add dielectric material

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

@@ -1,5 +1,6 @@
 package eu.jonahbauer.raytracing;
 
+import eu.jonahbauer.raytracing.material.DielectricMaterial;
 import eu.jonahbauer.raytracing.material.LambertianMaterial;
 import eu.jonahbauer.raytracing.material.MetallicMaterial;
 import eu.jonahbauer.raytracing.render.Camera;
@@ -16,7 +17,7 @@ public class Main {
         var scene = new Scene(
                 new Sphere(0, -100.5, 1, 100, new LambertianMaterial(new Color(0.8, 0.8, 0.0))),
                 new Sphere(0, 0, 1.2, 0.5, new LambertianMaterial(new Color(0.1, 0.2, 0.5))),
-                new Sphere(-1, 0, 1, 0.5, new MetallicMaterial(new Color(0.8, 0.8, 0.8), 0.3)),
+                new Sphere(-1, 0, 1, 0.5, new DielectricMaterial(1.5)),
                 new Sphere(1, 0, 1, 0.5, new MetallicMaterial(new Color(0.8, 0.6, 0.2), 1.0))
         );
         var camera = new Camera(512, 2, 16 / 9d);

src/main/java/eu/jonahbauer/raytracing/material/DielectricMaterial.java

@@ -0,0 +1,18 @@
+package eu.jonahbauer.raytracing.material;
+
+import eu.jonahbauer.raytracing.math.Ray;
+import eu.jonahbauer.raytracing.math.Vec3;
+import eu.jonahbauer.raytracing.render.Color;
+import eu.jonahbauer.raytracing.scene.HitResult;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.Optional;
+
+public record DielectricMaterial(double refractionIndex) implements Material {
+    @Override
+    public @NotNull Optional<ScatterResult> scatter(@NotNull Ray ray, @NotNull HitResult hit) {
+        var ri = hit.frontFace() ? (1 / refractionIndex) : refractionIndex;
+        var refracted = Vec3.refract(ray.direction(), hit.normal(), ri);
+        return Optional.of(new ScatterResult(new Ray(hit.position(), refracted), Color.WHITE));
+    }
+}

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

@@ -31,6 +31,14 @@ public record Vec3(double x, double y, double z) {
         return vec.minus(normal.times(2 * normal.times(vec)));
     }
 
+    public static @NotNull Vec3 refract(@NotNull Vec3 vec, @NotNull Vec3 normal, double index) {
+        vec = vec.unit();
+        var cosTheta = Math.min(- vec.times(normal), 1.0);
+        var rOutPerp = vec.plus(normal.times(cosTheta)).times(index);
+        var rOutParallel = normal.times(- Math.sqrt(Math.abs(1 - rOutPerp.squared())));
+        return rOutPerp.plus(rOutParallel);
+    }
+
     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/scene/HitResult.java

@@ -10,7 +10,8 @@ public record HitResult(
         double t,
         @NotNull Vec3 position,
         @NotNull Vec3 normal,
-        @NotNull Material material
+        @NotNull Material material,
+        boolean frontFace
 ) implements Comparable<HitResult> {
     public HitResult {
         if (t < 0 || !Double.isFinite(t)) throw new IllegalArgumentException("t must be non-negative");

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

@@ -39,7 +39,9 @@ public record Sphere(@NotNull Vec3 center, double radius, @NotNull Material mate
         if (!range.surrounds(t)) return Optional.empty();
 
         var position = ray.at(t);
-        return Optional.of(new HitResult(t, position, position.minus(center), material));
+        var normal = position.minus(center);
+        var frontFace = normal.times(ray.direction()) < 0;
+        return Optional.of(new HitResult(t, position, frontFace ? normal : normal.times(-1), material, frontFace));
     }
 
     public @NotNull Sphere withCenter(@NotNull Vec3 center) {