[Piglit] [PATCH] Test accuracy of dFdx and dFdy functions.

[Ian Romanick]

On 09/20/2013 10:15 AM, Paul Berry wrote:
> Mesa/i965 currently (as of commit 1cc3b90) has an inaccurate
> implementation of dFdy() which produces the same results for every
> pixel in an aligned 2x2 block.  This piglit test exposes the
> inaccuracy.

Two questions:

 - How did you come up with the error metric values?

 - Why are the different?

> ---
>  .../execution/fs-dfdx-accuracy.shader_test         | 35 ++++++++++++++++++++++
>  .../execution/fs-dfdy-accuracy.shader_test         | 35 ++++++++++++++++++++++
>  2 files changed, 70 insertions(+)
>  create mode 100644 tests/spec/glsl-1.10/execution/fs-dfdx-accuracy.shader_test
>  create mode 100644 tests/spec/glsl-1.10/execution/fs-dfdy-accuracy.shader_test
> 
> diff --git a/tests/spec/glsl-1.10/execution/fs-dfdx-accuracy.shader_test b/tests/spec/glsl-1.10/execution/fs-dfdx-accuracy.shader_test
> new file mode 100644
> index 0000000..b8d471c
> --- /dev/null
> +++ b/tests/spec/glsl-1.10/execution/fs-dfdx-accuracy.shader_test
> @@ -0,0 +1,35 @@
> +# The spec allows considerable leeway in how dFdx() is calculated,
> +# however the expected implementation is via forward or backward
> +# differencing.  (Implementations are permitted, for instance, to use
> +# forward differencing for some pixels and backward differencing for
> +# other pixels).
> +#
> +# This test computes dFdx(x*y), which should exactly equal y at every
> +# pixel, regardless of whether forward or backward differencing is
> +# used.
> +
> +[require]
> +GLSL >= 1.10
> +
> +[vertex shader]
> +void main()
> +{
> +	gl_Position = gl_Vertex;
> +}
> +
> +[fragment shader]
> +void main()
> +{
> +	float x = gl_FragCoord.x;
> +	float y = gl_FragCoord.y;
> +	float xy = x * y;
> +	float dxydx = dFdx(xy);
> +	if (distance(dxydx, y) > 1.0e-3)
> +		gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
> +	else
> +		gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0);
> +}
> +
> +[test]
> +draw rect -1 -1 2 2
> +probe all rgba 0.0 1.0 0.0 1.0
> diff --git a/tests/spec/glsl-1.10/execution/fs-dfdy-accuracy.shader_test b/tests/spec/glsl-1.10/execution/fs-dfdy-accuracy.shader_test
> new file mode 100644
> index 0000000..5c02680
> --- /dev/null
> +++ b/tests/spec/glsl-1.10/execution/fs-dfdy-accuracy.shader_test
> @@ -0,0 +1,35 @@
> +# The spec allows considerable leeway in how dFdy() is calculated,
> +# however the expected implementation is via forward or backward
> +# differencing.  (Implementations are permitted, for instance, to use
> +# forward differencing for some pixels and backward differencing for
> +# other pixels).
> +#
> +# This test computes dFdy(x*y), which should exactly equal x at every
> +# pixel, regardless of whether forward or backward differencing is
> +# used.
> +
> +[require]
> +GLSL >= 1.10
> +
> +[vertex shader]
> +void main()
> +{
> +	gl_Position = gl_Vertex;
> +}
> +
> +[fragment shader]
> +void main()
> +{
> +	float x = gl_FragCoord.x;
> +	float y = gl_FragCoord.y;
> +	float xy = x * y;
> +	float dxydy = dFdy(xy);
> +	if (distance(dxydy, x) > 1.0e-1)
> +		gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
> +	else
> +		gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0);
> +}
> +
> +[test]
> +draw rect -1 -1 2 2
> +probe all rgba 0.0 1.0 0.0 1.0
>