[Bf-cycles] Oren-Nayar model revisited

[Brecht Van Lommel]

Thanks, seems to be working well, and the code is simpler too.
Committed to svn now.

Brecht.

On Tue, Feb 21, 2012 at 3:17 PM, Yasuhiro Fujii
<y-fujii at mimosa-pudica.net> wrote:
> Hi all,
>
> A few month ago, I implemented a Oren-Nayar model for cycles.  It has a little
> wider range of roughness parameter, I noticed that it sometimes generates bad
> results. For example, The dark rings are appeared on the spheres.
> Essentially, it is caused a bad approximation of O-N model.
>
> In the original paper, Oren and Nayar have proposed two approximate functions
> (I call them full O-N and qualitative O-N, respectively). Qualitative O-N is
> well known and widely used in CG community but full O-N is less used. I think
> full O-N is too complex.
>
> So I developed a tiny improvement of qualitative O-N.  It has the same
> simplicity as qualitative O-N but yields very similar result as full O-N.
>
> The detail are described at:
> http://mimosa-pudica.net/improved-oren-nayar.html
>
> The implementation for cycles is very simple:
>
> __device float3 bsdf_oren_nayar_get_intensity(const ShaderClosure *sc,
> float3 n, float3 v, float3 l)
> {
>    float nl = max(dot(n, l), 0.0f);
>    float nv = max(dot(n, v), 0.0f);
>    float t = dot(l, v) - nl * nv;
>    if (t > 0.0f)
>        t /= max(nl, nv) + FLT_MIN;
>    float is = nl * (sc->data0 + sc->data1 * t);
>    return make_float3(is, is, is);
> }
>
> __device void bsdf_oren_nayar_setup(ShaderData *sd, ShaderClosure *sc,
> float sigma)
> {
>    sc->type = CLOSURE_BSDF_OREN_NAYAR_ID;
>    sd->flag |= SD_BSDF | SD_BSDF_HAS_EVAL;
>
>    sigma = clamp(sigma, 0.0f, 1.0f);
>    float div = 1.0f / (M_PI_F + ((3.0f * M_PI_F - 4.0f) / 6.0f) * sigma);
>
>    sc->data0 =  1.0f * div;
>    sc->data1 = sigma * div;
> }
>
> I've used this model in practice for 2, 3 month and no problems are
> found at the moment.
> I'd be happy if this improvement is reviewed.
>
> -- Yasuhiro Fujii
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