make Vec3#random generate unit vectors by default

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

@@ -17,29 +17,33 @@ public record Vec3(double x, double y, double z) {
         assert Double.isFinite(x) && Double.isFinite(y) && Double.isFinite(z) : "x, y and z must be finite";
     }
 
-    /**
-     * {@return a uniformly random vector with components in the range [-1, 1)}
-     */
-    public static @NotNull Vec3 random(@NotNull RandomGenerator random) {
-        return new Vec3(
-                Math.fma(2, random.nextDouble(), -1),
-                Math.fma(2, random.nextDouble(), -1),
-                Math.fma(2, random.nextDouble(), -1)
-        );
-    }
-
     /**
      * {@return a uniformly random unit vector}
      */
-    public static @NotNull Vec3 random(@NotNull RandomGenerator random, boolean unit) {
-        if (!unit) return random(random);
-        Vec3 vec;
+    public static @NotNull Vec3 random(@NotNull RandomGenerator random) {
+        double x, y, z;
         double squared;
         do {
-            vec = random(random);
-            squared = vec.squared();
+            x = Math.fma(2, random.nextDouble(), -1);
+            y = Math.fma(2, random.nextDouble(), -1);
+            z = Math.fma(2, random.nextDouble(), -1);
+            squared = x * x + y * y + z * z;
         } while (squared > 1);
-        return vec.div(Math.sqrt(squared));
+        var factor = 1 / Math.sqrt(squared);
+        return new Vec3(x * factor, y * factor, z * factor);
+    }
+
+    public static @NotNull Vec3 randomOppositeHemisphere(@NotNull RandomGenerator random, @NotNull Vec3 direction) {
+        double x, y, z;
+        double squared;
+        do {
+            x = Math.fma(2, random.nextDouble(), -1);
+            y = Math.fma(2, random.nextDouble(), -1);
+            z = Math.fma(2, random.nextDouble(), -1);
+            squared = x * x + y * y + z * z;
+        } while (squared > 1 || direction.x() * x + direction.y() * y + direction.z() * z >= 0);
+        var factor = 1 / Math.sqrt(squared);
+        return new Vec3(x * factor, y * factor, z * factor);
     }
 
     public static @NotNull Vec3 reflect(@NotNull Vec3 vec, @NotNull Vec3 normal) {

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

@@ -25,7 +25,7 @@ public record MetallicMaterial(@NotNull Texture texture, double fuzz) implements
     public @NotNull Optional<ScatterResult> scatter(@NotNull Ray ray, @NotNull HitResult hit, @NotNull RandomGenerator random) {
         var newDirection = Vec3.reflect(ray.direction(), hit.normal());
         if (fuzz > 0) {
-            newDirection = newDirection.unit().plus(Vec3.random(random, true).times(fuzz));
+            newDirection = newDirection.unit().plus(Vec3.random(random).times(fuzz));
         }
         var attenuation = texture.get(hit);
         return Optional.of(new SpecularScatterResult(attenuation, new Ray(hit.position(), newDirection)));

src/main/java/eu/jonahbauer/raytracing/render/renderer/pdf/CosineProbabilityDensityFunction.java

@@ -21,7 +21,7 @@ public record CosineProbabilityDensityFunction(@NotNull Vec3 normal) implements
 
     @Override
     public @NotNull Vec3 generate(@NotNull RandomGenerator random) {
-        var out = normal().plus(Vec3.random(random, true));
+        var out = normal().plus(Vec3.random(random));
         return out.isNearZero() ? normal() : out;
     }
 }

src/main/java/eu/jonahbauer/raytracing/render/renderer/pdf/SphereProbabilityDensityFunction.java

@@ -14,6 +14,6 @@ public record SphereProbabilityDensityFunction() implements ProbabilityDensityFu
 
     @Override
     public @NotNull Vec3 generate(@NotNull RandomGenerator random) {
-        return Vec3.random(random, true);
+        return Vec3.random(random);
     }
 }

src/main/java/eu/jonahbauer/raytracing/render/texture/PerlinTexture.java

@@ -53,7 +53,7 @@ public final class PerlinTexture implements Texture {
         this.mask = count - 1;
         this.randvec = new Vec3[count];
         for (int i = 0; i < count; i++) {
-            this.randvec[i] = Vec3.random(random, true);
+            this.randvec[i] = Vec3.random(random);
         }
         this.permX = generatePerm(count, random);
         this.permY = generatePerm(count, random);

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

@@ -143,7 +143,7 @@ public final class Box implements Hittable, Target {
 
     @Override
     public @NotNull Vec3 getTargetingDirection(@NotNull Vec3 origin, @NotNull RandomGenerator random) {
-        if (contains(origin)) return Vec3.random(random, true);
+        if (contains(origin)) return Vec3.random(random);
 
         // determine sides facing the origin and their solid angles
         int visible = 0;

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

@@ -89,13 +89,12 @@ public final class Sphere implements Hittable, Target {
     @Override
     public @NotNull Vec3 getTargetingDirection(@NotNull Vec3 origin, @NotNull RandomGenerator random) {
         var direction = center.minus(origin);
-
-        Vec3 target;
-        do {
-            target = Vec3.random(random, true);
-        } while (target.times(direction) >= 0);
-
-        return target.times(radius).plus(center).minus(origin);
+        var out = Vec3.randomOppositeHemisphere(random, direction);
+        return new Vec3(
+                Math.fma(radius, out.x(), center.x() - origin.x()),
+                Math.fma(radius, out.y(), center.y() - origin.y()),
+                Math.fma(radius, out.z(), center.z() - origin.z())
+        );
     }
 
     @Override