[Bf-cycles] Question about Oren-Nayar diffuse model

[Brecht Van Lommel]

This looks like a correct optimization to me, can't see any mistakes
in it. I'd say those references are wrong :).

Brecht.

On Mon, Oct 17, 2011 at 9:16 AM, Yasuhiro Fujii
<y-fujii at mimosa-pudica.net> wrote:
> Hi all,
> I am trying to implement Oren-Nayar reflectance model for the cycles renderer.
> I have some question about the implementation of O-N model (Sorry for not
> cycles specific question).
>
> I optimized the calculation of O-N model like this:
>
> original (from gpu_shader_material.glsl, modified for an explanation):
>  nl = max(dot(N, L), 0.0f);
>  nv = max(dot(N, E), 0.0f);
>  cos_nl = acos(nl);
>  cos_nv = acos(nv);
>  a = max(nl, nv);
>  b = min(nl, nv);
>  b *= 0.95f;
>  sin_a = sin(a);
>  tan_b = tan(b);
>  // use sin_a, tan_b
>
> optimized:
>  nl = max(dot(N, L), 0.0f);
>  nv = max(dot(N, E), 0.0f);
>  cos_a = min(nl, nv);
>  cos_b = max(nl, nv);
>  cos_b += FLT_MIN;
>  // FLT_MIN <= cos_b <= 1.0f because 1.0f + FLT_MIN == 1.0f
>  sin_a = sqrtf(1.0f - cos_a * cos_a);
>  tan_b = sqrtf(1.0f - cos_b * cos_b) / cos_b;
>  // use sin_a, tan_b
>
> But some references say "O-N model is expensive because the calculation of
> trigonometric function is unavoidable".  I doubt this optimization has
> something wrong though it seems to be worked on my implementation.
>
> Are there any suggestions?
>
> -- y.fujii
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