vk: rt: attenuate secondary bounces

`throughput` should be scaled based on BRDF. I.e. diffuse bounces should
be scales by base_color at the very least.

This fixes lighting being too bright after 3-5 bounces.

ref/vk/TODO.md

@@ -6,6 +6,9 @@
 - [ ] -vkdbgprintf or something
 - [ ] performance profiling and comparison
 
+## 2024-01-29 E370
+- [x] bounce > 1 brighness
+
 # Previously
 ## 2024-01-26 E369
 - [x] white furnace test

ref/vk/shaders/bounce.comp

@@ -106,6 +106,8 @@ struct MaterialEx {
 
 int max_bounces = 1;
 
+const vec3 kThrougputExtinctionThreshold = vec3(1e-3);
+
 ivec2 pix;
 void computeBounces(MaterialEx mat, vec3 pos, vec3 direction, inout vec3 diffuse, inout vec3 specular) {
 	const float ray_normal_fudge = .01;
@@ -124,6 +126,28 @@ void computeBounces(MaterialEx mat, vec3 pos, vec3 direction, inout vec3 diffuse
 		if (brdf_type == BRDF_TYPE_NONE)
 			return;
 
+		if (IS_INVALIDV(throughput)) {
+#ifdef SHADER_DEBUG_ENABLE
+			debugPrintfEXT("pix=(%d,%d) pos=(%f,%f,%f) dir=(%f,%f,%f) throughput=invalid",
+				pix.x, pix.y, pos.x, pos.y, pos.z, direction.x, direction.y, direction.z);
+#endif
+			throughput = vec3(0.);
+		}
+
+/*
+		if (any(lessThan(throughput, vec3(0.)))) {
+#ifdef SHADER_DEBUG_ENABLE
+			debugPrintfEXT("pix=(%d,%d) pos=(%f,%f,%f) dir=(%f,%f,%f) throughput=(%f, %f, %f)",
+				pix.x, pix.y, pos.x, pos.y, pos.z, direction.x, direction.y, direction.z,
+				throughput.r, throughput.g, throughput.b
+				);
+#endif
+		}
+*/
+
+		if (all(lessThan(throughput,kThrougputExtinctionThreshold)))
+			return;
+
 		if (first_brdf_type == BRDF_TYPE_NONE)
 			first_brdf_type = brdf_type;
 
@@ -133,7 +157,7 @@ void computeBounces(MaterialEx mat, vec3 pos, vec3 direction, inout vec3 diffuse
 		payload.material_rmxx = vec4(0.);
 		payload.normals_gs = vec4(0.);
 
-		vec3 lighting = vec3(0.);
+		vec3 contribution = vec3(0.);
 		pos += mat.geometry_normal * ray_normal_fudge;
 		const bool did_hit = getHit(pos, bounce_direction, payload);
 		bool hit_skybox = false;
@@ -179,7 +203,13 @@ void computeBounces(MaterialEx mat, vec3 pos, vec3 direction, inout vec3 diffuse
 			//background += lspecular * mix(vec3(1.), payload.base_color_a.rgb, hit_material.metalness);
 
 			const vec4 blend = traceLegacyBlending(pos, bounce_direction, payload.hit_t.w);
-			lighting = SRGBtoLINEAR(blend.rgb) + background * blend.a;
+			contribution = throughput*(SRGBtoLINEAR(blend.rgb) + background * blend.a);
+
+			// FIXME check this very early before doing any lighting stuff
+			hit_skybox = payload.hit_t.w < 0.;
+			if (hit_skybox)
+				/* FIXME: also if last bounce (i.e. w/o direct light sampling) or */
+				contribution += throughput * payload.emissive.rgb;
 
 			// Prepare next bounce state
 			pos = hit_pos;
@@ -188,57 +218,25 @@ void computeBounces(MaterialEx mat, vec3 pos, vec3 direction, inout vec3 diffuse
 			mat.geometry_normal = normalDecode(payload.normals_gs.xy);
 			mat.shading_normal = hit_shading_normal;
 
-			hit_skybox = payload.hit_t.w < 0.;
+			if (brdf_type == BRDF_TYPE_DIFFUSE) {
+				const vec3 diffuse_color = mix(payload.base_color_a.rgb, vec3(0.), payload.material_rmxx.g);
+				throughput *= diffuse_color;
+			}
 		} else {
 			// FIXME this no-hit path should still do traceLegacyBlending()
-			lighting = sampleSkybox(bounce_direction);
+			contribution = throughput * sampleSkybox(bounce_direction);
 			hit_skybox = true;
 		} // not hit
 
