Implement path tracing material

See https://github.com/SebLague/Ray-Tracing/blob/main/Assets/Scripts/Shaders/RayTracer.shader#L328

src/main/kotlin/Ray.kt

@@ -1,5 +1,6 @@
 import lights.PointLight
 import materials.MaterialColor
+import materials.PathTracedMaterial
 import materials.ReflectiveMaterial
 import materials.WhateverMaterial
 import materials.times
@@ -15,19 +16,46 @@ fun main() {
     val bmp = Bitmap(1000, 1000)
     val origin = Point(500f, 500f, -500f)
 
-    val sphere = Sphere(Point(500f, 500f, 500f), radius = 500f, material = ReflectiveMaterial(reflectivity = 0.2f, MaterialColor(0f, 1.0f, 0f)))
-    val sphere2 = Sphere(Point(0f, 500f, 50f), radius = 50f, material = ReflectiveMaterial(reflectivity = 0.2f, MaterialColor(1.0f, 0.0f, 0.1f)))
+    val material1 = PathTracedMaterial(
+        MaterialColor(1f, 0f, 0f),
+        specularProbability = 0.1f,
+        specularColour = MaterialColor.White,
+        smoothness = 1.0f,
+    )
 
+    val materialFloor = PathTracedMaterial(
+        MaterialColor(1f, 0f, 1f),
+        specularProbability = 0.1f,
+        specularColour = MaterialColor.White,
+        smoothness = 1.0f,
+    )
+
+    val lightMaterial = PathTracedMaterial(
+        color = MaterialColor.Black,
+        emissionColor = MaterialColor.White,
+        emissionStrength = 0.2f,
+        specularColour = MaterialColor.White
+    )
+
+    val material2 = ReflectiveMaterial(reflectivity = 0.2f, MaterialColor(0f, 1.0f, 0f))
+    val material3 = ReflectiveMaterial(reflectivity = 0.2f, MaterialColor(1.0f, 0.0f, 0.1f))
+
+    val sphere = Sphere(Point(500f, 500f, 500f), radius = 500f, material = material1)
+    val sphere2 = Sphere(Point(0f, 500f, 50f), radius = 50f, material = material1)
+    val sphere3 = Sphere(Point(1000f, 1000f, -500f), radius = 100f, material = lightMaterial)
+
+    val material4 = WhateverMaterial(MaterialColor(1f, 0.0f, 0.0f))
     val plane = Plane(
         Point(0f, -1000f, 0f),
         normal = Vector(0f, 1f, 0f),
-        material = WhateverMaterial(MaterialColor(1f, 0.0f, 0.0f))
+        material = materialFloor
     )
 
+    val material5 = WhateverMaterial(MaterialColor(0.0f, 1f, 0.0f))
     val leftPlane = Plane(
         Point(-1000f, 0f, 0f),
         Vector(1f, 0f, 0f),
-        material = WhateverMaterial(MaterialColor(0.0f, 1f, 0.0f))
+        material = materialFloor
     )
 
     val light = PointLight(Point(1000f, 1000f, -500f), color = 0.2f * MaterialColor(0.9f, 0.9f, 0.9f))
@@ -36,40 +64,30 @@ fun main() {
         sphere,
         sphere2,
         plane,
-        leftPlane
-    ), light = light)
+        leftPlane,
+        sphere3
+    ), light = light, ambientLight = MaterialColor.White * 0.1f)
 
 
     for (y in 0 until bmp.height) {
         for (x in 0 until bmp.width) {
-            var r = 0f
-            var g = 0f
-            var b = 0f
+            var color = MaterialColor.Black
 
             repeat(samplesPerPixel) {
                 val point = Point(x.toFloat(), y.toFloat(), 0f) + Vector.randomInCircleXY() * blurriness
                 val ray = origin.rayTo(point)
 
-                val color = scene.colorForRay(ray)
-                if (color != null) {
-                    r += color.r
-                    g += color.g
-                    b += color.b
-                }
+                color += scene.colorForRay(ray)
             }
 
             bmp.setPixel(
                 x, y,
-                MaterialColor(
-                    r = r / samplesPerPixel,
-                    g = g / samplesPerPixel,
-                    b = b / samplesPerPixel
-                ).toColor()
+                (color * (1.0f / samplesPerPixel)).toColor()
             )
         }
     }
     File("test.tga").writeBitmap(bmp)
 }
 
-const val samplesPerPixel = 5
+const val samplesPerPixel = 64
 const val blurriness = 1f

src/main/kotlin/materials/MaterialColor.kt

@@ -11,9 +11,11 @@ data class MaterialColor(val r: Float, val g: Float, val b: Float) {
 
     operator fun times(rhs: MaterialColor) = MaterialColor(r * rhs.r, g * rhs.g, b * rhs.b)
     operator fun plus(rhs: MaterialColor) = MaterialColor(r + rhs.r, g + rhs.g, b + rhs.b)
+    operator fun times(rhs: Float) = MaterialColor(rhs * r, rhs * g, rhs * b)
 
     companion object {
         val Black: MaterialColor = MaterialColor(0f, 0f, 0f)
+        val White = MaterialColor(1f, 1f, 1f)
     }
 }
 

