Name EXT_texture_compression_latc Name Strings GL_EXT_texture_compression_latc GL_NV_texture_compression_latc (legacy) Contributors Mark J. Kilgard, NVIDIA Pat Brown, NVIDIA Yanjun Zhang, S3 Attila Barsi, Holografika Daniel Koch, NVIDIA Contact Mark J. Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com) Status Shipping for GeForce 8 Series (November 2006) Version Last Modified Date: July 30, 2017 Revision: 2 Number 331 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 luminance and luminance-alpha textures (hence the name "latc" for Luminance-Alpha Texture Compression). These formats are designed to reduce the storage requirements and memory bandwidth required for luminance and luminance-alpha 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 luminance-alpha format is reasonably suited for storing compressed normal maps. New Procedures and Functions None. New Tokens Accepted by the parameter of TexImage2D, CopyTexImage2D, and CompressedTexImage2D and the parameter of CompressedTexSubImage2D: COMPRESSED_LUMINANCE_LATC1_EXT 0x8C70 COMPRESSED_SIGNED_LUMINANCE_LATC1_EXT 0x8C71 COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT 0x8C72 COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT 0x8C73 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_LUMINANCE_LATC1_EXT LUMINANCE COMPRESSED_SIGNED_LUMINANCE_LATC1_EXT LUMINANCE COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT LUMINANCE_ALPHA COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT LUMINANCE_ALPHA -- 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_LUMINANCE_LATC1_EXT, COMPRESSED_SIGNED_LUMINANCE_LATC1_EXT, COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT, or COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT, the texture is stored using one of the two LATC 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 LATC 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_LUMINANCE_LATC1_EXT, COMPRESSED_SIGNED_LUMINANCE_LATC1_EXT, COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT, or COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT, the compressed texture is stored using one of several LATC compressed texture image formats. The LATC texture compression algorithm supports only 2D images without borders. CompressedTexImage1D and CompressedTexImage3D produce an INVALID_ENUM error if is an LATC 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_LUMINANCE_LATC1_EXT, COMPRESSED_SIGNED_LUMINANCE_LATC1_EXT, COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT, or COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT, the texture is stored using one of the several LATC compressed texture image formats. Since the LATC texture compression algorithm supports only 2D images, CompressedTexSubImage1D and CompressedTexSubImage3D produce an INVALID_ENUM error if is an LATC format. Since LATC 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 LATC compressed texture image that does not intersect the area being modified are preserved during valid TexSubImage2D and CopyTexSubImage2D calls." 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_LUMINANCE_LATC1_EXT, COMPRESSED_SIGNED_LUMINANCE_LATC1_EXT, COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT, or COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT. INVALID_OPERATION is generated by CompressedTexImage2D if is COMPRESSED_LUMINANCE_LATC1_EXT, COMPRESSED_SIGNED_LUMINANCE_LATC1_EXT, COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT, or COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT and is not equal to zero. INVALID_ENUM is generated by CompressedTexSubImage1D or CompressedTexSubImage3D if is COMPRESSED_LUMINANCE_LATC1_EXT, COMPRESSED_SIGNED_LUMINANCE_LATC1_EXT, COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT, or COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT. INVALID_OPERATION is generated by TexSubImage2D or CopyTexSubImage2D if TEXTURE_INTERNAL_FORMAT is COMPRESSED_LUMINANCE_LATC1_EXT, COMPRESSED_SIGNED_LUMINANCE_LATC1_EXT, COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT, or COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT 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_LUMINANCE_LATC1_EXT, COMPRESSED_SIGNED_LUMINANCE_LATC1_EXT, COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT, or COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT 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 LATC 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 LATC Compressed Texture Image Formats Compressed texture images stored using the LATC 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 LATC 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 LATC 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 