WIP(gi): use medium stack

pablode · May 26, 2024 · 7b2a46d · 7b2a46d
1 parent 3a2c098
commit 7b2a46d
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Showing 2 changed files with 78 additions and 19 deletions.
diff --git a/src/gi/shaders/rp_main.chit b/src/gi/shaders/rp_main.chit
@@ -135,7 +135,6 @@ void main()
 {
     /* 1. Get hit info. */
     vec2 hit_bc = baryCoord;
-    float hit_t = gl_HitTEXT;
     BlasPayload payload = blas_payloads[gl_InstanceCustomIndexEXT];
 
     /* 2. Set up shading state. */
@@ -150,8 +149,7 @@ void main()
     vec3 throughput = vec3(rayPayload.throughput);
     vec3 radiance = vec3(rayPayload.radiance);
 
-    uint bounce = (rayPayload.bitfield & SHADE_RAY_PAYLOAD_BOUNCES_MASK);
-    bool inside = shadeRayPayloadGetMediumIdx(rayPayload) > 0;
+    uint mediumIdx = shadeRayPayloadGetMediumIdx(rayPayload);
     bool doubleSided = bool(payload.bitfield & BLAS_PAYLOAD_BITFLAG_IS_DOUBLE_SIDED);
 
     bool thinWalled = doubleSided;
@@ -162,9 +160,6 @@ void main()
     }
 #endif
 
-    const float iorCurrent = (inside && !thinWalled) ? BSDF_USE_MATERIAL_IOR : 1.0;
-    const float iorOther = (inside && !thinWalled) ? 1.0 : BSDF_USE_MATERIAL_IOR;
-
 #if AOV_ID == AOV_ID_DEBUG_OPACITY
 #ifndef HAS_CUTOUT_TRANSPARENCY
     rayPayload.radiance = vec3(1.0, 0.0, 0.0); // Distinct from viridis heatmap
@@ -216,14 +211,30 @@ void main()
 #endif
 
     /* 3. Apply volume attenuation */
-#ifdef HAS_VOLUME_ABSORPTION_COEFF
-    if (inside && !thinWalled)
+    vec3 mediumIor = vec3(1.0);
+    vec3 prevMediumIor = vec3(1.0);
+
+    if (mediumIdx > 0)
     {
-        vec3 abs_coeff = mdl_volume_absorption_coefficient(shading_state);
-        throughput *= exp(-abs_coeff * hit_t);
-    }
+#if MEDIUM_STACK_SIZE == 0
+        vec3 sigma_a = mdl_volume_absorption_coefficient(shading_state);
+#else
+        Medium m = rayPayload.media[mediumIdx - 1];
+        vec3 sigma_a = m.sigma_t - m.sigma_s;
+
+        mediumIor = m.ior;
+        if (mediumIdx > 1)
+        {
+            prevMediumIor = rayPayload.media[mediumIdx - 2].ior;
+        }
 #endif
 
+        throughput *= exp(-sigma_a * gl_HitTEXT);
+    }
+
+    const vec3 iorCurrent = (!is_backface || thinWalled) ? mediumIor : vec3(BSDF_USE_MATERIAL_IOR);
+    const vec3 iorOther = (!is_backface || thinWalled) ? vec3(BSDF_USE_MATERIAL_IOR) : prevMediumIor;
+
     /* 4. Add Emission */
 #ifdef IS_EMISSIVE
     {
@@ -311,6 +322,8 @@ void main()
         rayPayload.ray_dir = bsdf_sample_data.k2;
     }
 
+    bool isTransmissionEvent = (eventType & BSDF_EVENT_TRANSMISSION) != 0;
+
     /* 6. NEE light sampling */
 #ifdef NEXT_EVENT_ESTIMATION
     if ((eventType & (BSDF_EVENT_DIFFUSE | BSDF_EVENT_GLOSSY)) != 0)
@@ -364,23 +377,55 @@ void main()
     }
 #endif
 
-    // Update rest of path state
-    if (eventType == BSDF_EVENT_ABSORB)
+    // Modify medium stack
+    if (!thinWalled && isTransmissionEvent)
     {
-        rayPayload.bitfield |= SHADE_RAY_PAYLOAD_TERMINATE_FLAG;
+#if MEDIUM_STACK_SIZE > 0
+        if (!is_backface) // push
+        {
+            mediumIdx++;
+
+            if (mediumIdx <= MEDIUM_STACK_SIZE)
+            {
+                vec3 sigma_a = mdl_volume_absorption_coefficient(shading_state);
+                vec3 sigma_s = mdl_volume_scattering_coefficient(shading_state);
+                vec3 sigma_t = sigma_a + sigma_s;
+
+                Medium m;
+                m.ior = mdl_ior(shading_state);
+                m.sigma_s = sigma_s;
+                m.sigma_t = sigma_t;
+                m.bias = MEDIUM_DIRECTIONAL_BIAS;
+                rayPayload.media[mediumIdx - 1] = m;
+
+                rayPayload.sigma_t = sigma_t;
+            }
+        }
+        else if (mediumIdx > 0) // pop
+        {
+            mediumIdx--;
+        }
+
+        shadeRayPayloadSetMediumIdx(rayPayload, mediumIdx);
+
+        // medium changes -- reset walk
+        rayPayload.bitfield &= ~SHADE_RAY_PAYLOAD_WALK_MASK;
+#else
+        mediumIdx = (1 - mediumIdx); // toggle between inside and outside
+#endif
+
+        shadeRayPayloadSetMediumIdx(rayPayload, mediumIdx);
     }
 
-    bool isTransmissionEvent = (eventType & BSDF_EVENT_TRANSMISSION) != 0;
-    if (isTransmissionEvent && !thinWalled)
+    // Update rest of path state
+    if (eventType == BSDF_EVENT_ABSORB)
     {
-        inside = !inside;
+        rayPayload.bitfield |= SHADE_RAY_PAYLOAD_TERMINATE_FLAG;
     }
 
     vec3 geomNormal = shading_state.geom_normal * (isTransmissionEvent ? -1.0 /* undo flip */ : 1.0);
     rayPayload.ray_origin = offset_ray_origin(shading_state.position, geomNormal);
 
-    shadeRayPayloadSetMediumIdx(rayPayload, inside ? 1 : 0);
-
     rayPayload.throughput = throughput;
     rayPayload.radiance = radiance;
 }
diff --git a/src/gi/shaders/rp_main_payload.glsl b/src/gi/shaders/rp_main_payload.glsl
@@ -7,6 +7,14 @@
 #define SHADE_RAY_PAYLOAD_WALK_OFFSET 12
 #define SHADE_RAY_PAYLOAD_BOUNCES_MASK 0x00000fffu
 
+struct Medium
+{
+    vec3 ior;
+    vec3 sigma_s;
+    vec3 sigma_t; // sigma_a + sigma_s
+    float bias;
+};
+
 struct ShadeRayPayload
 {
     /* inout */ vec3 throughput;
@@ -19,7 +27,13 @@ struct ShadeRayPayload
     /* inout */ uint bitfield;
 
     /* inout */ vec3 radiance;
+
     /* inout */ RNG_STATE_TYPE rng_state;
+
+#if MEDIUM_STACK_SIZE > 0
+    /* inout */ Medium media[MEDIUM_STACK_SIZE];
+#endif
+
     /* out */   vec3 ray_origin;
     /* out */   vec3 ray_dir;
     /* out */   vec3 neeToLight;