Add russian roulette to kill rays that don't contribute much to the scene.

chunky/src/java/se/llbit/chunky/renderer/scene/PathTracer.java

@@ -127,6 +127,19 @@ public static boolean pathTrace(Scene scene, Ray ray, WorkerState state,
         break;
       }
       ray.depth += 1;
+
+      // Russian Roulette
+      if (!firstReflection && ray.depth >= scene.minRayDepth) {
+        double max = FastMath.max(ray.color.x, FastMath.max(ray.color.y, ray.color.z));
+        double p = QuickMath.clamp(max, 0.05, 1.0);
+        if (random.nextDouble() > p) {
+          ray.color.set(0, 0, 0, 0);
+          break;
+        } else {
+          ray.color.scale(1 / p);
+        }
+      }
+
       Vector4 cumulativeColor = new Vector4(0, 0, 0, 0);
       Ray next = new Ray();
       float pMetal = currentMat.metalness;

chunky/src/java/se/llbit/chunky/renderer/scene/Scene.java

@@ -191,6 +191,10 @@ public class Scene implements JsonSerializable {
    * Recursive ray depth limit (not including Russian Roulette).
    */
   protected int rayDepth = PersistentSettings.getRayDepthDefault();
+  /**
+   * Minimum number of ray bounces before Russian Roulette starts.
+   */
+  protected int minRayDepth = PersistentSettings.getMinRayDepthDefault();
   protected String worldPath = "";
   protected int worldDimension = 0;
   protected RenderMode mode = RenderMode.PREVIEW;
@@ -1685,13 +1689,32 @@ public synchronized void setRayDepth(int value) {
     }
   }
 
+  /**
+   * Set the minimum recursive ray depth before Russian Roulette starts
+   */
+  public synchronized void setMinRayDepth(int value) {
+    value = Math.max(1, value);
+    if (minRayDepth != value) {
+      minRayDepth = value;
+      PersistentSettings.setMinRayDepth(minRayDepth);
+      refresh();
+    }
+  }
+
   /**
    * @return Recursive ray depth limit
    */
   public int getRayDepth() {
     return rayDepth;
   }
 
+  /**
+   * @return Minimum ray depth before Russion Roulette is applied
+   */
+  public int getMinRayDepth() {
+    return minRayDepth;
+  }
+
   /**
    * Clear the scene refresh flag
    */
@@ -1939,6 +1962,7 @@ public synchronized void copyTransients(Scene other) {
     sppTarget = other.sppTarget;
     branchCount = other.branchCount;
     rayDepth = other.rayDepth;
+    minRayDepth = other.minRayDepth;
     mode = other.mode;
     pictureExportFormat = other.pictureExportFormat;
     cameraPresets = other.cameraPresets;
@@ -2624,6 +2648,7 @@ public void setUseCustomWaterColor(boolean value) {
     json.add("sppTarget", sppTarget);
     json.add("branchCount", branchCount);
     json.add("rayDepth", rayDepth);
+    json.add("minRayDepth", minRayDepth);
     json.add("pathTrace", mode != RenderMode.PREVIEW);
     json.add("dumpFrequency", dumpFrequency);
     json.add("saveSnapshots", saveSnapshots);
@@ -2880,6 +2905,7 @@ public synchronized void importFromJson(JsonObject json) {
     sppTarget = json.get("sppTarget").intValue(sppTarget);
     branchCount = json.get("branchCount").intValue(branchCount);
     rayDepth = json.get("rayDepth").intValue(rayDepth);
+    minRayDepth = json.get("minRayDepth").intValue(minRayDepth);
     if (!json.get("pathTrace").isUnknown()) {
       boolean pathTrace = json.get("pathTrace").boolValue(false);
       if (pathTrace) {

chunky/src/java/se/llbit/chunky/ui/render/tabs/AdvancedTab.java

@@ -64,6 +64,7 @@ public class AdvancedTab extends ScrollPane implements RenderControlsTab, Initia
   @FXML private IntegerAdjuster renderThreads;
   @FXML private IntegerAdjuster cpuLoad;
   @FXML private IntegerAdjuster rayDepth;
+  @FXML private IntegerAdjuster minRayDepth;
   @FXML private IntegerAdjuster branchCount;
   @FXML private Button mergeRenderDump;
   @FXML private CheckBox shutdown;
@@ -104,10 +105,15 @@ public void initialize(URL location, ResourceBundle resources) {
       controller.getRenderManager().setCPULoad(value);
     });
     rayDepth.setName("Ray depth");
-    rayDepth.setTooltip("Sets the minimum recursive ray depth.");
+    rayDepth.setTooltip("Sets the maximum recursive ray depth.");
     rayDepth.setRange(1, 25);
     rayDepth.clampMin();
     rayDepth.onValueChange(value -> scene.setRayDepth(value));
+    minRayDepth.setName("Min ray depth");
+    minRayDepth.setTooltip("Sets the minimum recursive ray depth before Russian Roulette is applied.");
+    minRayDepth.setRange(1, 25);
+    minRayDepth.clampMin();
+    minRayDepth.onValueChange(value -> scene.setMinRayDepth(value));
 
     branchCount.setName("Branch count");
     branchCount.setTooltip("Sets the number of rays cast after the first intersection, effectively reusing the first ray that many times." +
@@ -339,6 +345,7 @@ public void update(Scene scene) {
     renderThreads.set(PersistentSettings.getNumThreads());
     cpuLoad.set(PersistentSettings.getCPULoad());
     rayDepth.set(scene.getRayDepth());
+    minRayDepth.set(scene.getMinRayDepth());
     branchCount.set(scene.getBranchCount());
     octreeImplementation.getSelectionModel().select(scene.getOctreeImplementation());
     bvhMethod.getSelectionModel().select(scene.getBvhImplementation());

chunky/src/res/se/llbit/chunky/ui/render/tabs/AdvancedTab.fxml

@@ -18,6 +18,7 @@
     <IntegerAdjuster fx:id="cpuLoad" />
     <Separator prefWidth="200.0" />
     <IntegerAdjuster fx:id="rayDepth" />
+    <IntegerAdjuster fx:id="minRayDepth" />
     <IntegerAdjuster fx:id="branchCount" />
     <Separator layoutX="20.0" layoutY="90.0" prefWidth="200.0" />
     <Button fx:id="mergeRenderDump" mnemonicParsing="false" text="Merge render dumps" />

lib/src/se/llbit/chunky/PersistentSettings.java

@@ -63,6 +63,7 @@ public final class PersistentSettings {
   public static final double DEFAULT_FOG_BLUE = 1;
 
   public static final int DEFAULT_RAY_DEPTH = 5;
+  public static final int DEFAULT_MIN_RAY_DEPTH = 3;
   public static final int DEFAULT_BRANCH_COUNT = 10;
   public static final int DEFAULT_SPP_TARGET = 1000;
 
@@ -272,13 +273,22 @@ public static void setRayDepth(int rayDepth) {
     save();
   }
 
+  public static void setMinRayDepth(int rayDepth) {
+    settings.setInt("minRayDepth", rayDepth);
+    save();
+  }
+
   /**
    * @return the default configured ray depth
    */
   public static int getRayDepthDefault() {
     return settings.getInt("rayDepth", DEFAULT_RAY_DEPTH);
   }
 
+  public static int getMinRayDepthDefault() {
+    return settings.getInt("minRayDepth", DEFAULT_MIN_RAY_DEPTH);
+  }
+
   public static int get3DCanvasHeight() {
     return settings.getInt("3dcanvas.height", DEFAULT_3D_CANVAS_HEIGHT);
   }