[Bug 109519] Input sample mask has unexpected values

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=109519

--- Comment #2 from Lionel Landwerlin <lionel.g.landwerlin at linux.intel.com> ---
(In reply to Danylo from comment #1)
> Created attachment 143510 [details]
> resulting image on anv
> 
> Hi, sorry for the delay.
> 
> I checked an output of the test in RenderDoc and saw this (see attachment).
> There is a diagonal of pixels having a *wrong* mask.
> 
> It is happening because this test draws a big triangle which covers the
> whole render area and this triangle is being clipped which divides it and
> creates new geometry.
> 
> So on this diagonal there are two fragments per one pixel and to get the
> right coverage you should add their coverages together.
> 
> e.g. you can achieve this by having ssbo and atomic adding sample mask to
> the corresponding fragment's position (suggested by Lionel on irc).

Here is how we did it for a post depth coverage test :

https://gitlab.freedesktop.org/mesa/piglit/blob/master/tests/spec/arb_post_depth_coverage/multisampling.c#L75

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