OES_compressed_paletted_texture

GL_OES_compressed_paletted_texture

Aaftab Munshi, ATI (amunshi@ati.com)

Copyright (c) 2003-2013 The Khronos Group Inc. Copyright terms at
    http://www.khronos.org/registry/speccopyright.html

Khronos-approved extension specifications are updated in response to
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extensions which have been promoted to a core Specification, fixes will
first appear in the latest version of that core Specification, and will
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    https://www.khronos.org/registry/OpenGL/docs/update_policy.php

No known IP issues

Ratified by the Khronos BOP, July 23, 2003.

Last Modifed Date: 12 November 2005
Author Revision: 0.6

OpenGL ES Extension #6 (formerly OpenGL Extension #294)

Written based on the wording of the OpenGL ES 1.0 specification

The goal of this extension is to allow direct support of palettized
textures in OpenGL ES.

Palettized textures are implemented in OpenGL ES using the
CompressedTexImage2D call. The definition of the following parameters
"level" and "internalformat" in the CompressedTexImage2D call have
been extended to support paletted textures.

A paletted texture is described by the following data:

    palette format
        can be R5_G6_B5, RGBA4, RGB5_A1, RGB8, or RGBA8

    number of bits to represent texture data
        can be 4 bits or 8 bits per texel.  The number of bits
        also detemine the size of the palette.  For 4 bits/texel
        the palette size is 16 entries and for 8 bits/texel the
        palette size will be 256 entries.

        The palette format and bits/texel are encoded in the
        "internalformat" parameter.

    palette data and texture mip-levels
        The palette data followed by all necessary mip levels are
        passed in "data" parameter of CompressedTexImage2D.

        The size of palette is given by palette format and bits / texel.
        A palette format of RGB_565 with 4 bits/texel imply a palette
        size of 2 bytes/palette entry * 16 entries = 32 bytes.

        The level value is used to indicate how many mip levels
        are described.  Negative level values are used to define
        the number of miplevels described in the "data" component.
        A level of zero indicates a single mip-level.

*   Should glCompressedTexSubImage2D be allowed for modifying paletted
    texture data.

    RESOLVED:  No, this would then require implementations that do not
    support paletted formats internally to also store the palette
    per texture.  This can be a memory overhead on platforms that are
    memory constrained.

*   Should palette format and number of bits used to represent each
    texel be part of data or internal format.

    RESOLVED:  Should be part of the internal format since this makes
    the palette format and texture data size very explicit for the
    application programmer.

*   Should the size of palette be fixed i.e 16 entries for 4-bit texels
    and 256 entries for 8-bit texels or be programmable.

    RESOLVED:  Should be fixed.  The application can expand the palette
    to 16 or 256 if internally it is using a smaller palette.

None

Accepted by the <level> parameter of CompressedTexImage2D

    Zero and negative values.  |level| + 1 determines the number of
    mip levels defined for the paletted texture.

Accepted by the <internalformat> paramter of CompressedTexImage2D

    PALETTE4_RGB8_OES         0x8B90
    PALETTE4_RGBA8_OES        0x8B91
    PALETTE4_R5_G6_B5_OES     0x8B92
    PALETTE4_RGBA4_OES        0x8B93
    PALETTE4_RGB5_A1_OES      0x8B94
    PALETTE8_RGB8_OES         0x8B95
    PALETTE8_RGBA8_OES        0x8B96
    PALETTE8_R5_G6_B5_OES     0x8B97
    PALETTE8_RGBA4_OES        0x8B98
    PALETTE8_RGB5_A1_OES      0x8B99

None

Add to Table 3.17:  Specific Compressed Internal Formats

    Compressed Internal Format         Base Internal Format
    ==========================         ====================
    PALETTE4_RGB8_OES                     RGB
    PALETTE4_RGBA8_OES                    RGBA
    PALETTE4_R5_G6_B5_OES                 RGB
    PALETTE4_RGBA4_OES                    RGBA
    PALETTE4_RGB5_A1_OES                  RGBA
    PALETTE8_RGB8_OES                     RGB
    PALETTE8_RGBA8_OES                    RGBA
    PALETTE8_R5_G6_B5_OES                 RGB
    PALETTE8_RGBA4_OES                    RGBA
    PALETTE8_RGB5_A1_OES                  RGBA

