Name

    EXT_texture_compression_s3tc

Name Strings

    GL_EXT_texture_compression_s3tc

Contact

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

Status

    CANDIDATE FOR FINAL DRAFT -- NOT YET COMPLETE

Version
    
    Draft 0.7, 11 April 2000

Number

    198

Dependencies

    OpenGL 1.1 is required.

    GL_ARB_texture_compression is required.

    This extension is written against the OpenGL 1.2.1 Specification.

Overview

    This extension provides additional texture compression functionality
    specific to S3's S3TC format (called DXTC in Microsoft's DirectX API),
    subject to all the requirements and limitations described by the extension
    GL_ARB_texture_compression.

    This extension supports DXT1, DXT3, and DXT5 texture compression formats.
    For the DXT1 image format, this specification supports an RGB-only mode
    and a special RGBA mode with single-bit "transparent" 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.

Issues

    (1) Should DXT2 and DXT4 (premultiplied alpha) formats be supported?

        RESOLVED:  No -- insufficient interest.  Supporting DXT2 and DXT4
        would require some rework to the TexEnv definition (maybe add a new
        base internal format RGBA_PREMULTIPLIED_ALPHA) for these formats.
        Note that the EXT_texture_env_combine extension (which extends normal
        TexEnv modes) can be used to support textures with premultipled alpha.

    (2) Should generic "RGB_S3TC_EXT" and "RGBA_S3TC_EXT" enums be supported
        or should we use only the DXT<n> enums?  

        RESOLVED:  No.  A generic RGBA_S3TC_EXT is problematic because DXT3
        and DXT5 are both nominally RGBA (and DXT1 with the 1-bit alpha is
        also) yet one format must be chosen up front.

    (3) Should TexSubImage support all block-aligned edits or just the minimal
        functionality required by the the ARB_texture_compression extension?

        RESOLVED:  Allow all valid block-aligned edits.

    (4) A pre-compressed image with a DXT1 format can be used as either an
        RGB_S3TC_DXT1 or an RGBA_S3TC_DXT1 image.  If the image has
        transparent texels, how are they treated in each format?

        RESOLVED:  The renderer has to make sure that an RGB_S3TC_DXT1 format
        is decoded as RGB (where alpha is effectively one for all texels),
        while RGBA_S3TC_DXT1 is decoded as RGBA (where alpha is zero for all
        texels with "transparent" encodings).  Otherwise, the formats are
        identical.

    (5) Is the encoding of the RGB components for DXT1 formats correct in this
        spec?  MSDN documentation does not specify an RGB color for the
        "transparent" encoding.  Is it really black?

        RESOLVED:  Yes.  The specification for the DXT1 format initially
        required black, but later changed that requirement to a
        recommendation.  All vendors involved in the definition of this
        specification support black.  In addition, specifying black has a
        useful behavior.

        When blending multiple texels (GL_LINEAR filtering), mixing opaque and
        transparent samples is problematic.  Defining a black color on
        transparent texels achieves a sensible result that works like a
        texture with premultiplied alpha.  For example, if three opaque white
        and one transparent sample is being averaged, the result would be a
        75% intensity gray (with an alpha of 75%).  This is the same result on
        the color channels as would be obtained using a white color, 75%
        alpha, and a SRC_ALPHA blend factor.

    (6) Is the encoding of the RGB components for DXT3 and DXT5 formats
        correct in this spec?  MSDN documentation suggests that the RGB blocks
        for DXT3 and DXT5 are decoded as described the the DXT1 format.

        RESOLVED:  Yes -- this appears to be a bug in the MSDN documentation.
        The specification for the DXT2-DXT5 formats require decoding using the
        opaque block encoding, regardless of the relative values of "color0"
        and "color1".

New Procedures and Functions

    None.

