On Mon, Mar 29, 2010 at 5:50 PM, Ian Romanick <i...@freedesktop.org> wrote:
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> Philipp Klaus Krause wrote:
>
>> 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.
>
> Ah. Good catch. My best guess is that there are few, if any, apps that
> do that. Such apps would be easy to detect. We could enable the
> non-conformant behavior by default, and provide a driconf switch to
> disable it. We'd then need to blacklist apps that use unsupported
> cases. Since we can detect these cases, we can log a message when the
> occur.
>
> Does that seem like a reasonable compromise?
We don't have to compromise at all. If the image is already compressed
internally, then updating it with TexSubImage or CompressedTexSubImage
must be done along the block boundaries, and must be done with
pre-compressed blocks, so we are never decompressing and recompressing
the texture.
I've pushed a branch, s3tc-by-the-book, to my personal repo
(http://cgit.freedesktop.org/~csimpson/mesa/?h=s3tc-by-the-book), that
changes to this newer behavior. I haven't written up test cases for
these delightful corners and edges we're finding, but they shouldn't
be too hard to handle. The basic idea behind this branch is that if
the internal format request indicates that GL should compress the
texture with S3TC, but we don't have libdxtn present, we just change
the internal format to something more sensible and refuse to compress.
Comments and flames welcome.
~ C.
--
When the facts change, I change my mind. What do you do, sir? ~ Keynes
Corbin Simpson
<mostawesomed...@gmail.com>
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