commit 1f02ac9a68ef25e76833bb15253da15d98b45b06
parent c2b41c82fcd97a70809fd6cab03a29b618046c8e
Author: Vincent Forest <vincent.forest@meso-star.com>
Date: Wed, 5 Oct 2016 10:48:05 +0200
Comment the built-in BxDFs, Fresnel terms and Microfacet distributions
Diffstat:
2 files changed, 69 insertions(+), 18 deletions(-)
diff --git a/src/ssf.h b/src/ssf.h
@@ -143,29 +143,82 @@ SSF_MICROFACET_DISTRIBUTION_TYPE_NULL = SSF_MICROFACET_DISTRIBUTION_TYPE_NULL__;
BEGIN_DECLS
-/* Reflects the incoming direction with respect to the surface normal */
+/*******************************************************************************
+ * Built-in BxDFs
+ ******************************************************************************/
+/* Dirac distribution whose incoming direction `wi' is the reflection of the
+ * supplied direction `wo' with respect to the surface normal `N'. The
+ * directional reflectance is controlled by the Fresnel term `Fr'.
+ * fr(wo, wi) = Fr(|wi.N|) * delta(wo - Reflect(wi, N)) / |wi.N|
+ * Since it is a dirac distribution, the returned value of the `eval' and `pdf'
+ * function is always 0 while the pdf returned by `sample' function is
+ * infinity. */
SSF_API const struct ssf_bxdf_type ssf_specular_reflection;
-/* Reflects the same intensity independent of the incoming direction */
+
+/* Reflects the same intensity in all direction independently of the incoming
+ * direction; fr(wo, wr) = R/PI */
SSF_API const struct ssf_bxdf_type ssf_lambertian_reflection;
-/* Glossy reflections with respect to a microfacet distribution */
+
+/* Glossy reflections with respect to a microfacet distribution. It is based on
+ * the Torrance Sparrow BRDF to provide a general Microfacet-based BRDF model
+ * fr(wo, wi) = Fr(|wi.wh|) * G(wo, wi) * D(wh) / (4 * |wo.N| * |wi.N|)
+ * with `wh' the half vector between `wi' and `wo', `Fr(|wi.wh|)' the Fresnel
+ * term that controls the directional reflectance, `G(wo, wi)' the geometric
+ * attenuation term that describes the masking and the shadowing of the
+ * microfacets and `D(wh)' the distribution area of the microfacets whose
+ * normal is `wh'. Note that common BRDFs based on this model ignore multiple
+ * scattering of lights into the microfacet structure and remove them with the
+ * 'G' term. As a consequence, this model does not satisfy the energy
+ * conservation property */
SSF_API const struct ssf_bxdf_type ssf_microfacet_reflection;
+
/* Glossy reflections with respect to a microfacet distribution. In contrast to
- * the common microfacet reflection, this BRDF ensures, by design, the energy
- * conservation property without requiring the normalization of the BRDF. As a
- * counterpart, it only provides the sample function. The others functions
- * return invalid value. */
+ * the ssp_microfacet_reflection model, this BRDF ensures, by design, the
+ * energy conservation property without requiring the normalization of the
+ * BRDF. As a counterpart, it only provides the sample function. The others
+ * functions return invalid value. However, it is well suited for Monte Carlo
+ * integrations that only need to sample a direction and to define its
+ * associated directional reflectance */
SSF_API const struct ssf_bxdf_type ssf_microfacet2_reflection;
-/* Fresnel term for perfect reflection */
+/*******************************************************************************
+ * Built-in Fresnel terms
+ ******************************************************************************/
+/* Fresnel term for perfect reflection; Fr = 1 */
SSF_API const struct ssf_fresnel_type ssf_fresnel_no_op;
-/* Fresnel term between 2 dielectric mediums */
+
+/* Fresnel term between 2 dielectric mediums.
+ * Fr = 1/2 * (Rs^2 + Rp^2)
+ * with `Rs' and `Rp' the reflectance for the light polarized with its electric
+ * field perpendicular or parallel to the plane of incidence, respectively.
