iOS 8 Metal Tutorial with Swift Part 3: Adding Texture

Andriy Kharchyshyn
Note: Updated for Xcode 7.3, iOS 9.3, and Swift 2.2 on 04-01-2016
Learn how add texture to a 3D cube with Metal!

Learn how add texture to a 3D cube with Metal!

Welcome back to our iOS 8 Metal tutorial series!

In the first part of the series, you learned how to get started with Metal and render a simple 2D triangle.

In the second part of the series, you learned how to set up a series of transformations to move from a triangle to a full 3D cube.

In this third part of the series, you’ll learn how to add a texture to the cube. As you work through this tutorial, you’ll learn:

  • How to reuse uniform buffers
  • A few useful and simple texture concepts
  • How to apply textures to a 3D model
  • How to add touch input to your app
  • How to debug Metal

Dust off your guitars — it’s time to rock Metal!

Getting Started

First, download the starter project. It’s very similar to the app at the end of part two, but with a few modifications, explained below.

Previously, ViewController was a heavy lifter. Even though you’d refactored it, it still had more than one responsibility. Now in the updated starter project, ViewController is now split into two classes:

  • MetalViewController: The base class that contains generic Metal setup code.
  • MySceneViewController: A subclass where that contains code specific to this app – i.e. creating and rendering the cube model.

The most important part is the new protocol MetalViewControllerDelegate:

protocol MetalViewControllerDelegate : class{
  func updateLogic(timeSinceLastUpdate:CFTimeInterval)
  func renderObjects(drawable:CAMetalDrawable)

This establishes callbacks from MetalViewController so that your app knows when to update logic and when to actually render.

In MySceneViewController you set yourself to be a delegate and then implement MetalViewControllerDelegate methods, and this is where all the cube rendering and updating action happens.

Now that you’re up to speed on the changes from part two, it’s time to move forward and delve deeper into the world of Metal.

Reusing Uniform Buffers (optional)

Note: This next section is theory driven and gives you more context about how Metal works under the hood. If you’re eager to move into exercises, feel free to skip ahead to the next section (Texturing). But reading this will make you at least 70 percent smarter. ;-]

In the previous part of this series, you learned about allocating new uniform buffers for every new frame, and you also learned that it’s not always the best way to use Metal since it’s not very efficient.

So, the time has come to change your ways and make Metal sing, like an epic hair-band guitar solo — starting with identifying the problem.

The Problem

In the render method in Node.swift, find:

let uniformBuffer = device.newBufferWithLength(sizeof(Float) * Matrix4.numberOfElements() * 2, options: [])

Take a good look at this monster! This method is called 60 times per second, and you create new buffer each time.

Since this is a performance issue, you’ll want to compare stats before and after optimization.
Build and run the app, open the Debug Navigator tab and select the FPS row.

Screen Shot 2015-01-08 at 4.35.43 PM

You should have numbers similar to these:


You’ll return to those numbers after optimization, so you may want to grab a screencap or jot down the stats before you move on.

The Solution

The solution is that instead of allocating a buffer each time, you’ll reuse a pool of buffers.

To keep your code clean, you’ll encapsulate all of the logic to create and reuse buffers into a helper class called BufferProvider that you’ll create.

You can visualize the class as follows:


As you can see, the BufferProvider will be responsible for creating a pool of buffers, and it’ll have a method to get the next available reusable buffer. This is kinda like UITableViewCell!

Now it’s time to dig in and make magic happen. Create a new Swift class named BufferProvider, and make it a subclass of NSObject.

First import Metal at the top of the file:

import Metal

Now, add these properties to the class:

// 1
let inflightBuffersCount: Int 
// 2
private var uniformsBuffers: [MTLBuffer]
// 3
private var avaliableBufferIndex: Int = 0

You’ll get some errors at the moment due to a missing initializer, but you’ll fix those shortly. For now, let’s review each property you just added:

  1. An Int that will store the number of buffers stored by BufferProvider — in the diagram above, this equals 3.
  2. An array that will store the buffers themselves.
  3. The index of the next available buffer. In your case it’ll change like this: 0 -> 1 -> 2 -> 0 -> 1 -> 2 -> 0 -> …

Now add an initializer:

init(device:MTLDevice, inflightBuffersCount: Int, sizeOfUniformsBuffer: Int) {
    self.inflightBuffersCount = inflightBuffersCount
    uniformsBuffers = [MTLBuffer]()
    for _ in 0...inflightBuffersCount-1 {
      let uniformsBuffer = device.newBufferWithLength(sizeOfUniformsBuffer, options: [])

Here you create a number of buffers, equal to the inflightBuffersCount parameter passed in to this initializer, and append them to the array.