-		/* TODO: also if last bounce (i.e. w/o direct light sampling) or */
-		if (hit_skybox)
-			lighting += payload.emissive.rgb;
-
 		if (first_brdf_type == BRDF_TYPE_DIFFUSE)
-			diffuse += throughput * lighting;
+			diffuse += contribution;
 		else
-			specular += throughput * lighting;
-
-		if (IS_INVALIDV(throughput)) {
-#ifdef SHADER_DEBUG_ENABLE
-			debugPrintfEXT("pix=(%d,%d) pos=(%f,%f,%f) dir=(%f,%f,%f) throughput=invalid",
-				pix.x, pix.y, pos.x, pos.y, pos.z, direction.x, direction.y, direction.z);
-#endif
-			throughput = vec3(0.);
-			//diffuse = 10.*vec3(1., 0., 0.);
-			//specular = 10.*vec3(0., 0., 1.);
-		} else {
-			//diffuse = throughput;
-		}
-
-/*
-		if (any(lessThan(throughput, vec3(0.)))) {
-#ifdef SHADER_DEBUG_ENABLE
-			debugPrintfEXT("pix=(%d,%d) pos=(%f,%f,%f) dir=(%f,%f,%f) throughput=(%f, %f, %f)",
-				pix.x, pix.y, pos.x, pos.y, pos.z, direction.x, direction.y, direction.z,
-				throughput.r, throughput.g, throughput.b
-				);
-#endif
-		}
-*/
+			specular += contribution;
 
 		if (hit_skybox) {
-			//diffuse = vec3(1.,1.,0.);
 			break;
-		} else {
-			//diffuse = vec3(0.,1.,1.);
 		}
 
-		//if (should_include_emissive)
-			//diffuse = lighting;// payload.emissive.rgb;
-		//diffuse = bounce_direction * .5 + .5;
-		//diffuse = throughput;
-
 	} // for bounces
 }
 

ref/vk/shaders/brdf.glsl

@@ -147,7 +147,7 @@ void brdfComputeGltfModel(vec3 N, vec3 L, vec3 V, MaterialProperties material, o
 
 	// Specular does get the real color, as its contribution is light-direction-dependent
 	const vec3 f0 = mix(vec3(.04), material.base_color, material.metalness);
-	float fresnel_factor = max(0., pow(1. - abs(h_dot_v), 5.));
+	const float fresnel_factor = max(0., pow(1. - abs(h_dot_v), 5.));
 	const vec3 fresnel = vec3(1.) * fresnel_factor + f0 * (1. - fresnel_factor);
 
 	// This is taken directly from glTF 2.0 spec. It seems incorrect to me: it should not include the base_color twice.
@@ -349,18 +349,12 @@ vec3 SampleGGXReflection ( vec3 i , vec2 alpha , vec2 rand ) {
 
 int brdfGetSample(vec2 rnd, MaterialProperties material, vec3 view, vec3 geometry_normal, vec3 shading_normal, /*float alpha, */out vec3 out_direction, inout vec3 inout_throughput) {
 #if 1
-	// Idiotic sampling, super noisy, bad distribution, etc etc
-	// But we need to start somewhere
-	// TODO pick diffuse-vs-specular based on expected contribution
-	// TODO fresnel factor
-	// TODO base_color also might play a role
-
 	// See SELECTING BRDF LOBES in 14.3.6 RT Gems 2
 	// TODO DRY brdfComputeGltfModel
 	// Use shading_normal as H estimate
 	const float h_dot_v = max(0., dot(shading_normal, view));
 	const vec3 f0 = mix(vec3(.04), material.base_color, material.metalness);
-	float fresnel_factor = max(0., pow(1. - abs(h_dot_v), 5.));
+	const float fresnel_factor = max(0., pow(1. - abs(h_dot_v), 5.));
 	const vec3 fresnel = vec3(1.) * fresnel_factor + f0 * (1. - fresnel_factor);
 	const float est_spec = luminance(fresnel);
 	const float est_diff = (1. - fresnel_factor) * (1. - material.metalness);
@@ -393,8 +387,7 @@ if (g_mat_gltf2) {
 
 		const vec3 H = normalize(out_direction + view);
 		const float h_dot_v = max(0., dot(H, view));
-		const vec3 f0 = mix(vec3(.04), material.base_color, material.metalness);
-		float fresnel_factor = max(0., pow(1. - abs(h_dot_v), 5.));
+		const float fresnel_factor = max(0., pow(1. - abs(h_dot_v), 5.));
 		// TODO use mix(), higher chance for it to be optimized
 		const vec3 fresnel = vec3(1.) * fresnel_factor + f0 * (1. - fresnel_factor);