add diffuse material

src/main/java/eu/jonahbauer/raytracing/Main.java

@@ -14,7 +14,7 @@ public class Main {
                 new Sphere(0, 0, 1, 0.5),
                 new Sphere(0, -100.5, 1, 100)
         );
-        var camera = new Camera(256, 2, 16 / 9d);
+        var camera = new Camera(512, 2, 16 / 9d);
 
         var image = camera.render(scene);
         ImageIO.write(image, Path.of("scene-" + System.currentTimeMillis() + ".ppm"));

src/main/java/eu/jonahbauer/raytracing/math/Vec3.java

@@ -13,7 +13,20 @@ public record Vec3(double x, double y, double z) {
             throw new IllegalArgumentException("x, y and z must be finite");
         }
     }
-    
+
+    public static @NotNull Vec3 random() {
+        return random(false);
+    }
+
+    public static @NotNull Vec3 random(boolean unit) {
+        var random = new Vec3(
+                2 * Math.random() - 1,
+                2 * Math.random() - 1,
+                2 * Math.random() - 1
+        );
+        return unit ? random.unit() : random;
+    }
+
     public @NotNull Vec3 plus(@NotNull Vec3 b) {
         return new Vec3(this.x + b.x, this.y + b.y, this.z + b.z);
     }

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

@@ -23,6 +23,7 @@ public final class Camera {
 
     // antialiasing
     private final int samplesPerPixel = 100;
+    private final int maxDepth = 10;
 
     // internal properties
     private final @NotNull Vec3 pixelU;
@@ -123,10 +124,23 @@ public final class Camera {
     }
 
     private @NotNull Color getColor(@NotNull Scene scene, @NotNull Ray ray) {
-        var result = scene.hit(ray, Range.NON_NEGATIVE);
-        if (result.isPresent()) {
-            var normal = result.get().normal();
-            return getNormalColor(normal);
+        return getColor(scene, ray, maxDepth);
+    }
+
+    private @NotNull Color getColor(@NotNull Scene scene, @NotNull Ray ray, int depth) {
+        if (depth <= 0) return Color.BLACK;
+
+        var optional = scene.hit(ray, Range.NON_NEGATIVE);
+        if (optional.isPresent()) {
+            var result = optional.get();
+
+            var newDirection = Vec3.random(true);
+            if (result.normal().times(newDirection) < 0) {
+                newDirection = newDirection.times(-1);
+            }
+            var scattered = new Ray(result.position(), newDirection);
+
+            return Color.lerp(Color.BLACK, getColor(scene, scattered, depth - 1), 0.5);
         } else {
             return getSkyboxColor(ray);
         }

src/main/java/eu/jonahbauer/raytracing/scene/HitResult.java

@@ -3,9 +3,12 @@ package eu.jonahbauer.raytracing.scene;
 import eu.jonahbauer.raytracing.math.Vec3;
 import org.jetbrains.annotations.NotNull;
 
-public record HitResult(double t, @NotNull Vec3 normal) implements Comparable<HitResult> {
+import java.util.Objects;
+
+public record HitResult(double t, @NotNull Vec3 position, @NotNull Vec3 normal) implements Comparable<HitResult> {
     public HitResult {
         if (t < 0 || !Double.isFinite(t)) throw new IllegalArgumentException("t must be non-negative");
+        Objects.requireNonNull(position, "position");
         normal = normal.unit();
     }
 

src/main/java/eu/jonahbauer/raytracing/scene/Sphere.java

@@ -36,7 +36,9 @@ public record Sphere(@NotNull Vec3 center, double radius) implements Hittable {
         double t = (- h - sd) / a;
         if (!range.surrounds(t)) t = (- h + sd) / a;
         if (!range.surrounds(t)) return Optional.empty();
-        return Optional.of(new HitResult(t, ray.at(t).minus(center)));
+
+        var position = ray.at(t);
+        return Optional.of(new HitResult(t, position, position.minus(center)));
     }
 
     public @NotNull Sphere withCenter(@NotNull Vec3 center) {