Now add a method to fetch the next available buffer and copy some data into it:

func nextUniformsBuffer(projectionMatrix: Matrix4, modelViewMatrix: Matrix4) -> MTLBuffer {
    // 1
    let buffer = uniformsBuffers[avaliableBufferIndex]
    // 2
    let bufferPointer = buffer.contents()
    // 3
    memcpy(bufferPointer, modelViewMatrix.raw(), sizeof(Float)*Matrix4.numberOfElements())
    memcpy(bufferPointer + sizeof(Float)*Matrix4.numberOfElements(), projectionMatrix.raw(), sizeof(Float)*Matrix4.numberOfElements())
    // 4
    avaliableBufferIndex += 1
    if avaliableBufferIndex == inflightBuffersCount{
      avaliableBufferIndex = 0
    return buffer

Let’s review this section by section:

  1. Fetch MTLBuffer from uniformsBuffers array at avaliableBufferIndex index.
  2. Get void * pointer from MTLBuffer.
  3. Copy the passed-in matrices data into the buffer, using memcpy.
  4. Increment avaliableBufferIndex.

Almost done – just need to set up the rest of the code to use this now.

To do this, open Node.swift, and add this new property:

var bufferProvider: BufferProvider

Find init and add this at the end of the method:

self.bufferProvider = BufferProvider(device: device, inflightBuffersCount: 3, sizeOfUniformsBuffer: sizeof(Float) * Matrix4.numberOfElements() * 2)

Finally, inside render, replace this code:

let uniformBuffer = device.newBufferWithLength(sizeof(Float) * Matrix4.numberOfElements() * 2, options: nil)
var bufferPointer = uniformBuffer?.contents()
memcpy(bufferPointer!, nodeModelMatrix.raw(), UInt(sizeof(Float)*Matrix4.numberOfElements()))
memcpy(bufferPointer! + sizeof(Float)*Matrix4.numberOfElements(), projectionMatrix.raw(), UInt(sizeof(Float)*Matrix4.numberOfElements()))

With this far more elegant code:

let uniformBuffer = bufferProvider.nextUniformsBuffer(projectionMatrix, modelViewMatrix: nodeModelMatrix)

Build and run. Just as it did before adding bufferProvider, everything is working just fine!


A Wild Race Condition Appears!

Things are running smoothly, but there is a problem that could cause you some major pain later.

Have a look at this graph (and the explanation below):


Currently, the CPU gets the “next available buffer”, fills it with data, and then sends it to the GPU for processing.

But since there’s no guarantee about how long the GPU takes to render each frame, there could be a situation where you’re filling buffers on the CPU faster than the GPU can deal with them. In that case, you could find a scenario where you need a buffer on the CPU even though it’s in use on the GPU.

On the graph above, the CPU wants to encode the third frame while the GPU draws the first frame, but its uniform buffer is still in use.

So how do you fix this? The easiest way is to increase the number of buffers in the reuse pool so that it’s unlikely for the CPU to be ahead of the GPU. This would probably fix it but wouldn’t be 100% safe, and I know you’re here because you want to code in Metal like a ninja.

Patience. That’s what you need to solve this problem like a real ninja.

Like A Ninja

Like an undisciplined ninja, the CPU lacks patience, and that’s the problem. It’s good that the CPU can encode commands so quickly, but it wouldn’t hurt the CPU to wait a bit to avoid this racing condition.

feel like a ninja

Fortunately, it’s easy to “train” the CPU to wait when the buffer it wants is still in use.

For this task you’ll use semaphores, a low-level synchronization primitive. Basically, semaphores allow you to keep track of how many of a limited amount of resources are available, and block when no more resources are available.

Here’s how you’ll use a semaphore in this example:

  • Initialize with number of buffers. You’ll be using the semaphore to keep track of how many buffers are currently in use on the GPU, so you’ll initialize the semaphore with the number of buffers that are available (3 to start in this case).
  • Wait before accessing a buffer. Every time you need to access a buffer, you’ll ask the semaphore to “wait”. If a buffer is available, you’ll continue running as usual (but decrement the count on the semaphore). If all buffers are in use, this will block the thread until one becomes available. This should be a very short wait in practice as the GPU is fast.
  • Signal when done with a buffer. When the GPU is done with a buffer, you will “signal” the semaphore to track that it’s available again.

