Sampling and BRDF improvements:

- Random number generator now produces enough different sequences to cover 500 frames of reference mode accumulation;
- Hemisphere sampling for indirect diffuse tuned to make the results better match the cosine-weighted sampling in reference mode;
- Direct diffuse lighting is now correctly divided by PI and matches same lighting computed by random sampling of emissive surfaces;
- Spotlight term removed from sky polygons and added to other non-analytic emissive surfaces;
- Suspicious 0.5 term removed from sky polygons.
Also the `pt_direct_polygon_lights` cvar has a new meaning when set to -1: all polygon lights are sampled through the indirect lighting shader, for comparison purposes.
Inspired by #37

src/refresh/vkpt/shader/asvgf_seed_rng.comp

@@ -52,8 +52,7 @@ main()
 
 	rng_seed |= (uint(ipos.x) % BLUE_NOISE_RES)  <<  0u;
 	rng_seed |= (uint(ipos.y) % BLUE_NOISE_RES)  << 10u;
-	rng_seed |= checkerboard << 20;
-	rng_seed |= uint(frame_num) << 21;
+	rng_seed |= uint(frame_num + checkerboard) << 20;
 
 	imageStore(IMG_ASVGF_RNG_SEED_A, ipos, uvec4(rng_seed));
 }

src/refresh/vkpt/shader/direct_lighting.rgen

@@ -577,8 +577,8 @@ direct_lighting(ivec2 ipos, bool is_odd_checkerboard, out vec3 high_freq, out ve
     	spec_enable_caustics != 0, 
     	primary_specular, 
     	direct_specular_weight, 
-    	global_ubo.pt_direct_polygon_lights != 0,
-    	global_ubo.pt_direct_sphere_lights != 0,
+    	global_ubo.pt_direct_polygon_lights > 0,
+    	global_ubo.pt_direct_sphere_lights > 0,
     	direct_diffuse,
     	direct_specular);
 

src/refresh/vkpt/shader/indirect_lighting.rgen

@@ -177,7 +177,7 @@ trace_secondary_ray(
 
 	if(is_sky(ray_payload_brdf))
 	{
-		bool is_analytic_light = (triangle.material_id & MATERIAL_FLAG_LIGHT) != 0 && (spec_bounce_index == 0);
+		bool is_analytic_light = (triangle.material_id & MATERIAL_FLAG_LIGHT) != 0 && (spec_bounce_index == 0) && (global_ubo.pt_direct_polygon_lights >= 0);
 		if(!is_analytic_light || is_specular_ray)
 		{
 			vec3 env = env_map(bounce_direction, true);
@@ -226,7 +226,11 @@ trace_secondary_ray(
 		{        
 			emissive *= bounce_throughput;
 
-			bool is_analytic_light = (bounce_material_id & MATERIAL_FLAG_LIGHT) != 0;
+			// spotlight term that matches the sample_light_lists() function in light_lists.h
+			float spotlight = sqrt(max(0, -dot(bounce_direction, bounce_normal)));
+			emissive *= spotlight;
+
+			bool is_analytic_light = (bounce_material_id & MATERIAL_FLAG_LIGHT) != 0 && (global_ubo.pt_direct_polygon_lights >= 0);
 
 			if(is_specular_ray)
 			{

src/refresh/vkpt/shader/light_lists.h

@@ -129,7 +129,7 @@ sample_light_list(
 
 		float light_lum = luminance(light.color);
 		if(light_lum < 0 && global_ubo.environment_type == ENVIRONMENT_DYNAMIC)
-			m *= sun_color_ubo.sky_luminance * 0.5;
+			m *= sun_color_ubo.sky_luminance;
 		else
 			m *= abs(light_lum); // abs because sky lights have negative color
 
@@ -181,7 +181,7 @@ sample_light_list(
 		if(light.color.r >= 0)
 			light_color = light.color * area * spotlight;
 		else
-			light_color = env_map(L, true) * area * spotlight * global_ubo.pt_env_scale * 0.5;
+			light_color = env_map(L, true) * area * global_ubo.pt_env_scale;
 	}
 }
 

src/refresh/vkpt/shader/path_tracer_rgen.h

@@ -177,7 +177,7 @@ float
 get_rng(uint idx)
 {
 	uvec3 p = uvec3(rng_seed, rng_seed >> 10, rng_seed >> 20);
-	p.z = (p.z * NUM_RNG_PER_FRAME + idx);
+	p.z = (p.z + idx);
 	p &= uvec3(BLUE_NOISE_RES - 1, BLUE_NOISE_RES - 1, NUM_BLUE_NOISE_TEX - 1);
 
 	return min(texelFetch(TEX_BLUE_NOISE, ivec3(p), 0).r, 0.9999999999999);
@@ -488,7 +488,7 @@ get_direct_illumination(
 
 	float NdotL = max(0, dot(normal, L));
 
-	diffuse *= NdotL;
+	diffuse *= NdotL / M_PI;
 }
 
 void
@@ -566,7 +566,7 @@ get_sunlight(
     	specular = diffuse * GGX(view_direction, global_ubo.sun_direction, normal, roughness, NoH_offset);
 	}
 
-	diffuse *= NdotL;
+	diffuse *= NdotL / M_PI;
 }
 
 vec3 clamp_output(vec3 c)

src/refresh/vkpt/shader/utils.glsl

@@ -291,7 +291,7 @@ sample_cos_hemisphere(vec2 uv)
 }
 
 #define HEMISPHERE_COSINE 0.5
-#define HEMISPHERE_UNIFORMISH 0.25
+#define HEMISPHERE_UNIFORMISH 0.4
 
 vec3 
 sample_cos_hemisphere_multi(