[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #11 from Andreas Boll <andreas.boll.dev at gmail.com> 2012-09-12 13:56:46 UTC ---
(In reply to comment #10)
> 
> It's so long ago I don't remember the exact details. You can easily figure it
> out though. The test program creates a 4x4 checker board texture and then uses
> a single point on it to draw the triangles. The range [0.25,0.5[ should all be
> the same color. The bug was that as you approached 0.5, it started sampling the
> next pixel prematurely.
> 
> So change the fragment program to use coordinate 0.4 or something like that.
> That colour should be the same as 0.4999... . Looking at the code, white seems
> to be the correct colour.

Ok I've got it.

With coordinate 0.4 I get a white color and with 0.4999 I get black.

Additionally I checked the other end of the range:
Between the range [0.2480468676,0.498046860099] the color is white.
If I change the fragment program to use coordinate 0.2480468675 then the color
is black. With 0.4980468601 I get also black.

So the issue persists in git mesa master 9.1-devel (git-e81ee67)

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