LATC 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 LATC image formats: COMPRESSED_LUMINANCE_LATC1: Each 4x4 block of texels consists of 64 bits of unsigned luminance image data. Each luminance image data block is encoded as a sequence of 8 bytes, called (in order of increasing address): lum0, lum1, 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)))) lum0 and lum1 are 8-bit unsigned integers that are unpacked to luminance values LUM0 and LUM1 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 luminance value L for a texel at location (x,y) in the block is given by: LUM0, if lum0 > lum1 and code(x,y) == 0 LUM1, if lum0 > lum1 and code(x,y) == 1 (6*LUM0+ LUM1)/7, if lum0 > lum1 and code(x,y) == 2 (5*LUM0+2*LUM1)/7, if lum0 > lum1 and code(x,y) == 3 (4*LUM0+3*LUM1)/7, if lum0 > lum1 and code(x,y) == 4 (3*LUM0+4*LUM1)/7, if lum0 > lum1 and code(x,y) == 5 (2*LUM0+5*LUM1)/7, if lum0 > lum1 and code(x,y) == 6 ( LUM0+6*LUM1)/7, if lum0 > lum1 and code(x,y) == 7 LUM0, if lum0 <= lum1 and code(x,y) == 0 LUM1, if lum0 <= lum1 and code(x,y) == 1 (4*LUM0+ LUM1)/5, if lum0 <= lum1 and code(x,y) == 2 (3*LUM0+2*LUM1)/5, if lum0 <= lum1 and code(x,y) == 3 (2*LUM0+3*LUM1)/5, if lum0 <= lum1 and code(x,y) == 4 ( LUM0+4*LUM1)/5, if lum0 <= lum1 and code(x,y) == 5 MINLUM, if lum0 <= lum1 and code(x,y) == 6 MAXLUM, if lum0 <= lum1 and code(x,y) == 7 MINLUM and MAXLUM are 0.0 and 1.0 respectively. Since the decoded texel has a luminance format, the resulting RGBA value for the texel is (L,L,L,1). COMPRESSED_SIGNED_LUMINANCE_LATC1: Each 4x4 block of texels consists of 64 bits of signed luminance image data. The luminance values of a texel are extracted in the same way as COMPRESSED_LUMINANCE_LATC1 except lum0, lum1, LUM0, LUM1, MINLUM, and MAXLUM are signed values defined as follows: lum0 and lum1 are 8-bit signed (two's complement) integers. { lum0 / 127.0, lum0 > -128 LUM0 = { { -1.0, lum0 == -128 { lum1 / 127.0, lum1 > -128 LUM1 = { { -1.0, lum1 == -128 MINLUM = -1.0 MAXLUM = 1.0 CAVEAT for signed lum0 and lum1 values: the expressions "lum0 > lum1" and "lum0 <= lum1" above are considered undefined (read: may vary by implementation) when lum0 equals -127 and lum1 equals -128, This is because if lum0 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 lum0 equals -127 and lum1 equals -128. COMPRESSED_LUMINANCE_ALPHA_LATC2: Each 4x4 block of texels consists of 64 bits of compressed unsigned luminance image data followed by 64 bits of compressed unsigned alpha image data. The first 64 bits of compressed luminance are decoded exactly like COMPRESSED_LUMINANCE_LATC1 above. The second 64 bits of compressed alpha are decoded exactly like COMPRESSED_LUMINANCE_LATC1 above except the decoded value L for this second block is considered the resulting alpha value A. Since the decoded texel has a luminance-alpha format, the resulting RGBA value for the texel is (L,L,L,A). COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2: Each 4x4 block of texels consists of 64 bits of compressed signed luminance image data followed by 64 bits of compressed signed alpha image data. The first 64 bits of compressed luminance are decoded exactly like COMPRESSED_SIGNED_LUMINANCE_LATC1 above. The second 64 bits of compressed alpha are decoded exactly like COMPRESSED_SIGNED_LUMINANCE_LATC1 above except the decoded value L for this second block is considered the resulting alpha value A. Since this image has a luminance-alpha format, the resulting RGBA value is (L,L,L,A). Issues 1) What should these new formats be called? RESOLVED: "latc" for Luminance-Alpha Texture Compression. 2) How should the uncompressed and filtered texels be returned by texture fetches? RESOLVED: Luminance values show up as they do conventionally as (L,L,L,1) where the luminance value L is replicated in the red, green, and blue components and alpha is forced to 1. Likewise, luminance-alpha values show up as (L,L,L,A) where A is the alpha value. Alternatively, 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. Such formats have not proven popular. In particular, they interact awkwardly with the pixel path and conventional texture environment modes. 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. However, this niceness is mostly conceptual however since the same effect can be accomplished with swizzling as shown in this GLSL code: vec2 texLA = texture2D(samplerLA, gl_TexCoord[0]).xw; vec3 normal = vec3(texLA.x, texLA.y, sqrt(abs(1-texLA.x*texLA.x-texLA.y*texLA.y))); The most important reason to make these new compressed formats show up identically to conventional luminance and luminance-alpha texels is to allow applications to seamlessly substitute the new compressed formats for existing GL_LUMINANCE and GL_LUMINANCE_ALPHA textures. Alternative component arrangements would make it more cumbersome for existing applications to switch over luminance and luminance-alpha textures to these compressed formats. 