Fix MIS weights

Author: yozhijk

App/CL/integrator_pt.cl

@@ -411,7 +411,7 @@ __kernel void ShadeSurface(
             // Sample light
             float3 le = Light_Sample(light_idx, &scene, &diffgeo, TEXTURE_ARGS, Sampler_Sample2D(&sampler, SAMPLER_ARGS), &lightwo, &lightpdf);
             lightbxdfpdf = Bxdf_GetPdf(&diffgeo, wi, normalize(lightwo), TEXTURE_ARGS);
-            lightweight = Light_IsSingular(&scene.lights[light_idx]) ? 1.f : BalanceHeuristic(1, lightpdf, 1, lightbxdfpdf);
+            lightweight = Light_IsSingular(&scene.lights[light_idx]) ? 1.f : BalanceHeuristic(1, lightpdf / selection_pdf, 1, lightbxdfpdf);
 
             // Apply MIS to account for both
             if (NON_BLACK(le) && lightpdf > 0.0f && !Bxdf_IsSingular(&diffgeo))   

App/CL/light.cl

@@ -219,7 +219,7 @@ float3 AreaLight_Sample(// Emissive object
 
     if (ndotv > 0.f)
     {
-        float dist2 = (length(*wo) * length(*wo));
+        float dist2 = dot(*wo, *wo);
         float denom = fabs(ndotv) * area;
         *pdf = denom > 0.f ? dist2 / denom : 0.f;
         return dist2 > 0.f ? ke * ndotv / dist2 : 0.f;