[Piglit] [PATCH] shaders: Make glsl-fs-lots-of-tex less susceptable to rounding error.

[Jason Ekstrand]

n Fri, Jul 11, 2014 at 1:06 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:

> Kinda like http://patchwork.freedesktop.org/patch/17718/ ? :)


Yeah, kinda like that :-).  Only my patch solves it a little more for-sure


> The
> feedback I got (although not apparently in e-mail) was that it was
> shady. OTOH, the test doesn't do what it claims to due to the CSE that
> now happens, so perhaps it should just get deleted.
>

If it's not doing anything, then let's get rid of it.  In any case, I won't
care about it failing.
--Jason


>
> On Fri, Jul 11, 2014 at 4:01 PM, Jason Ekstrand <jason at jlekstrand.net>
> wrote:
> > The only reason this test passed before is because we were rounding wrong
> > in texture upload.  Previously, the shader started with 0.01, effectively
> > multiplied it by 34, and then compared it to 0.4.  The problem is that,
> > when converting from float to unsigned byte, you get 0.01 * 255 = 2.55.
> > Previously, we were rounding this up to 3.  Then 3 * 34 = 102 and
> > 102 / 255 = 0.4.  However, the OpenGL spec specifies that you should
> simply
> > cast the floating point value to the integer value and this rounds down
> to
> > 2.  The result is that you get 2 * 34 = 68 and 68 / 255 = 2.667 which is
> > nowhere close to 0.4.
> >
> > The new version simply starts with 0.4 and then divides by 4 at the end
> of
> > the shader to get back to 0.4.
> >
> > Signed-off-by: Jason Ekstrand <jason.ekstrand at intel.com>
> > ---
> >  tests/shaders/glsl-fs-lots-of-tex.shader_test | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/tests/shaders/glsl-fs-lots-of-tex.shader_test
> b/tests/shaders/glsl-fs-lots-of-tex.shader_test
> > index a31778f..9056774 100644
> > --- a/tests/shaders/glsl-fs-lots-of-tex.shader_test
> > +++ b/tests/shaders/glsl-fs-lots-of-tex.shader_test
> > @@ -34,12 +34,13 @@ void main()
> >         vec4 p = texture2D(tex, cst.xy) + texture2D(tex, cst.xy);
> >         vec4 q = texture2D(tex, cst.xy) + texture2D(tex, cst.xy);
> >
> > -       gl_FragColor = a + b + c + d + e + f + g + h + i + j + k + l + m
> + n + o + p + q;
> > +       vec4 sum = a + b + c + d + e + f + g + h + i + j + k + l + m + n
> + o + p + q;
> > +       gl_FragColor = sum / 34.0f;
> >  }
> >
> >  [test]
> >  uniform int tex 0
> >  uniform vec4 cst 0.0 0.0 0.0 0.0
> > -texture checkerboard 0 0 (8, 8) (0.01, 0.0, 0.0, 0.0) (0.01, 0.0, 0.0,
> 0.0)
> > +texture checkerboard 0 0 (8, 8) (0.4, 0.0, 0.0, 0.0) (0.4, 0.0, 0.0,
> 0.0)
> >  draw rect -1 -1 2 2
> >  relative probe rgb (0.75, 0.75) (0.4, 0.0, 0.0)
> > --
> > 2.0.0
> >
> > _______________________________________________
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> > Piglit at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/piglit
>
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