EXT_texture_compression_s3tc_srgb

GL_EXT_texture_compression_s3tc_srgb

Christophe Riccio, Unity
Kai Ninomiya, Google
Kenneth Russell, Google
Contributors to EXT_texture_compression_s3tc

Christophe Riccio, (christophe 'dot' riccio 'at' unity3d 'dot' com)

FINAL, implemented by ANGLE

1 October 2016

OpenGL ES Extension #289

OpenGL ES 2.0 is required.

OpenGL ES 3.0 or EXT_sRGB are required.

EXT_texture_compression_s3tc is required.

This extension is written against the OpenGL ES 3.0.4
specification with EXT_texture_compression_s3tc extension.

This extension is written against the OpenGL ES 2.0.25
specification with EXT_texture_compression_s3tc extension.

EXT_texture_storage affects the definition of this
extension.

This extension adds new compressed color texture formats using S3TC with
nonlinear sRGB color components.

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.

None

This extension introduces new tokens:

    COMPRESSED_SRGB_S3TC_DXT1_EXT                  0x8C4C
    COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT            0x8C4D
    COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT            0x8C4E
    COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT            0x8C4F

In extended OpenGL ES 2.0.25 these new tokens are accepted by the
<internalformat> parameter of TexImage2D, CompressedTexImage2D, TexStorage2DEXT,
TextureStorage2DEXT and the <format> parameter of CompressedTexSubImage2D.

In extended OpenGL ES 3.0.4 these new tokens are also accepted by the
<internalformat> parameter of TexImage2D, TexImage3D, CompressedTexImage3D,
TexStorage2D, TexStorage3D, TexStorage3DEXT, TextureStorage3DEXT and the <format>
parameter of CompressedTexSubImage3D.

Modify Section 3.7.1, Texture Image Specification:

Change last paragraph on Page 67 as follows
(modified by EXT_texture_compression_s3tc):

Components are then selected from the resulting R, G, B, or A values
to obtain a texture with the base internal format specified by
<internalformat>, which must match <format> except when <target> is
TEXTURE_2D and <internalformat> is one of the following compressed
formats: COMPRESSED_RGB_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT1_EXT,
COMPRESSED_RGBA_S3TC_DXT3_EXT, COMPRESSED_RGBA_S3TC_DXT5_EXT,
COMPRESSED_SRGB_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT, or COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT.
In this case, conversion from only RGB and RGBA formats are supported
during texture image processing. <format> values other than RGB or RGBA
will result in the INVALID_OPERATION error. In all other cases where
<internalformat> does not match <format>, the error INVALID_OPERATION is
generated. Table 3.8 summarizes the mapping of R, G, B, and A values to
texture components, as a function of the base internal format of the
texture image. <internalformat> may be one of the five internal format
symbolic constants listed in table 3.8 or the four compressed
formats: COMPRESSED_RGB_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT1_EXT,
COMPRESSED_RGBA_S3TC_DXT3_EXT, COMPRESSED_RGBA_S3TC_DXT5_EXT,
COMPRESSED_SRGB_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT, or COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT.
Specifying a value for <internalformat> that is not one of the above values
generates the error INVALID_VALUE. When a compressed <internalformat> is
specified, a compressed texture is created and all the associated
restrictions mentioned in Section 3.7.3 are imposed.

Note that 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 or
COMPRESSED_SRGB_ALPHA_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 or
COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT.

Modify Section 3.7.2, Alternate Texture Image Specification Commands
(modified by EXT_texture_compression_s3tc):

Modify last paragraph with:

When the internal format of the texture object is
COMPRESSED_RGB_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT1_EXT,
COMPRESSED_RGBA_S3TC_DXT3_EXT, COMPRESSED_RGBA_S3TC_DXT5_EXT,
COMPRESSED_SRGB_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT, or COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT,
the update region specified in TexSubImage2D must be aligned to 4x4
pixel blocks. If <xoffset> or <yoffset> are not multiples of 4 an
INVALID_OPERATION error is generated. If <width> is not a multiple
of 4 and <xoffset> + <width> is not equal to the width of the LOD
then an INVALID_OPERATION error is generated.  If <height> is not
a multiple of 4 and <yoffset> + <height> is not equal to the
height of the LOD then an INVALID_OPERATION error is generated.

Modify Section 3.7.3, "Compressed Texture Images"

Add 4 new rows to "Specific compressed texture formats" Table 3.X:

    Compressed Internal Format           Base Internal Format
    -----------------------------------  --------------------
    COMPRESSED_SRGB_S3TC_DXT1_EXT        RGB
    COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT  RGBA
    COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT  RGBA
    COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT  RGBA

Replace last paragraph with:

If the internal format is one of COMPRESSED_RGB_S3TC_DXT1_EXT,
COMPRESSED_RGBA_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT3_EXT,
COMPRESSED_RGBA_S3TC_DXT5_EXT, COMPRESSED_SRGB_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT,
or COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT 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,
CompressedTexSubImage2D 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.

