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Name
NV_texture_compression_s3tc_update
Name Strings
GL_NV_texture_compression_s3tc_update
Contact
Ian Stewart, NVIDIA Corporation (istewart 'at' nvidia.com)
Contributors
Nicholas Haemel, NVIDIA Corporation
Acorn Pooley, NVIDIA Corporation
Antti Rasmus, NVIDIA Corporation
Musawir Shah, NVIDIA Corporation
Status
Complete.
Version
Last Modified Date: Sep 27, 2012
NVIDIA Revision: 2
Number
OpenGL ES Extension #95
Dependencies
This specification is written against the OpenGL ES 2.0.25
specification.
Depends on the GL_NV_texture_compression_s3tc extension.
Overview
This extension allows for full or partial image updates to a
compressed 2D texture from an uncompressed texel data buffer using
TexImage2D and TexSubImage2D. Consquently, if a compressed internal
format is used, all the restrictions associated with compressed
textures will apply. These include sub-image updates aligned to 4x4
pixel blocks and the restriction on usage as render targets.
IP Status
NVIDIA Proprietary
New Procedures and Functions
None
New Tokens
None
Additions to Chapter 3 of the OpenGL ES 2.0.25 Specification (Rasterization)
Modify Section 3.7.1, Texture Image Specification
(change last paragraph on Page 67 as follows)
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_NV,
COMPRESSED_RGBA_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT3_NV, or
COMPRESSED_RGBA_S3TC_DXT5_NV. In this case, conversion from only
RGB and RGBA formats are supported during texture image processing.
<format> values other than RBA 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_NV,
COMPRESSED_RGBA_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT3_NV, or
COMPRESSED_RGBA_S3TC_DXT5_NV. 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_NV
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_NV.
Modify Section 3.7.2, Alternate Texture Image Specification Commands
(add to the end of section)
When the internal format of the texture object is
COMPRESSED_RGB_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT1_NV,
COMPRESSED_RGBA_S3TC_DXT3_NV, or COMPRESSED_RGBA_S3TC_DXT5_NV, 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.
Additions to Chapter 4 of the OpenGL ES 2.0.25 Specification (Per-Fragment
Operations and the Framebuffer)
Modify Section 4.4.3, Attaching Texture Images to a Framebuffer
(add after last paragraph on Page 113)
If <texture> is not zero and the internal format of the
corresponding texture object is a compressed format, an
INVALID_OPERATION error is generated.
Errors
INVALID_OPERATION is generated by TexImage2D and TexSubImage2D if
TEXTURE_INTERNAL_FORMAT is COMPRESSED_RGB_S3TC_DXT1_NV,
COMPRESSED_RGBA_S3TC_DXT1_NV, COMPRESSED_RGBA_S3TC_DXT3_NV, or
COMPRESSED_RGBA_S3TC_DXT5_NV and <format> is not RGB or RGBA.
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.
Issues
1) Should sub-image updates be aligned to 4x4 pixel blocks?
RESOLUTION: Yes. Relaxing this restriction would otherwise require
CPU read-back of pixels at the 4x4 compression block boundary.
2) Should we support binding of compressed textures as render targets?
RESOLUTION: No. Writing to directly to compressed formats not
possible using hardware. Supporting this feature would involve
read-back of the texture and re-compression on the CPU, which would
be prohibitively slow.
3) Should we support mip map generation?
RESOLUTION: No. Mip map generation is not supported for compressed
textures. However, uploading to specific mip level is supported.
Revision History
Rev. Date Author Changes
---- -------- --------- -------------------------------------
2 09/27/12 istewart Changed all EXT references to NV.
Added note about DXT1 alpha encoding.
Fixed TexSubImage2D error conditions.
1 04/06/11 mshah First revision.