Add to Section 3.8.3, Alternate Image Specification 

If <internalformat> is PALETTE4_RGB8, PALETTE4_RGBA8, PALETTE4_R5_G6_B5,
PALETTE4_RGBA4, PALETTE4_RGB5_A1, PALETTE8_RGB8, PALETTE8_RGBA8,
PALETTE8_R5_G6_B5, PALETTE8_RGBA4 or PALETTE8_RGB5_A1, the compressed
texture is a compressed paletted texture.  The texture data contains the
palette data following by the mip-levels where the number of mip-levels
stored is given by |level| + 1.  The number of bits that represent a
texel is 4 bits if <interalformat> is given by PALETTE4_xxx and is 8
bits if <internalformat> is given by PALETTE8_xxx.

The number of bits that represent each palette entry is:

    Compressed Internal Format         # of bits / palette entry
    ==========================         =========================
    PALETTE4_RGB8_OES                     24
    PALETTE4_RGBA8_OES                    32
    PALETTE4_R5_G6_B5_OES                 16
    PALETTE4_RGBA4_OES                    16
    PALETTE4_RGB5_A1_OES                  16
    PALETTE8_RGB8_OES                     24
    PALETTE8_RGBA8_OES                    32
    PALETTE8_R5_G6_B5_OES                 16
    PALETTE8_RGBA4_OES                    16
    PALETTE8_RGB5_A1_OES                  16

Compressed paletted textures support only 2D images without
borders. CompressedTexImage2D will produce an INVALID_OPERATION
error if <border> is non-zero.


To determine palette format refer to tables 3.10 and 3.11 of Chapter
3 where the data ordering for different <type> formats are described.

Add table 3.17.1:  Texel Data Formats for compressed paletted textures

PALETTE4_xxx:

     7 6 5 4 3 2 1 0
     ---------------
    |  1st  |  2nd  |
    | texel | texel |
     ---------------

PALETTE8_xxx

None

None

None

None

None

INVALID_OPERATION is generated by TexImage2D, CompressedTexSubImage2D,
CopyTexSubImage2D if <internalformat> is PALETTE4_RGB8_OES,
PALETTE4_RGBA8_OES, PALETTE4_R5_G6_B5_OES, PALETTE4_RGBA4_OES,
PALETTE4_RGB5_A1_OES, PALETTE8_RG8_OES, PALETTE8_RGBA8_OES,
PALETTE8_R5_G6_B5_OES, PALETTE8_RGBA4_OES, or PALETTE8_RGB5_A1_OES.

INVALID_VALUE is generated by CompressedTexImage2D if
if <internalformat> is PALETTE4_RGB8_OES, PALETTE4_RGBA8_OES,
PALETTE4_R5_G6_B5_OES, PALETTE4_RGBA4_OES, PALETTE4_RGB5_A1_OES,
PALETTE8_RGB8_OES, PALETTE8_RGBA8_OES, PALETTE8_R5_G6_B5_OES,
PALETTE8_RGBA4_OES, or PALETTE8_RGB5_A1_OES and <level> value is
neither zero or a negative value.

The queries for NUM_COMPRESSED_TEXTURE_FORMATS and 
COMPRESSED_TEXTURE_FORMATS include these ten new formats.

05/29/2003    0.2    (David Blythe)
     - Use paletted rather than palettized.  Change naming of internal
       format tokens to match scheme used for other internal formats.

07/08/2003    0.3    (David Blythe)
     - Add official enumerant values and extension number.

07/09/2003    0.4    (David Blythe)
     - Note that [NUM_]COMPRESSED_TEXTURE_FORMAT queries include the
       new formats.

07/21/2004    0.5    (Aaftab Munshi)
       - Fixed PALETTE_8xxx drawing

11/12/2005    0.6    (Aaftab Munshi)
   - Corrections