from the EXT_texture_compression_s3tc specification. When that extension was originally written, non-bordered textures were required to have widths and heights that were powers of two. Therefore, the only cases where partial blocks could occur were if the width or height of the texture image was one or two. The original spec language allowed partial block edits only if the width or height of the region edited was equal to the full texture size. That language didn't handle cases such as the 70x50 example above. This specification was updated in May, 2009 to allow such edits. Multiple OpenGL implementers correctly implemented the original restriction, and partial edits that include partially covered tiles will result in INVALID_OPERATION errors on older drivers. Revision History Revision 1.1, April 24, 2007: mjk - Add caveat about how signed LA decompression happens when lum0 equals -127 and lum1 equals -128. This caveat matches a decoding allowance in DirectX 10. Revision 1.2, January 21, 2008: mjk - Add issues #18 and #19. Revision 1.3, June 30, 2008: js - trivial conversion to ARB from EXT Revision 1.4, August 7, 2008: jleech - Remove ARB suffixes. Revision 1.5, May 29, 2009: jleech - Sync with pbrown's fixes to the EXT version of this extension: Add interaction with non-power-of-two textures from OpenGL 2.0 / ARB_texture_non_power_of_two. Allow CompressedTexSubImage2D to perform edits that include partial tiles at the edge of the image as long as the specified width/height parameters line up with the edge. Thanks to Emil Persson for finding this issue. Revision 1.6, April 17, 2014: jleech - Fix table 3.17 so base type for COMRPESSED*RG_RGTC2 formats is RG instead of RGB, matching core GL spec (Bug 7812). Revision 1.7, January 27, 2015: jleech - Fix table 3.17 so base type for COMRPESSED*RED_RGTC1 formats is RED instead of RGB, matching core GL spec (Bug 13260).