Note: To learn more about semaphores, check out this great explanation.

This will make more sense in code than in prose, let’s try this out. Go to BufferProvider.swift and add the following property:

var avaliableResourcesSemaphore:dispatch_semaphore_t

Now add this to the top of init:

avaliableResourcesSemaphore = dispatch_semaphore_create(inflightBuffersCount)

Here you create your semaphore with an initial count equal to the number of available buffers.

Now open Node.swift and add this at the top of render method:

dispatch_semaphore_wait(bufferProvider.avaliableResourcesSemaphore, DISPATCH_TIME_FOREVER)

This will make the CPU to wait in case bufferProvider.avaliableResourcesSemaphore has no free resources.

Now you need to signal the semaphore when the resource becomes available.

While you’re still in the render method, find:

let commandBuffer = commandQueue.commandBuffer()

And add this below:

commandBuffer.addCompletedHandler { (commandBuffer) -> Void in

This makes it so that when the GPU finishes rendering, it executes a completion handler to signal the semaphore (and bumps its count back up again).

Also in BufferProvider.swift, add this method:

  for _ in 0...self.inflightBuffersCount{

deinit simply does a little cleanup before object deletion, and without this your app would crash when the semaphore waits and when you delete BufferProvider.

Build and run. Everything should work as before – but now ninja style!


Performance Results

Now you must be eager to see if there’s been any performance improvement. As you did before, open the Debug Navigator tab, select the FPS row.


These are my stats – you’ll see that for me the CPU Frame Time decreased from 1.7ms to 1.2ms. It looks like a small win now, but the more objects you draw, the more value it gains. Please note that your actual results will depend on the device you’re using.



Note: If you skipped the previous section, start with this version of the project.

So, what is texture? Simply put, textures are 2D images that are typically mapped to 3D models.

For better understanding, think about some real life objects, for example, a mandarin. How would mandarin texture look in Metal? Something like this:


If you rendered a mandarin, first you would create a sphere-like 3D model, then you would use texture similar to one above, and Metal would map it.

Texture Coordinates

Contrary to the bottom-left origination of OpenGL, Metal’s textures originate in the top-left corner.

Here’s a sneak peek of the texture you’ll use in this tutorial.


With 3D graphics, it’s typical to see the texture coordinate axis marked with letter s for horizontal and t for vertical, just like the image above.

Going forward in this tutorial, to differentiate iOS device pixels and texture pixels, you’ll refer to texture pixels as texels.

So, your texture has 512×512 texels. In this tutorial, you’ll use normalized coordinates, which means that coordinates within the texture are always within the range of 0->1. So the:

  • Top-left corner has the coordinates (0.0, 0.0)
  • Top-right corner has the coordinates (1.0, 0.0)
  • Bottom-left corner has the coordinates (0.0, 1.0)
  • Bottom-right corner has the coordinates (1.1, 1.1)

When you map this texture to your cube, normalized coordinates will be important to understand.

Using normalized coordinates isn’t mandatory, but it has some advantages. For example, you want to switch texture with one that has resolution 256×256 texels. If you use normalized coordinates, it’ll “just work” (if new texture maps correctly).

Using Textures in Metal

In Metal, an object that represents texture is any object that conforms to MTLTexture protocol. There are countless texture types in Metal, but for now all you need is a type called MTLTexture2D.

Another important protocol is MTLSamplerState. An object that conforms to this protocol basically instructs the GPU how to use the texture.

So, when you pass a texture, you’ll pass the sampler as well. When using multiple textures that need to be treated similarly, you use the same sampler.

Here is a small visual to help illustrate how you’ll work with textures:


For your convenience, the project file contains a special, handcrafted class named MetalTexture that holds all the code to create MTLTexture from the image file in bundle.

Note: I’m not going to delve into it here, but if you want to learn how to create MTLTexture, refer to this post on


Now that you understand how this will work, it’s time to bring this texture to life. Download and copy MetalTexture.swift to your project and open it.