New Tokens

    Accepted by the <internalformat> parameter of TexImage2D, CopyTexImage2D,
    and CompressedTexImage2DARB and the <format> parameter of
    CompressedTexSubImage2DARB:

        COMPRESSED_RGB_S3TC_DXT1_EXT                   0x83F0
        COMPRESSED_RGBA_S3TC_DXT1_EXT                  0x83F1
        COMPRESSED_RGBA_S3TC_DXT3_EXT                  0x83F2
        COMPRESSED_RGBA_S3TC_DXT5_EXT                  0x83F3

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

    None.

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

    Add to Table 3.16.1:  Specific Compressed Internal Formats

        Compressed Internal Format         Base Internal Format
        ==========================         ====================
        COMPRESSED_RGB_S3TC_DXT1_EXT       RGB
        COMPRESSED_RGBA_S3TC_DXT1_EXT      RGBA
        COMPRESSED_RGBA_S3TC_DXT3_EXT      RGBA
        COMPRESSED_RGBA_S3TC_DXT5_EXT      RGBA

    
    Modify Section 3.8.2, Alternate Image Specification

    (add to end of TexSubImage discussion, p.123 -- after edit from the
    ARB_texture_compression spec)

    If the internal format of the texture image being modified is
    COMPRESSED_RGB_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT1_EXT,
    COMPRESSED_RGBA_S3TC_DXT3_EXT, or COMPRESSED_RGBA_S3TC_DXT5_EXT, the
    texture is stored using one of the several S3TC compressed texture image
    formats.  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:

        * <width> is not a multiple of four or equal to TEXTURE_WIDTH.
        * <height> is not a multiple of four or equal to TEXTURE_HEIGHT.
        * <xoffset> or <yoffset> is not a multiple of four.


    Add to Section 3.8.2, Alternate Image Specification (adding to the end of
    the CompressedTexImage section introduced by the ARB_texture_compression
    spec)

    If <internalformat> is COMPRESSED_RGB_S3TC_DXT1_EXT,
    COMPRESSED_RGBA_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT3_EXT, or
    COMPRESSED_RGBA_S3TC_DXT5_EXT, the compressed texture is stored using one
    of several S3TC compressed texture image formats.  The S3TC texture
    compression algorithm supports only 2D images without borders.
    CompressedTexImage1DARB and CompressedTexImage3DARB produce an
    INVALID_ENUM error if <internalformat> is an S3TC format.
    CompressedTexImage2DARB will produce an INVALID_OPERATION error if
    <border> is non-zero.


    Add to Section 3.8.2, Alternate Image Specification (adding to the end of
    the CompressedTexSubImage section introduced by the
    ARB_texture_compression spec)

    If the internal format of the texture image being modified is
    COMPRESSED_RGB_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT1_EXT,
    COMPRESSED_RGBA_S3TC_DXT3_EXT, or COMPRESSED_RGBA_S3TC_DXT5_EXT, the
    texture is stored using one of the several S3TC compressed texture image
    formats.  Since the S3TC texture compression algorithm supports only 2D
    images, CompressedTexSubImage1DARB and CompressedTexSubImage3DARB produce
    an INVALID_ENUM error if <format> is an S3TC format.  Since S3TC 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:

        * <width> is not a multiple of four or equal to TEXTURE_WIDTH.
        * <height> is not a multiple of four or equal to TEXTURE_HEIGHT.
        * <xoffset> or <yoffset> is not a multiple of four.


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

    None.

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

    None.

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

    None.

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

    None.

Additions to the AGL/GLX/WGL Specifications

    None.

GLX Protocol

    None.

Errors

    INVALID_ENUM is generated by CompressedTexImage1DARB or
    CompressedTexImage3DARB if <internalformat> is
    COMPRESSED_RGB_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT1_EXT,
    COMPRESSED_RGBA_S3TC_DXT3_EXT, or COMPRESSED_RGBA_S3TC_DXT5_EXT.

    INVALID_OPERATION is generated by CompressedTexImage2DARB if if
    <internalformat> is COMPRESSED_RGB_S3TC_DXT1_EXT,
    COMPRESSED_RGBA_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT3_EXT, or
    COMPRESSED_RGBA_S3TC_DXT5_EXT and <border> is not equal to zero.

