Class GGXDist

src/testrender/shading.cpp:297–328

The anisotropic variant of GGX and Beckmann comes from * "Understanding the Masking-Shadowing Function in * Microfacet-Based BRDFs" by Eric Heitz, JCGT 2014 (section 5.4) * * We use the height correlated masking and shadowing function * instead of the separable form as it is more realistic and * reduces energy loss at grazing angles. * * The sampling method is derived from "Importance Samp

 * is sufficient).
 */
struct GGXDist {
	static float F(const float tan_m2) {
        return 1 / (float(M_PI) * (1 + tan_m2) * (1 + tan_m2));
    }

    static float Lambda(const float a2) {
        return 0.5f * (-1.0f + sqrtf(1.0f + 1.0f / a2));
    }

    static Vec2 sampleSlope(float cos_theta, float randu, float randv) {
        // GGX
        Vec2 slope;
        /* sample slope_x */

        float c = cos_theta < 1e-6f ? 1e-6f : cos_theta;
        float Q = (1 + c) * randu - c;
        float num = c * sqrtf((1 - c) * (1 + c)) - Q * sqrtf((1 - Q) * (1 + Q));
        float den = (Q - c) * (Q + c);
        float eps = 1.0f / 4294967296.0f;
        den = fabsf(den) < eps ? copysignf(eps, den) : den;
        slope.x = num / den;

        /* sample slope_y */
        float Ru = 1 - 2 * randv;
        float u2 = fabsf(Ru);
        float z = (u2 * (u2 * (u2 * 0.27385f - 0.73369f) + 0.46341f)) /
                  (u2 * (u2 * (u2 * 0.093073f + 0.309420f) - 1.0f) + 0.597999f);
        slope.y = copysignf(1.0f, Ru) * z * sqrtf(1.0f + slope.x * slope.x);

        return slope;
    }
};

struct BeckmannDist {
	static float F(const float tan_m2) {