[Mesa-dev] [Bug 46631] It's really hard to hit the fast path for the fallback glReadPixels code

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Sun Feb 26 09:40:14 PST 2012


--- Comment #6 from Christoph Bumiller <e0425955 at student.tuwien.ac.at> 2012-02-26 09:40:14 PST ---
(In reply to comment #5)
> Created attachment 57667 [details]
> Test case showing reading luminance values
> I'm attaching a test case which shows what happens when luminance values are
> read from an FBO wrapping a luminance texture. The texture is created with a
> single pixel with the value 1. When the pixel is read back from glReadPixels it
> becomes 3, which implies Mesa really is adding the components together.

This seems wrong. If you specify luminance as 1 and read back luminance, you
should receive 1.

If you're reading GL_LUMINANCE from an RGB texture, adding the values L = R + G
+ B is certainly conformant, but if the texture *is* LUMINANCE, the sum has
already been performed.

If you supply RGB values to TexImage, you would have to supply (for instance) R
= G = B = 1/3 to achieve luminance 1, and adding those 3 * 1/3 back together
... it's a bug in mesa.

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