[Mesa-dev] [PATCH] st/mesa: fix WPOS adjustment, with more comments

[Marek Olšák]

Tested-by: Marek Olšák <maraeo at gmail.com>

On Wed, Apr 27, 2011 at 8:53 PM, Christoph Bumiller
<e0425955 at student.tuwien.ac.at> wrote:
> Again, with a more detailed explanation added as comments.
>
> On 20.04.2011 17:33, Christoph Bumiller wrote:
>
> On 04/19/2011 04:00 AM, Christoph Bumiller wrote:
>
>   On 16.04.2011 18:50, Christoph Bumiller wrote:
>
> I hope the new version is correct, the commit message describes why I
> did the first change, and the second change is described in a comment.
>
> Note that the MAD for inversion uses Height - 1 instead of Height.
>
> With this, piglit glsl-arb-fragment-coord-conventions and
> fbo-depth-sample-compare pass on nvc0.
>
> I was assuming that integer pixel centers for size 100 range from 0 to
> 99 and half-integer pixel centers from 0.5 to 99.5.
>
> Attached a better version of the patch, potentially saving an
> instruction and avoiding precision issues with NEAREST filtering that
> made piglit's blending-in-shader fail.
>
> The only other location I found STATE_FB_WPOS_Y_TRANSFORM used was
> r600 classic, which should also work more correctly now since (at
> least according to gallium caps) it uses half-integer pixel center,
> and for H=100, 0.5 * -1 + 99 obviously isn't the desired 99.5.
>
> Please review.
>
> Unfortunately I found another small error making a more precise piglit
> test that draws to a float buffer instead of mangling the coordinate
> like the original test does, which conceals errors.
>
> Now there's a dependency again on whether inversion is actually done or
> not, so I added a check on the inversion constant in the shader, but
> maybe adding additional uniforms or even adjusting
> STATE_FB_WPOS_Y_TRANSFORM dependent on the coordinat convention might be
> nicer ?
>
> Sorry for not noticing right away,
> Christoph
>
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