There are two important methods in this file. The first is:

init(resourceName: String,ext: String, mipmaped:Bool)

Here you pass the name of the file and its extension, and you also have choose whether you want mipmaps or not.

But wait, what’s a mipmap?

For through this tutorial, all you need to know is that when mipmaped is set to true, it generates an array of images instead of one when the texture loads, and each image is two times smaller than the previous. Lastly, the GPU automatically selects the best mip level from which to read texels.

The other method to note is this:

func loadTexture(device device: MTLDevice, commandQ: MTLCommandQueue, flip: Bool)

This method is called when MetalTexture actually creates MTLTexture. To create this object, you need a device object (similar to buffers). Also, you pass in MTLCommandQueue, and it’s used when mipmap levels are generated. Usually textures are loaded upside down, so this also has a flip param to deal with that.

Okay, now it’s time to put it all together.

Open Node.swift, and add 2 new variables:

var texture: MTLTexture
lazy var samplerState: MTLSamplerState? = Node.defaultSampler(self.device)

For now, Node holds just one texture and one sampler.

Now add the following method at the end of the file:

class func defaultSampler(device: MTLDevice) -> MTLSamplerState
    let pSamplerDescriptor:MTLSamplerDescriptor? = MTLSamplerDescriptor();
    if let sampler = pSamplerDescriptor
      sampler.minFilter             = MTLSamplerMinMagFilter.Nearest
      sampler.magFilter             = MTLSamplerMinMagFilter.Nearest
      sampler.mipFilter             = MTLSamplerMipFilter.Nearest
      sampler.maxAnisotropy         = 1
      sampler.sAddressMode          = MTLSamplerAddressMode.ClampToEdge
      sampler.tAddressMode          = MTLSamplerAddressMode.ClampToEdge
      sampler.rAddressMode          = MTLSamplerAddressMode.ClampToEdge
      sampler.normalizedCoordinates = true
      sampler.lodMinClamp           = 0
      sampler.lodMaxClamp           = FLT_MAX
      print(">> ERROR: Failed creating a sampler descriptor!")
    return device.newSamplerStateWithDescriptor(pSamplerDescriptor!)

This method generates a simple texture sampler that basically just holds a bunch of flags. Here you are enabling “nearest-neighbor” filtering, which is the fastest method of filtering (opposed to “linear”), and “clamp to edge” which instructs Metal how to deal with out-of-range values (which won’t happen in this tutorial, so it doesn’t matter).

Now in the render method, find this code:

renderEncoder.setVertexBuffer(vertexBuffer, offset: 0, atIndex: 0)

And add this below it:

renderEncoder.setFragmentTexture(texture, atIndex: 0)
if let samplerState = samplerState{
  renderEncoder.setFragmentSamplerState(samplerState, atIndex: 0)

This simply passes the texture and sampler to the shaders. It’s similar to what you did with vertex and uniform buffers, except that now you pass them to a fragment shader because you want to map texels to fragments.

Now you need to modify init. Change its declaration so it matches this:

init(name: String, vertices: Array<Vertex>, device: MTLDevice, texture: MTLTexture) {

Find this:

vertexCount = vertices.count

And add this just below it:

self.texture = texture

Each vertex needs to map to some coordinates on the texture. So open Vertex.swift and replace its contents with the following:

struct Vertex{
  var x,y,z: Float     // position data
  var r,g,b,a: Float   // color data
  var s,t: Float       // texture coordinates
  func floatBuffer() -> [Float] {
    return [x,y,z,r,g,b,a,s,t]

This adds two floats that hold texture coordinates.

Now open Cube.swift, and change init so it looks like this:

init(device: MTLDevice, commandQ: MTLCommandQueue){
    // 1
    let A = Vertex(x: -1.0, y:   1.0, z:   1.0, r:  1.0, g:  0.0, b:  0.0, a:  1.0, s: 0.25, t: 0.25)
    let B = Vertex(x: -1.0, y:  -1.0, z:   1.0, r:  0.0, g:  1.0, b:  0.0, a:  1.0, s: 0.25, t: 0.50)
    let C = Vertex(x:  1.0, y:  -1.0, z:   1.0, r:  0.0, g:  0.0, b:  1.0, a:  1.0, s: 0.50, t: 0.50)
    let D = Vertex(x:  1.0, y:   1.0, z:   1.0, r:  0.1, g:  0.6, b:  0.4, a:  1.0, s: 0.50, t: 0.25)
    let E = Vertex(x: -1.0, y:   1.0, z:  -1.0, r:  1.0, g:  0.0, b:  0.0, a:  1.0, s: 0.00, t: 0.25)
    let F = Vertex(x: -1.0, y:  -1.0, z:  -1.0, r:  0.0, g:  1.0, b:  0.0, a:  1.0, s: 0.00, t: 0.50)
    let G = Vertex(x: -1.0, y:  -1.0, z:   1.0, r:  0.0, g:  0.0, b:  1.0, a:  1.0, s: 0.25, t: 0.50)
    let H = Vertex(x: -1.0, y:   1.0, z:   1.0, r:  0.1, g:  0.6, b:  0.4, a:  1.0, s: 0.25, t: 0.25)
    let I = Vertex(x:  1.0, y:   1.0, z:   1.0, r:  1.0, g:  0.0, b:  0.0, a:  1.0, s: 0.50, t: 0.25)
    let J = Vertex(x:  1.0, y:  -1.0, z:   1.0, r:  0.0, g:  1.0, b:  0.0, a:  1.0, s: 0.50, t: 0.50)
    let K = Vertex(x:  1.0, y:  -1.0, z:  -1.0, r:  0.0, g:  0.0, b:  1.0, a:  1.0, s: 0.75, t: 0.50)
    let L = Vertex(x:  1.0, y:   1.0, z:  -1.0, r:  0.1, g:  0.6, b:  0.4, a:  1.0, s: 0.75, t: 0.25)
    let M = Vertex(x: -1.0, y:   1.0, z:  -1.0, r:  1.0, g:  0.0, b:  0.0, a:  1.0, s: 0.25, t: 0.00)
    let N = Vertex(x: -1.0, y:   1.0, z:   1.0, r:  0.0, g:  1.0, b:  0.0, a:  1.0, s: 0.25, t: 0.25)
    let O = Vertex(x:  1.0, y:   1.0, z:   1.0, r:  0.0, g:  0.0, b:  1.0, a:  1.0, s: 0.50, t: 0.25)
    let P = Vertex(x:  1.0, y:   1.0, z:  -1.0, r:  0.1, g:  0.6, b:  0.4, a:  1.0, s: 0.50, t: 0.00)
    let Q = Vertex(x: -1.0, y:  -1.0, z:   1.0, r:  1.0, g:  0.0, b:  0.0, a:  1.0, s: 0.25, t: 0.50)
    let R = Vertex(x: -1.0, y:  -1.0, z:  -1.0, r:  0.0, g:  1.0, b:  0.0, a:  1.0, s: 0.25, t: 0.75)
    let S = Vertex(x:  1.0, y:  -1.0, z:  -1.0, r:  0.0, g:  0.0, b:  1.0, a:  1.0, s: 0.50, t: 0.75)
    let T = Vertex(x:  1.0, y:  -1.0, z:   1.0, r:  0.1, g:  0.6, b:  0.4, a:  1.0, s: 0.50, t: 0.50)
    let U = Vertex(x:  1.0, y:   1.0, z:  -1.0, r:  1.0, g:  0.0, b:  0.0, a:  1.0, s: 0.75, t: 0.25)
    let V = Vertex(x:  1.0, y:  -1.0, z:  -1.0, r:  0.0, g:  1.0, b:  0.0, a:  1.0, s: 0.75, t: 0.50)
    let W = Vertex(x: -1.0, y:  -1.0, z:  -1.0, r:  0.0, g:  0.0, b:  1.0, a:  1.0, s: 1.00, t: 0.50)
    let X = Vertex(x: -1.0, y:   1.0, z:  -1.0, r:  0.1, g:  0.6, b:  0.4, a:  1.0, s: 1.00, t: 0.25)
    // 2
    let verticesArray:Array<Vertex> = [
      A,B,C ,A,C,D,   //Front
      E,F,G ,E,G,H,   //Left
      I,J,K ,I,K,L,   //Right
      M,N,O ,M,O,P,   //Top
      Q,R,S ,Q,S,T,   //Bot
      U,V,W ,U,W,X    //Back
    let texture = MetalTexture(resourceName: "cube", ext: "png", mipmaped: true)
    texture.loadTexture(device: device, commandQ: commandQ, flip: true)
    super.init(name: "Cube", vertices: verticesArray, device: device, texture: texture.texture)

Let’s go over the changes here section by section:

  1. Now as you create each vertex, you also specify the texture coordinate for each vertex. For a bit more clarity look at this image, and make sure you understand the s and t values of each vertex.


