added particles part 3

README.md

@@ -25,10 +25,11 @@ Repository to accompany the following blog posts:
 - [Shadows in Metal part 1](http://metalkit.org/2017/01/31/shadows-in-metal-part-1.html)
 - [Shadows in Metal part 2](http://metalkit.org/2017/02/28/shadows-in-metal-part-2.html)
 - [Ambient Occlusion in Metal](http://metalkit.org/2017/03/22/ambient-occlusion-in-metal.html)
-- [Working with memory in Metal](http://metalkit.org/2017/04/30/working-with-memory-in-metal.html)
+- [Working with memory in Metal part 1](http://metalkit.org/2017/04/30/working-with-memory-in-metal.html)
 - [Working with memory in Metal part 2](http://metalkit.org/2017/05/26/working-with-memory-in-metal-part-2.html)
 - [Introducing Metal 2](http://metalkit.org/2017/06/30/introducing-metal-2.html)
-- [Using ARKit with Metal](http://metalkit.org/2017/07/29/using-arkit-with-metal.html)
+- [Using ARKit with Metal part 1](http://metalkit.org/2017/07/29/using-arkit-with-metal.html)
 - [Using ARKit with Metal part 2](http://metalkit.org/2017/08/31/using-arkit-with-metal-part-2.html)
-- [Working with Particles in Metal](http://metalkit.org/2017/09/30/working-with-particles-in-metal.html)
+- [Working with Particles in Metal part 1](http://metalkit.org/2017/09/30/working-with-particles-in-metal.html)
 - [Working with Particles in Metal part 2](http://metalkit.org/2017/10/31/working-with-particles-in-metal-part-2.html)
+- [Working with Particles in Metal part 3](http://metalkit.org/2017/11/30/working-with-particles-in-metal-part-3.html)

particles/particle3.playground/Contents.swift

@@ -0,0 +1,9 @@
+
+import MetalKit
+import PlaygroundSupport
+
+let frame = NSRect(x: 0, y: 0, width: 600, height: 600)
+let mView = MetalView()
+let view = MTKView(frame: frame, device: mView.device)
+view.delegate = mView
+PlaygroundPage.current.liveView = view

particles/particle3.playground/Resources/Shaders.metal

@@ -0,0 +1,35 @@
+
+#include <metal_stdlib>
+using namespace metal;
+
+struct Particle {
+    float2 position;
+    float2 velocity;
+};
+
+kernel void firstPass(texture2d<half, access::write> output [[texture(0)]],
+                      uint2 id [[thread_position_in_grid]]) {
+    output.write(half4(0., 0., 0., 1.), id);
+}
+
+kernel void secondPass(texture2d<half, access::write> output [[texture(0)]],
+                       device Particle *particles [[buffer(0)]],
+                       uint id [[thread_position_in_grid]]) {
+    Particle particle = particles[id];
+    float2 position = particle.position;
+    float2 velocity = particle.velocity;
+    int width = output.get_width();
+    int height = output.get_height();
+    if (position.x < 0 || position.x > width) { velocity.x *= -1; }
+    if (position.y < 0 || position.y > height) { velocity.y *= -1; }
+    position += velocity;
+    particle.position = position;
+    particle.velocity = velocity;
+    particles[id] = particle;
+    uint2 pos = uint2(position.x, position.y);
+    output.write(half4(1.), pos);
+    output.write(half4(1.), pos + uint2( 1, 0));
+    output.write(half4(1.), pos + uint2( 0, 1));
+    output.write(half4(1.), pos - uint2( 1, 0));
+    output.write(half4(1.), pos - uint2( 0, 1));
+}

particles/particle3.playground/Sources/MetalView.swift

@@ -0,0 +1,74 @@
+
+import MetalKit
+
+struct Particle {
+    var position: float2
+    var velocity: float2
+}
+
+public class MetalView: NSObject, MTKViewDelegate {
+
+    public var device: MTLDevice!
+    var queue: MTLCommandQueue!
+    var firstState: MTLComputePipelineState!
+    var secondState: MTLComputePipelineState!
+    var particleBuffer: MTLBuffer!
+    let particleCount = 10000
+    var particles = [Particle]()
+    let side = 1200
+
+    override public init() {
+        super.init()
+        initializeMetal()
+        initializeBuffers()
+    }
+
+    func initializeBuffers() {
+        for _ in 0 ..< particleCount {
+            let particle = Particle(position: float2(Float(arc4random() %  UInt32(side)), Float(arc4random() % UInt32(side))), velocity: float2((Float(arc4random() %  10) - 5) / 10, (Float(arc4random() %  10) - 5) / 10))
+            particles.append(particle)
+        }
+        let size = particles.count * MemoryLayout<Particle>.size
+        particleBuffer = device.makeBuffer(bytes: &particles, length: size, options: [])
+    }
+
+    func initializeMetal() {
+        device = MTLCreateSystemDefaultDevice()
+        queue = device.makeCommandQueue()
+        guard let path = Bundle.main.path(forResource: "Shaders", ofType: "metal") else { return }
+        do {
+            let input = try String(contentsOfFile: path, encoding: String.Encoding.utf8)
+            let library = try device.makeLibrary(source: input, options: nil)
+            guard let firstPass = library.makeFunction(name: "firstPass") else { return }
+            firstState = try device.makeComputePipelineState(function: firstPass)
+            guard let secondPass = library.makeFunction(name: "secondPass") else { return }
+            secondState = try device.makeComputePipelineState(function: secondPass)
+        } catch let e { print(e) }
+    }
+
+    public func mtkView(_ view: MTKView, drawableSizeWillChange size: CGSize) {}
+
+    public func draw(in view: MTKView) {
+        if let drawable = view.currentDrawable,
+           let commandBuffer = queue.makeCommandBuffer(),
+           let commandEncoder = commandBuffer.makeComputeCommandEncoder() {
+            // first pass
+            commandEncoder.setComputePipelineState(firstState)
+            commandEncoder.setTexture(drawable.texture, index: 0)
+            let w = firstState.threadExecutionWidth
+            let h = firstState.maxTotalThreadsPerThreadgroup / w
+            let threadsPerGroup = MTLSizeMake(w, h, 1)
+            var threadsPerGrid = MTLSizeMake(side, side, 1)
+            commandEncoder.dispatchThreads(threadsPerGrid, threadsPerThreadgroup: threadsPerGroup)
+            // second pass
+            commandEncoder.setComputePipelineState(secondState)
+            commandEncoder.setTexture(drawable.texture, index: 0)
+            commandEncoder.setBuffer(particleBuffer, offset: 0, index: 0)
+            threadsPerGrid = MTLSizeMake(particleCount, 1, 1)
+            commandEncoder.dispatchThreads(threadsPerGrid, threadsPerThreadgroup: threadsPerGroup)
+            commandEncoder.endEncoding()
+            commandBuffer.present(drawable)
+            commandBuffer.commit()
+        }
+    }
+}

particles/particle3.playground/contents.xcplayground

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='macos' executeOnSourceChanges='false'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>

particles/particle3.playground/timeline.xctimeline

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Timeline
+   version = "3.0">
+   <TimelineItems>
+   </TimelineItems>
+</Timeline>