src/main/kotlin/materials/PathTracedMaterial.kt

@@ -0,0 +1,85 @@
+package materials
+
+import Hit
+import math.Ray
+import math.Vector
+import things.Scene
+import kotlin.math.cos
+import kotlin.math.ln
+import kotlin.math.log
+import kotlin.math.sqrt
+import kotlin.random.Random
+
+data class PathTracedMaterial(
+    val color: MaterialColor,
+    val emissionColor: MaterialColor = MaterialColor(0f, 0f, 0f),
+    val specularProbability: Float = 0f,
+    val smoothness: Float = 0.5f,
+    val emissionStrength: Float = 0f,
+    val specularColour: MaterialColor,
+) : Material {
+    override fun shade(ray: Ray, hit: Hit, scene: Scene): MaterialColor {
+        // source: https://github.com/SebLague/Ray-Tracing/blob/main/Assets/Scripts/Shaders/RayTracer.shader#L328
+
+        var incomingLight = MaterialColor.Black
+        var rayColor = MaterialColor.White
+
+        var hit: Hit? = hit
+        var ray = ray
+
+        repeat(maxBounces) {
+            if (hit != null && hit.material is PathTracedMaterial) {
+                val isSpecular = hit.material.specularProbability >= Random.Default.nextFloat()
+
+                val diffuseDir = (hit.normal + Random.Default.direction()).normalized()
+                val specularDir = reflect(ray.direction, hit.normal)
+
+                val rayDir = if (isSpecular) {
+                    lerp(diffuseDir, specularDir, hit.material.smoothness).normalized()
+                } else {
+                    diffuseDir
+                }
+
+                val emittedLight = hit.material.emissionColor * hit.material.emissionStrength;
+                incomingLight += emittedLight * rayColor;
+                rayColor *= if (isSpecular) hit.material.specularColour else hit.material.color
+
+                val p = maxOf(rayColor.r, rayColor.g, rayColor.b)
+                if (Random.Default.nextFloat() >= p) {
+                    return@repeat
+                }
+
+                rayColor *= 1.0f / p;
+
+                ray = Ray(hit.point, rayDir)
+
+            } else if (hit != null) {
+                incomingLight += hit.material.shade(ray, hit, scene) * rayColor
+                return@repeat
+            } else {
+                incomingLight += scene.ambientLight * rayColor
+                return@repeat
+            }
+
+            hit = scene.intersects(ray)
+        }
+
+        return incomingLight
+    }
+
+}
+
+const val maxBounces = 4
+
+fun Random.normalDistributedFloat(): Float {
+    val theta = nextFloat() * 2 * Math.PI.toFloat()
+    val rho = sqrt(-2 * ln(nextFloat()))
+    return rho * cos(theta)
+}
+
+fun Random.direction(): Vector =
+    Vector(normalDistributedFloat(), normalDistributedFloat(), normalDistributedFloat()).normalized()
+
+fun lerp(a: Float, b: Float, c: Float): Float = a * (1 - c) + b * c
+fun lerp(a: MaterialColor, b: MaterialColor, c: Float): MaterialColor = a * (1 - c) + b * c
+fun lerp(a: Vector, b: Vector, c: Float): Vector = a * (1 - c) + b * c

src/main/kotlin/materials/ReflectiveMaterial.kt

@@ -2,6 +2,7 @@ package materials
 
 import Hit
 import math.Ray
+import math.Vector
 import things.Scene
 import math.times
 import kotlin.math.pow
@@ -29,9 +30,14 @@ data class ReflectiveMaterial(
             color += intensity * MaterialColor(0.3f, 0f, 0f)
         }
 
-        val reflectionDirection = ray.direction - 2 * (ray.direction dot hit.normal) * hit.normal
-        val reflectedColor = scene.colorForRay(Ray(offsetPoint, reflectionDirection)) ?: MaterialColor.Black
+        val reflectionDirection = reflect(ray.direction, hit.normal)
+        val reflectedColor = scene.colorForRay(Ray(offsetPoint, reflectionDirection))
 
         return  (1.0f - reflectivity) * color + reflectivity * reflectedColor
     }
-}
+}
+
+fun reflect(
+    rayDirection: Vector,
+    normal: Vector
+): Vector = rayDirection - 2 * (rayDirection dot normal) * normal

src/main/kotlin/things/Scene.kt

@@ -3,9 +3,10 @@ package things
 import Hit
 import materials.MaterialColor
 import lights.PointLight
+import materials.Material
 import math.Ray
 
-data class Scene(val things: List<Thing>, val light: PointLight): Thing {
+data class Scene(val things: List<Thing>, val light: PointLight, val ambientLight: MaterialColor = MaterialColor.Black): Thing {
     override fun intersects(ray: Ray): Hit? {
         var closest: Hit? = null
         for (thing in things) {
@@ -18,11 +19,11 @@ data class Scene(val things: List<Thing>, val light: PointLight): Thing {
         return closest
     }
 
-    fun colorForRay(ray: Ray): MaterialColor? {
+    fun colorForRay(ray: Ray): MaterialColor {
         val hit = intersects(ray)
         if (hit != null) {
             return hit.material.shade(ray, hit, this)
         }
-        return null
+        return ambientLight
     }
 }