    Note that you also need to create vertices for each side of the cube individually now (rather than reusing vertices), because the texture coordinates might not match up correctly otherwise. It’s okay if adding extra vertices is a little confusing at this stage — your brain will grasp it soon enough.

  2. Here you form triangles, just like you did in part two of this tutorial series.
  3. You create and load the texture using the MetalTexture helper class.

Handling Texture on the GPU

At this point, you’re done working on the CPU side of things, and it’s all GPU from here.

Add this image to your project.

Open Shaders.metal and replace the entire file with this:

#include <metal_stdlib>
using namespace metal;
// 1
struct VertexIn{
  packed_float3 position;
  packed_float4 color;
  packed_float2 texCoord;  
struct VertexOut{
  float4 position [[position]];
  float4 color;
  float2 texCoord; 
struct Uniforms{
  float4x4 modelMatrix;
  float4x4 projectionMatrix;
vertex VertexOut basic_vertex(
                              const device VertexIn* vertex_array [[ buffer(0) ]],
                              const device Uniforms&  uniforms    [[ buffer(1) ]],
                              unsigned int vid [[ vertex_id ]]) {
  float4x4 mv_Matrix = uniforms.modelMatrix;
  float4x4 proj_Matrix = uniforms.projectionMatrix;
  VertexIn VertexIn = vertex_array[vid];
  VertexOut VertexOut;
  VertexOut.position = proj_Matrix * mv_Matrix * float4(VertexIn.position,1);
  VertexOut.color = VertexIn.color;
  // 2
  VertexOut.texCoord = VertexIn.texCoord; 
  return VertexOut;
// 3
fragment float4 basic_fragment(VertexOut interpolated [[stage_in]],
                              texture2d<float>  tex2D     [[ texture(0) ]],    
// 4
                              sampler           sampler2D [[ sampler(0) ]]) {  
// 5
  float4 color = tex2D.sample(sampler2D, interpolated.texCoord);               
  return color;

Now let’s go through all the stuff you changed:

  1. The vertex structs now contain texture coordinates.
  2. You now pass texture coordinates from VertexIn to VertexOut.
  3. Here you receive the texture you passed in.
  4. Here you receive the sampler.
  5. You use sample() on the texture to get color for the specific texture coordinate from the texture, by using rules specified in sampler.

Almost done! Open MySceneViewController.swift and replace this line:

objectToDraw = Cube(device: device)

With this:

objectToDraw = Cube(device: device, commandQ:commandQueue)

Build and run. Your cube should now be texturized.


Colorizing a Texture (Optional)

At this point, you’re ignoring the cube’s color values and just using color values from the texture. But what if you need to texturize the object’s color instead of covering it up?

In the fragment shader, replace this line:

float4 color = tex2D.sample(sampler2D, interpolated.texCoord);


float4 color =  interpolated.color * tex2D.sample(sampler2D, interpolated.texCoord);

You should get something like this:


You did this just to see how you combine colors inside the fragment shader. And yes, it’s as simple as doing a little multiplication.

But don’t continue until you revert that last change because it doesn’t look that good.

Adding User Input

All this texturing is cool, but it’s rather static. Wouldn’t it be cool if you could rotate the cube with your finger and see your beautiful texturing work at every angle?

Let’s do this! You’ll use UIPanGestureRecognizer to detect user interactions.

Open MySceneViewController.swift, and add these two new properties:

let panSensivity:Float = 5.0
var lastPanLocation: CGPoint!

Now add two new methods:

//MARK: - Gesture related
  // 1
  func setupGestures(){
    let pan = UIPanGestureRecognizer(target: self, action: #selector(MySceneViewController.pan(_:)))
  // 2
  func pan(panGesture: UIPanGestureRecognizer){
    if panGesture.state == UIGestureRecognizerState.Changed{
      let pointInView = panGesture.locationInView(self.view)
      // 3
      let xDelta = Float((lastPanLocation.x - pointInView.x)/self.view.bounds.width) * panSensivity
      let yDelta = Float((lastPanLocation.y - pointInView.y)/self.view.bounds.height) * panSensivity
      // 4
      objectToDraw.rotationY -= xDelta
      objectToDraw.rotationX -= yDelta
      lastPanLocation = pointInView
    } else if panGesture.state == UIGestureRecognizerState.Began{
      lastPanLocation = panGesture.locationInView(self.view)

Let’s review this section by section:

  1. Create a pan gesture recognizer and add it to your view.
  2. Check if the touch moved.
  3. When touch moves, you calculate by how much in normalized coordinates. You also apply panSensivity to control rotation speed.
  4. Apply the changes to the cube by setting the rotation properties.

