Ian Romanick schrieb:
> Philipp Klaus Krause wrote:
>> Corbin Simpson schrieb:
>>> After re-reading ARB_texture_compression, it does seem like it is
>>> legal for extensions such as EXT_texture_compression_s3tc to fall
>>> back, using a basic mechanism: glGetTexLevelParameter can return
>>> GL_FALSE for GL_TEXTURE_COMPRESSED_ARB (now GL_TEXTURE_COMPRESSED) if
>>> the texture could not successfully be compressed. Issue (10) of
>>> ARB_texture_compression deals with this explicitly:
>> Yes, ARB_texture_compression allows to support decompression only. That
>> could probably be used to have decompression support for the S3TC
>> formats in Mesa. However, this does mean that
>> EXT_texture_compression_s3tc would be supported, since
>> EXT_texture_compression_s3tc does require support for encoding and the
>> wording in ARB_texture_compression does not change that.
>
> EXT_texture_compression_s3tc is layered on top of
> ARB_texture_compression. I don't see anything in the spec that adds
> this requirement. In fact, I don't see *anything* in that spec that
> mentions TexImage2D. Am I just overlooking some wording in
> EXT_texture_compression_s3tc that adds this?
Well, there is TexSubImage2D. Assuming we have a compressed texture
stored internally as some S3TC format and then the application replaces
part of it using TexSubImage2D. According to ARB_texture_compression we
may not go to uncompressed ("the allocation and chosen compressed image
format must not be a function of any other state and cannot be changed
once they are established". And while ARB_texture_compression does not
require TexSubImage2D support, EXT_texture_compression_s3tc does.
Philipp
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