add a material that looks different on both sides

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

@@ -22,7 +22,7 @@ public record DielectricMaterial(double refractionIndex, @NotNull Texture textur
 
     @Override
     public @NotNull Optional<ScatterResult> scatter(@NotNull Ray ray, @NotNull HitResult hit, @NotNull RandomGenerator random) {
-        var ri = hit.frontFace() ? (1 / refractionIndex) : refractionIndex;
+        var ri = hit.isFrontFace() ? (1 / refractionIndex) : refractionIndex;
 
         var cosTheta = Math.min(- ray.direction().unit().times(hit.normal()), 1.0);
         var reflectance = reflectance(cosTheta);

src/main/java/eu/jonahbauer/raytracing/render/material/DirectionalMaterial.java

@@ -0,0 +1,67 @@
+package eu.jonahbauer.raytracing.render.material;
+
+import eu.jonahbauer.raytracing.math.Ray;
+import eu.jonahbauer.raytracing.math.Vec3;
+import eu.jonahbauer.raytracing.render.texture.Color;
+import eu.jonahbauer.raytracing.render.texture.Texture;
+import eu.jonahbauer.raytracing.scene.HitResult;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.Optional;
+import java.util.random.RandomGenerator;
+
+public final class DirectionalMaterial implements Material {
+    private final @Nullable Material front;
+    private final @Nullable Material back;
+
+    private final @NotNull Texture texture;
+
+    public DirectionalMaterial(@Nullable Material front, @Nullable Material back) {
+        this.front = front;
+        this.back = back;
+        this.texture = new DirectionalTexture(
+                front != null ? front.texture() : null,
+                back != null ? back.texture() : null
+        );
+    }
+
+    @Override
+    public @NotNull Texture texture() {
+        return texture;
+    }
+
+    @Override
+    public @NotNull Optional<ScatterResult> scatter(@NotNull Ray ray, @NotNull HitResult hit, @NotNull RandomGenerator random) {
+        if (hit.isFrontFace()) {
+            if (front != null) return front.scatter(ray, hit, random);
+        } else {
+            if (back != null) return back.scatter(ray, hit, random);
+        }
+        // let the ray pass through without obstruction
+        return Optional.of(new ScatterResult(new Ray(ray.at(hit.t()), ray.direction()), Color.WHITE));
+    }
+
+    @Override
+    public @NotNull Color emitted(@NotNull HitResult hit) {
+        if (hit.isFrontFace()) {
+            if (front != null) return front.emitted(hit);
+        } else {
+            if (back != null) return back.emitted(hit);
+        }
+        return Color.BLACK;
+    }
+
+    private record DirectionalTexture(@Nullable Texture front, @Nullable Texture back) implements Texture {
+
+        @Override
+        public @NotNull Color get(double u, double v, @NotNull Vec3 p) {
+            throw new UnsupportedOperationException();
+        }
+
+        @Override
+        public boolean isUVRequired() {
+            return front() != null && front().isUVRequired() || back() != null && back().isUVRequired();
+        }
+    }
+}

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

@@ -8,7 +8,7 @@ import java.util.Objects;
 
 public record HitResult(
         double t, @NotNull Vec3 position, @NotNull Vec3 normal,
-        @NotNull Material material, double u, double v, boolean frontFace
+        @NotNull Material material, double u, double v, boolean isFrontFace
 ) implements Comparable<HitResult> {
     public HitResult {
         Objects.requireNonNull(position, "position");
@@ -16,7 +16,7 @@ public record HitResult(
     }
 
     public @NotNull HitResult withPositionAndNormal(@NotNull Vec3 position, @NotNull Vec3 normal) {
-        return new HitResult(t, position, normal, material, u, v, frontFace);
+        return new HitResult(t, position, normal, material, u, v, isFrontFace);
     }
 
     @Override

src/main/java/eu/jonahbauer/raytracing/scene/hittable3d/ConstantMedium.java

@@ -32,7 +32,7 @@ public record ConstantMedium(@NotNull Hittable boundary, double density, @NotNul
         if (hitDistance > distance) return Optional.empty();
 
         var t = tmin + hitDistance / length;
-        return Optional.of(new HitResult(t, ray.at(t), Vec3.UNIT_X, material, 0, 0, true)); // arbitrary normal and frontFace
+        return Optional.of(new HitResult(t, ray.at(t), Vec3.UNIT_X, material, 0, 0, true)); // arbitrary normal, u, v and isFrontFace
     }
 
     @Override