3) Should luminance and luminance-alpha compression formats with signed components be introduced when the core specification lacked uncompressed luminance and luminance-alpha texture formats? RESOLVED: Yes, signed luminance and luminance-alpha compression formats should be added. Signed luminance-alpha formats are suited for compressed normal maps. Compressed normal maps may well be the dominant use of this extension. Unsigned luminance-alpha formats require an extra "expand normal" operation to convert [0,1] to [-1,+1]. Direct support for signed luminance-alpha formats avoids this step in a shader program. 4) Should there be a mix of signed luminance and unsigned alpha 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_LUMINANCE_UNSIGNED_ALPHA 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_LUMINANCE_LATC1_EXT and GL_COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT 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, EXT_fragment_program, EXT_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 luminance-alpha format. The GL_COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT and GL_COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT formats are sufficient for normal maps with additional shader instructions used to generate the 3rd component. 8) Should uncompressed signed luminance and luminance-alpha 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 LATC1 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 LATC2 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 LATC 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) Why is GL_NV_texture_compression_latc also listed in the Name Strings section? The very first GeForce 8800 driver shipped with the extension designated as NV before EXT-ization with S3 was agreed. Subsequent NVIDIA drivers will rename the extension to its EXT name only. 15) Should the the generic formats GL_COMPRESSED_LUMINANCE and GL_COMPRESSED_LUMINANCE_ALPHA correspond to COMPRESSED_LUMINANCE_LATC1_EXT and COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT respecitively when this extension is supported? RESOLVED: Yes. While no generic compression is strictly required for an implementation and there might exist superior compression schemes for luminance and luminance-alpha textures in the future, an application should reasonably expect that an implementation that supports EXT_texture_compression_latc will also use these formats for the generic compressed luminance and luminance-alpha formats. The COMPRESSED_LUMINANCE_LATC1_EXT and COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT are generic enough in their respective luminance and luminance-alpha behavior that these compression formats are acceptable generic compressed formats for luminance and luminance-alpha generic compressed formats. 16) Should the GL_NUM_COMPRESSED_TEXTURE_FORMATS and GL_COMPRESSED_TEXTURE_FORMATS queries return the LATC 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." Historically, OpenGL implementation have advertised the RGB and RGBA versions of the S3TC extensions compressed format tokens through this mechanism. The specification is not sufficiently clear about what "suitable for general-purpose usage" means. Historically that seems to mean unsigned RGB or unsigned RGBA. The DXT1 format supporting alpha (GL_COMPRESSED_RGBA_S3TC_DXT1_EXT) is not exposed in the list (at least for NVIDIA drivers) because the alpha is always 1.0 expect when it is 0.0 when RGB is required to be black. NVIDIA's even limits itself to true linear RGB or RGBA formats, specifically not including EXT_texture_sRGB's sRGB S3TC compressed formats. Adding luminance and luminance-alpha texture formats (and certainly signed versions of luminance and luminance-alpha formats!) invites potential comptaibility problems with old applications using this mechanism since old applications are unlikely to expect non-RGB or non-RGBA formats to be advertised through this mechanism. However no specific misinteractions with old applications is known. Applications that seek to use the LATC 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. 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 April, 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 #15 and #16. Revision 1.3, April 14, 2009: pbrown - 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 2, July 30, 2017: dgkoch - replace references to COMPRESSED_LUMINANCE_LACT1_EXT with references to the correct spelling COMPRESSED_LUMINANCE_LATC1_EXT.