[Mesa-dev] [Bug 45913] [bisected] piglit vs-clip-vertex-const-reject fails

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=45913

Paul Berry <stereotype441 at gmail.com> changed:

           What    |Removed                     |Added
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           Keywords|regression                  |

--- Comment #1 from Paul Berry <stereotype441 at gmail.com> 2012-02-10 15:22:48 PST ---
Before commit 7d68c639ddb0f9df45cf698b8e8227bf1860b5fe, Mesa's support for gl_ClipVertex was completely broken, because Mesa couldn't distinguish between
gl_ClipVertex and gl_Position.  The bugs in Mesa happened to line up in such a way that the Piglit test vs-clip-vertex-const-reject passed, but this was not a
sign that gl_ClipVertex in any way worked; it was just a coincidence.

Commit 7d68c639ddb0f9df45cf698b8e8227bf1860b5fe laid the groundwork for actual non-buggy support for gl_ClipVertex, and a later patch.  However, it was
impossible to lay this groundwork without causing vs-clip-vertex-const-reject to stop passing.  This is not a true regression, because
vs-clip-vertex-const-reject was only passing by dumb luck before this commit; it wasn't passing because of any correct functionality in Mesa.

In order for gl_ClipVertex to work on any particular driver (for example r300g or llvmpipe), additional back-end work needs to be done.  I have done this
back-end work for i965 (see commit d912669034eb7bf5c162358a7a574ec7a4c963c7).  However, I don't have the expertise to do it for other back-ends.  If anyone who
is knowledgeable about r300g RV530 or llvmpipe would like to do the corresponding back-end work for those drivers, I would be happy to help--feel free to email
me at stereotype441 at gmail.com if commit d912669034eb7bf5c162358a7a574ec7a4c963c7 doesn't provide enough information.

Incidentally, the same situation also exists for i915.

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