add stratification to pixel sampling

src/main/java/eu/jonahbauer/raytracing/render/camera/Camera.java

@@ -18,7 +18,13 @@ public interface Camera {
 
     /**
      * Casts a ray through the given pixel.
+     * @param x the pixel x coordinate
+     * @param y the pixel y coordinate
+     * @param i the subpixel x coordinate
+     * @param j the subpixel y coordinate
+     * @param n the subpixel count (per side)
+     * @param random a random number generator
      * @return a new ray
      */
-    @NotNull Ray cast(int x, int y, @NotNull RandomGenerator random);
+    @NotNull Ray cast(int x, int y, int i, int j, int n, @NotNull RandomGenerator random);
 }

src/main/java/eu/jonahbauer/raytracing/render/camera/SimpleCamera.java

@@ -79,13 +79,20 @@ public final class SimpleCamera implements Camera {
 
     /**
      * {@inheritDoc}
+     * @param x {@inheritDoc}
+     * @param y {@inheritDoc}
+     * @param i {@inheritDoc}
+     * @param j {@inheritDoc}
+     * @param n {@inheritDoc}
+     * @param random {@inheritDoc}
      */
-    public @NotNull Ray cast(int x, int y, @NotNull RandomGenerator random) {
+    @Override
+    public @NotNull Ray cast(int x, int y, int i, int j, int n, @NotNull RandomGenerator random) {
         Objects.checkIndex(x, width);
         Objects.checkIndex(y, height);
 
         var origin = getRayOrigin(random);
-        var target = getRayTarget(x, y, random);
+        var target = getRayTarget(x, y, i, j, n, random);
         return new Ray(origin, target.minus(origin));
     }
 
@@ -98,8 +105,8 @@ public final class SimpleCamera implements Camera {
         if (blurRadius <= 0) return origin;
 
         while (true) {
-            var du = 2 * random.nextDouble() - 1;
+            var du = Math.fma(2, random.nextDouble(), -1);
-            var dv = 2 * random.nextDouble() - 1;
+            var dv = Math.fma(2, random.nextDouble(), -1);
             if (du * du + dv * dv >= 1) continue;
 
             var ru = blurRadius * du;
@@ -116,9 +123,10 @@ public final class SimpleCamera implements Camera {
     /**
      * {@return the target vector for a ray through the given pixel} The position is randomized within the pixel.
      */
-    private @NotNull Vec3 getRayTarget(int x, int y, @NotNull RandomGenerator random) {
+    private @NotNull Vec3 getRayTarget(int x, int y, int i, int j, int n, @NotNull RandomGenerator random) {
-        double dx = x + random.nextDouble() - 0.5;
+        var factor = 1d / n;
-        double dy = y + random.nextDouble() - 0.5;
+        var dx = x + Math.fma(factor, i + random.nextDouble(), -0.5);
+        var dy = y + Math.fma(factor, j + random.nextDouble(), -0.5);
         return pixel00.plus(pixelU.times(dx)).plus(pixelV.times(dy));
     }
 

src/main/java/eu/jonahbauer/raytracing/render/renderer/SimpleRenderer.java

@@ -14,7 +14,7 @@ import java.util.random.RandomGenerator;
 import java.util.stream.IntStream;
 
 public final class SimpleRenderer implements Renderer {
-    private final int samplesPerPixel;
+    private final int sqrtSamplesPerPixel;
     private final int maxDepth;
     private final double gamma;
 
@@ -30,7 +30,7 @@ public final class SimpleRenderer implements Renderer {
     }
 
     private SimpleRenderer(@NotNull Builder builder) {
-        this.samplesPerPixel = builder.samplesPerPixel;
+        this.sqrtSamplesPerPixel = (int) Math.sqrt(builder.samplesPerPixel);
         this.maxDepth = builder.maxDepth;
         this.gamma = builder.gamma;
 
@@ -38,6 +38,9 @@ public final class SimpleRenderer implements Renderer {
         this.iterative = builder.iterative;
     }
 
+    /**
+     * {@inheritDoc}
+     */
     @Override
     public void render(@NotNull Camera camera, @NotNull Scene scene, @NotNull Canvas canvas) {
         if (canvas.getWidth() != camera.getWidth() || canvas.getHeight() != camera.getHeight()) {
@@ -45,19 +48,35 @@ public final class SimpleRenderer implements Renderer {
         }
 
         if (iterative) {
+            renderIterative(camera, scene, canvas);
+        } else {
+            renderNonIterative(camera, scene, canvas);
+        }
+    }
+
+    /**
+     * Renders the {@code scene} as seen by the {@code camera} to the {@code canvas}, taking one sample per pixel at
+     * a time and updating the canvas after each sample.
+     */
+    private void renderIterative(@NotNull Camera camera, @NotNull Scene scene, @NotNull Canvas canvas) {
         var random = new Random();
 
         // render one sample after the other
-            for (int i = 1 ; i <= samplesPerPixel; i++) {
+        int i = 0;
-                var sample = i;
+        for (int sj = 0; sj < sqrtSamplesPerPixel; sj++) {
+            for (int si = 0; si < sqrtSamplesPerPixel; si++) {
+                var sample = ++i;
+                var sif = si;
+                var sjf = sj;
                 getScanlineStream(camera.getHeight(), parallel).forEach(y -> {
                     for (int x = 0; x < camera.getWidth(); x++) {
-                        var ray = camera.cast(x, y, random);
+                        var ray = camera.cast(x, y, sif, sjf, sqrtSamplesPerPixel, random);
                         var c = getColor(scene, ray, random);
                         canvas.set(x, y, Color.average(canvas.get(x, y), c, sample));
                     }
                 });
             }
+        }
 
         // apply gamma correction
         getScanlineStream(camera.getHeight(), parallel).forEach(y -> {
@@ -65,23 +84,31 @@ public final class SimpleRenderer implements Renderer {
                 canvas.set(x, y, Color.gamma(canvas.get(x, y), gamma));
             }
         });
-        } else {
+    }
+
+    /**
+     * Renders the {@code scene} as seen by the {@code camera} to the {@code canvas}, taking some amount of samples
+     * per pixel and updating the canvas after each pixel.
+     */
+    private void renderNonIterative(@NotNull Camera camera, @NotNull Scene scene, @NotNull Canvas canvas) {
         var splittable = new SplittableRandom();
         // render one pixel after the other
         getScanlineStream(camera.getHeight(), parallel).forEach(y -> {
             var random = splittable.split();
             for (int x = 0; x < camera.getWidth(); x++) {
                 var color = Color.BLACK;
-                    for (int i = 1; i <= samplesPerPixel; i++) {
+                int i = 0;
-                        var ray = camera.cast(x, y, random);
+                for (int sj = 0; sj < sqrtSamplesPerPixel; sj++) {
+                    for (int si = 0; si < sqrtSamplesPerPixel; si++) {
+                        var ray = camera.cast(x, y, si, sj, sqrtSamplesPerPixel, random);
                         var c = getColor(scene, ray, random);
-                        color = Color.average(color, c, i);
+                        color = Color.average(color, c, ++i);
+                    }
                 }
                 canvas.set(x, y, Color.gamma(color, gamma));
             }
         });
     }
-    }
 
     /**
      * {@return the color of the given ray in the given scene}