+ * Rs = (n1*|wi.N| - n2*|wt.N|) / (n1*|wi.N| + n2*|wt.N)
+ * Rp = (n2*|wi.N| - n1*|wt.N|) / (n2*|wi.N| + n1*|wt.N)
+ * with `n1' and `n2' the indices of refraction of the incident and transmitted
+ * media, and `wi' and `wt' the incident and transmitted direction. */
SSF_API const struct ssf_fresnel_type ssf_fresnel_dielectric_dielectric;
-/* Fresnel term between a dielectric and a conductor */
+
+/* Fresnel term between a dielectric and a conductor.
+ * Fr = 1/2 * (Rs^2 + Rp^2)
+ * with `Rs' and `Rp' the reflectance for the light polarized with its electric
+ * field perpendicular or parallel to the plane of incidence, respectively.
+ * Rs = (n1*|wi.N| - (n2 - i*k2)*|wt.N|) / (n1*|wi.N| + (n2 - i*k2)*|wt.N|)
+ * Rp = ((n2 - i*k2)*|wt.N| - n1*|wi.N|) / ((n2 - i*k2)*|wt.N| + n1*|wi.N|)
+ * with `n1' the index of refraction of the incident media, `n2' and `k2' the
+ * real and imaginary part of the index of refraction of the transmitted media,
+ * and `wi' and `wt' the incident and transmitted direction */
SSF_API const struct ssf_fresnel_type ssf_fresnel_dielectric_conductor;
-/* Beckmann microfacet distribution TODO add comments */
+/*******************************************************************************
+ * Built-in microfacet distributions
+ ******************************************************************************/
+/* Beckmann microfacet distribution.
+ * D(wh) = exp(-tan^2(a)/m^2) / (PI*m^2*cos^4(a))
+ * with `a' = arccos(wh.N) and `m' the rougness in ]0, 1] of the surface */
SSF_API const struct ssf_microfacet_distribution_type ssf_beckmann_distribution;
-/* Blinn microfacet distribution TODO add comments */
+
+/* Blinn microfacet distribution.
+ * D(wh) = (e + 2) / (2*PI) * (wh.N)^e
+ * with `e' an exponent in [0, SSF_BLINN_DISTRIBUTION_MAX_EXPONENT] */
SSF_API const struct ssf_microfacet_distribution_type ssf_blinn_distribution;
/*******************************************************************************
@@ -378,9 +431,9 @@ ssf_microfacet_distribution_pdf
const double N[3], /* Normalized Z-direction of the distribution */
const double wh[3]); /* Normalized half vector */
-/* Retrieve the internal data of the microfacet distribution term. Usefull for
- * user defined distributions on which the caller has to retrieve their data to
- * setup their parameters */
+/* Retrieve the internal data of the microfacet distribution. Usefull for user
+ * defined distributions on which the caller has to retrieve their data to
+ * setup their parameters. */
SSF_API res_T
ssf_microfacet_distribution_get_data
(struct ssf_microfacet_distribution* distrib,
diff --git a/src/ssf_fresnel_dielectric_dielectric.c b/src/ssf_fresnel_dielectric_dielectric.c
@@ -66,14 +66,12 @@ fresnel_dielectric_dielectric_eval(void* fresnel, const double cos_theta_i)
* parallel to the plane of incidence */
Rp = (fd->eta_i*cos_theta_t - fd->eta_t*cos_theta_i)
/ (fd->eta_i*cos_theta_t + fd->eta_t*cos_theta_i);
- Rp = fabs(Rp);
Rp = Rp * Rp;
/* Compute the reflectance for the light polarized with its electric field
* perpendicular to the plane of incidence */
Rs = (fd->eta_i*cos_theta_i - fd->eta_t*cos_theta_t)
/ (fd->eta_i*cos_theta_i + fd->eta_t*cos_theta_t);
- Rs = fabs(Rs);
Rs = Rs * Rs;
return 0.5 * (Rp + Rs);
@@ -95,7 +93,7 @@ ssf_fresnel_dielectric_dielectric_setup
(struct ssf_fresnel* fresnel, const double eta_i, const double eta_t)
{
struct fresnel_dielectric_dielectric* fd;
- if(!fresnel
+ if(!fresnel
|| !FRESNEL_TYPE_EQ(&fresnel->type, &ssf_fresnel_dielectric_dielectric)) {
return RES_BAD_ARG;
}
