Gradients are broken with glamor when RepeatReflect is set

[Chris Wilson]

Quoting Adam Jackson (2018-01-22 20:09:52)
> On Sat, 2017-12-23 at 19:26 +0100, Clemens Eisserer wrote:
> > Hi there,
> > 
> > Glamor's gradient acceleration code is broken in case RepeatReflect is
> > used, please see: https://bugs.freedesktop.org/show_bug.cgi?id=98508
> > I've filed the bug report over a year ago, but except for a
> > confirmation from Michel Dänzer nothing happend.
> > 
> > Unfourntunatly I lack the expertise to fix it myself - however instead
> > of leaving it broken forever, could we fall back to software for
> > RepeatReflect.
> > I guess slow is better than completly broken?
> 
> Just want to note that this isn't forgotten. I got as far as testing
> the reproducer with Xephyr and verifying glamor was wrong and fb was
> right, but don't yet get what the RepeatReflect math is getting wrong.
> I'll definitely have a fix for 1.20 one way or another, but that may
> just be forcing a fallback.
> 
> If anyone wanted to investigate this, I think this is the guilty
> conditional:
> 
> https://cgit.freedesktop.org/xorg/xserver/tree/glamor/glamor_gradient.c#n296

-t = abs(fract(t * 0.5 + 0.5) * 2.0 - 1.0);
+t = abs(fract(abs(t) * 0.5 + 0.5) * 2.0 - 1.0);

#include <stdio.h>
#include <math.h>

#define abs(t) fabs(t)
#define fract(t) fmod((t), 1.0)

static float repeat(float t)
{
        return abs(fract(abs(t) * 0.5 + 0.5) * 2.0 - 1.0);
}

int main(void)
{
        float t;

        for (t = -3; t <= 3; t += .5)
                printf("%5.1f  ", t);
        printf("\n");
        for (t = -3; t <= 3; t += .5)
                printf("%5.1f  ", repeat(t));
        printf("\n");
}

 -3.0   -2.5   -2.0   -1.5   -1.0   -0.5    0.0    0.5    1.0    1.5    2.0    2.5    3.0  
  1.0    0.5    0.0    0.5    1.0    0.5    0.0    0.5    1.0    0.5    0.0    0.5    1.0 
-Chris