Modify Section 3.8.3, "Texture Image Specification":

Modify paragraph starting with "For internalformat different than "
(modified by EXT_texture_compression_s3tc):

For internalformat different than COMPRESSED_RGB_S3TC_DXT1_EXT,
COMPRESSED_RGBA_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT3_EXT, 
COMPRESSED_RGBA_S3TC_DXT5_EXT, COMPRESSED_SRGB_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT,
or COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT specifying a combination of values
for format, type, and internalformat that is not listed as a valid
combination in tables 3.2 or 3.3 generates the error INVALID_OPERATION.

For internalformat equal to COMPRESSED_RGB_S3TC_DXT1_EXT,
COMPRESSED_RGBA_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT3_EXT,
COMPRESSED_RGBA_S3TC_DXT5_EXT, COMPRESSED_SRGB_S3TC_DXT1_EXT, 
COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT, or
COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT only RGB and RGBA formats are supported
during texture image processing and for these internalformat values
<format> values other than RBA or RGBA will result in the INVALID_OPERATION
error.

Modify Section 3.8.6. "Compressed Texture Images":

Add 4 new rows to "Compressed internal formats" Table 3.19:

    Compressed Internal Format           Base Internal Format
    -----------------------------------  --------------------
    COMPRESSED_SRGB_S3TC_DXT1_EXT        RGB
    COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT  RGBA
    COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT  RGBA
    COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT  RGBA

Modify paragraph starting with "If the internal format is one of "
(added by EXT_texture_compression_s3tc):

If the internal format is one of COMPRESSED_RGB_S3TC_DXT1_EXT,
COMPRESSED_RGBA_S3TC_DXT1_EXT, COMPRESSED_RGBA_S3TC_DXT3_EXT,
COMPRESSED_RGBA_S3TC_DXT5_EXT, COMPRESSED_SRGB_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT,
or COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT 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,
CompressedTexSubImage2D/CompressedTexSubImage3D 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/CompressedTexSubImage3D
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.

Modify Section 3.8.16, "sRGB Texture Color Conversion":

Change the first sentence to:

If the currently bound texture's internal format is one of SRGB8,
SRGB8_ALPHA8, COMPRESSED_SRGB8_ETC2, COMPRESSED_SRGB8_ALPHA8_ETC2_EAC,
COMPRESSED_SRGB8_PUNCHTHROUGH_ALPHA1_ETC2, COMPRESSED_SRGB_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT,
or COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT, the red, green, and blue components
are converted from an sRGB color space to a linear color space as part of
filtering described in sections 3.8.10 and 3.8.11.

If EXT_texture_storage is not supported, ignore all references to
TexStorage2DEXT and TexStorage3DEXT functions.

INVALID_OPERATION is generated by CopyTexSubImage2D if the texture
image <level> bound to <target> has internal format
COMPRESSED_SRGB_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT, or COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT.

INVALID_OPERATION is generated by CompressedTexSubImage2D if
<format> is COMPRESSED_SRGB_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT, or
COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT 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.

INVALID_OPERATION is generated by TexImage2D and TexSubImage2D if
texture has internal format COMPRESSED_SRGB_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT, or
COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT and any of the following apply:

    * <xoffset> or <yoffset> are not multiples of 4

    * <width> is not a multiple of 4 and <xoffset> + <width> is not equal
      to the width of the LOD

    * if <height> is not a multiple of 4 and <yoffset> + <height> is not
      equal to the height of the LOD

INVALID_OPERATION is generated by CopyTexSubImage2D / CopyTexSubImage3D if
the texture image <level> bound to <target> has internal format
COMPRESSED_SRGB_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT, or COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT.

INVALID_OPERATION is generated by CompressedTexSubImage2D /
CompressedTexSubImage3D if <format> is COMPRESSED_SRGB_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT, or
COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT 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.

INVALID_OPERATION is generated by TexSubImage2D/TexSubImage3D if texture
has internal format COMPRESSED_SRGB_S3TC_DXT1_EXT, 
COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT,
or COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT and <format> is not RGB or RGBA, or
any of the following apply:

    * <xoffset> or <yoffset> are not multiples of 4

    * <width> is not a multiple of 4 and <xoffset> + <width> is not equal
      to the width of the LOD

    * if <height> is not a multiple of 4 and <yoffset> + <height> is not
      equal to the height of the LOD

The queries for NUM_COMPRESSED_TEXTURE_FORMATS and
COMPRESSED_TEXTURE_FORMATS include COMPRESSED_SRGB_S3TC_DXT1_EXT,
COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT,
and COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT.

None

(1) Should this be an extension for OpenGL too?

    sRGB DXT formats are already exposed through OpenGL EXT_texture_sRGB
    extension.

    RESOLVED: No

(2) Can we use the new compression formats with TexImage2D/TexImage3D?

    EXT_texture_compression_s3tc supports DXT formats as internalformat of 
    TexImage2D and TexImage3D hence this extension should follow this
    precedent.

    RESOLVED: Yes

2016-10-01 - criccio
   + Fixed missing formats in Section 3.8.3

2016-09-30 - criccio
   + Added issue 2

2016-09-10 - criccio
   + Initial draft