    INVALID_ENUM is generated by CompressedTexSubImage1DARB or
    CompressedTexSubImage3DARB if <format> is COMPRESSED_RGB_S3TC_DXT1_EXT,
    COMPRESSED_RGBA_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT3_EXT, or
    COMPRESSED_RGBA_S3TC_DXT5_EXT.

    INVALID_OPERATION is generated by TexSubImage2D CopyTexSubImage2D, or
    CompressedTexSubImage2D if INTERNAL_FORMAT is
    COMPRESSED_RGB_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT1_EXT,
    COMPRESSED_RGBA_S3TC_DXT3_EXT, or COMPRESSED_RGBA_S3TC_DXT5_EXT and any of
    the following apply: <width> is not a multiple of four or equal to
    TEXTURE_WIDTH; <height> is not a multiple of four or equal to
    TEXTURE_HEIGHT; <xoffset> or <yoffset> is not a multiple of four.


    The following restrictions from the ARB_texture_compression specification
    do not apply to S3TC 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 <xoffset>, <yoffset>, or <zoffset> does not equal
    DELETE: -b, where b is value of TEXTURE_BORDER.

    DELETE: INVALID_VALUE is generated by CompressedTexSubImage1DARB,
    DELETE: CompressedTexSubImage2DARB, or CompressedTexSubImage3DARB if the
    DELETE: entire texture image is not being edited:  if <xoffset>,
    DELETE: <yoffset>, or <zoffset> is greater than -b, <xoffset> + <width> is
    DELETE: less than w+b, <yoffset> + <height> is less than h+b, or <zoffset>
    DELETE: + <depth> 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

    None.

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 less than 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 beginning of the
    image of:

        blocksize * (ceil(<w>/4) * floor(<y>/4) + floor(<x>/4)).
    

    There are four distinct S3TC image formats:

    COMPRESSED_RGB_S3TC_DXT1_EXT:  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_EXT:  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_EXT.
 
        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

        IMPORTANT:  When encoding an RGBA image into a format using 1-bit
        alpha, any texels with an alpha component less than 0.5 end up with an
        alpha of 0.0 and any texels with an alpha component greater than or
        equal to 0.5 end up with an alpha of 1.0.  When encoding an RGBA image
        into the COMPRESSED_RGBA_S3TC_DXT1_EXT format, the resulting red,
        green, and blue components of any texels with a final alpha of 0.0
        will automatically be zero (black).  If this behavior is not desired
        by an application, it should not use COMPRESSED_RGBA_S3TC_DXT1_EXT.
        This format will never be used when a generic compressed internal
        format (Table 3.16.2) is specified, although the nearly identical
        format COMPRESSED_RGB_S3TC_DXT1_EXT (above) may be.


    COMPRESSED_RGBA_S3TC_DXT3_EXT:  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_EXT 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_EXT:  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_EXT 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 bytesw 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)+1..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


Revision History

    0.7,  04/11/00 prbrown1:  Added issues on DXT1, DXT3, and DXT5 encodings
                              where the MSDN documentation doesn't match what
                              is really done.  Added enum values from the
                              extension registry.

    0.4,  03/28/00 prbrown1:  Updated to reflect final version of the
                              ARB_texture_compression extension.  Allowed
                              block-aligned TexSubImage calls.

    0.3,  03/07/00 prbrown1:  Resolved issues pertaining to the format of RGB
                              blocks in the DXT3 and DXT5 formats (they don't
                              ever use the "transparent" encoding).  Fixed
                              decoding of DXT1 blocks.  Pointed out issue of
                              "transparent" texels in DXT1 encodings having
                              different behaviors for RGB and RGBA internal
                              formats.

    0.2,  02/23/00 prbrown1:  Minor revisions; added several issues.

    0.11, 02/17/00 prbrown1:  Slight modification to error semantics
      (INVALID_ENUM instead of INVALID_OPERATION).

    0.1,  02/15/00 prbrown1:  Initial revision.
