fixed refraction direction generation

src/main/java/basics/math/algebra/Vector.java

@@ -100,14 +100,31 @@ public class Vector {
         return this.sub(normal.scale(this.dot(normal) * 2.0));
     }
 
+    // based on https://bheisler.github.io/post/writing-raytracer-in-rust-part-3/
     public Vector refract(Vector normal, Vector incoming, double ior) {
-        double len2 = normal.dot(incoming);
-        double k = 1.0 - ior * ior * (1.0 - len2 * len2);
 
-        if (k < 0)
+        double iDotN = incoming.dot(normal);
+        double etaI = 1;
+        double etaT = ior;
+
+        Vector refraction_n = normal;
+
+        if (iDotN < 0) {
+            iDotN = -iDotN;
+        } else {
+            refraction_n = normal.negate();
+            etaT = 1.0;
+            etaI = ior;
+        }
+
+        double eta = etaI / etaT;
+        double k = 1.0 - (eta * eta) * (1.0 - iDotN * iDotN);
+
+        if (k < 0.0) {
             return  Vector.origin();
-
+        } else {
-        return incoming.scale(ior).sub(normal.scale(ior * len2 + Math.sqrt(k)));
+            return incoming.add(refraction_n.scale(iDotN)).scale(eta).sub(refraction_n).scale(Math.sqrt(k));
+        }
     }
 
     public Vector lerp(Vector other, double k) {

src/main/java/geometry/scene/Scene.java

@@ -32,13 +32,14 @@ public class Scene {
     public static Scene generateExampleScene() {
         Scene scene = new Scene(new LinearList());
 
-        BasicMaterial mirror = new BasicMaterial(Color.BLACK, 1.0, Color.WHITE, true);
+        BasicMaterial glass = new BasicMaterial(Color.BLACK, 1.0, Color.WHITE, true);
+        BasicMaterial mirror = new BasicMaterial(Color.BLACK, 1.0, Color.WHITE, false);
         BasicMaterial white  = new BasicMaterial(new Color(0.8, 0.8, 0.8), 0.1, new Color(0.8, 0.8, 0.8), false);
         BasicMaterial red    = new BasicMaterial(new Color(0.8, 0.0, 0.0), 0.1, new Color(0.8, 0.8, 0.8), false);
         BasicMaterial green  = new BasicMaterial(new Color(0.0, 0.8, 0.0), 0.1, new Color(0.8, 0.8, 0.8), false);
         BasicMaterial blue   = new BasicMaterial(new Color(0.0, 0.0, 0.8), 0.1, new Color(0.8, 0.8, 0.8), false);
 
-        scene.addMesh(new BasicMesh(mirror, new Sphere(new Vector(0,1.5,0), 1.0)));
+        scene.addMesh(new BasicMesh(glass, new Sphere(new Vector(0,0,0), 1.0)));
         scene.addMesh(new BasicMesh(white, new Sphere(new Vector(3,2,0), 0.25)));
         scene.addMesh(new BasicMesh(red, new Sphere(new Vector(2.5,2,-3), 0.33)));
 
@@ -49,7 +50,7 @@ public class Scene {
         scene.addMesh(new BasicMesh(green, new Plane(new Vector(1, 0, 0), 4)));
 
         scene.addMesh(new BasicMesh(blue, new Plane(new Vector(0, 0, 1), 5)));
-        scene.addMesh(new BasicMesh(white, new Plane(new Vector(0, 0, 1), -4)));
+        scene.addMesh(new BasicMesh(mirror, new Plane(new Vector(0, 0, 1), -4)));
 
         scene.addLight(new Point(Color.WHITE.scale(0.5), new Vector(0, 4, 0)));
         scene.addLight(new Point(Color.WHITE.scale(0.5), new Vector(1, 3, 1)));