Now find the end of viewDidLoad() and add:


Build and run.

Hmmm, the cube spins all by itself. Why is that? Think through what you just did and see if you can identify the problem here. Open the spoiler to check if your assumption is correct.

Solution Inside SelectShow

Debugging Metal

Like any code, you’ll need to do a little debugging to make sure your work is free of errors. And if you look closely, you’ll notice that at some angles, the sides are a little “crispy”.


To fully understand the problem, you’ll need to debug. Fortunately, Metal comes with some stellar tools to help you.

While app is running, press the Capture the GPU Frame button.

Screen Shot 2015-01-14 at 1.23.27 PM

Pressing the button will automatically pause the app on a breakpoint, and then Xcode will collect all values and states of this single frame.

Xcode may put you into assistant mode, meaning that it splits your main area into two, but you don’t need all that, so feel free to return to regular mode. Also, in the debug area, select All MTL Objects, just like on the screenshot:

Screen Shot 2015-01-14 at 1.46.25 PM

At last, you have proof that you’re actually drawing in triangles, not squares!

Anyways, in the debug area, find and open the Textures group.

Screen Shot 2015-01-14 at 1.54.18 PM

You’re probably asking why you have two textures, because you only passed in one.

One texture is for the cube image, and the other is the one formed with the fragment shader and the one shown to the screen.

Dive deeper; you’re getting close to explaining the mystery.

The weird part is this other texture has non-Retina resolution. Ah-ha!!! So the reason why you cube was a bit crispy is because the non-Retina texture stretched to fill screen. You’ll fix this in a moment.

Fixing Drawable Texture Resizing

There is one more problem to debug and solve before you can officially declare your mastery of Metal. Run your app again and rotate the device into landscape mode.


Not the best view, eh?

The problem here is that when the device rotates, its bounds change. However, the displayed texture dimensions don’t have any reason to change.

But it’s pretty easy to fix. Open MetalViewController.swift and take a look at this setup code in viewDidLoad:

  device = MTLCreateSystemDefaultDevice()
  metalLayer = CAMetalLayer()
  metalLayer.device = device
  metalLayer.pixelFormat = .BGRA8Unorm
  metalLayer.framebufferOnly = true
  metalLayer.frame = view.layer.frame

The important line is metalLayer.frame = view.layer.frame, which sets the layer frame once. You just need to update it when the device rotates.

So override viewDidLayoutSubviews like this:

override func viewDidLayoutSubviews() {
  if let window = view.window {
    let scale = window.screen.nativeScale  
    let layerSize = view.bounds.size
    view.contentScaleFactor = scale
    metalLayer.frame = CGRectMake(0, 0, layerSize.width, layerSize.height)
    metalLayer.drawableSize = CGSizeMake(layerSize.width * scale, layerSize.height * scale)  
  1. Gets the display nativeScale for the device (2 for iPhone 5s, 6 and iPads, 3 for iPhone 6 Plus)
  2. Applies the scale to increase the drawable texture size

Now delete following line in viewDidLoad:

metalLayer.frame = view.layer.frame

Build and run. Here is a classic before-and-after comparison.


The difference is even more obvious when you’re on an iPhone 6+.

Now rotate to landscape.


It’s all flat now, but at least the background is a rich green and the edges are pretty. :]

If you repeat the steps from the debug section, you’d see the texture’s dimensions are now correct. So, what’s the problem?

It’s something, that’s for sure. Think through what you just did and try to figure out what’s causing the pain. Then check the answer below to see if you figured it out.

Solution Inside SelectShow

Wrap Up

Nicely done. Take a moment to review what you’ve done in this tutorial.

  1. You created BufferProvider to cleverly reuse uniform buffers instead of creating new buffers every time.
  2. You added MetalTexture and loaded a MTLTexture with it.
  3. You modified the structure of Vertex so it also stores corresponding texture coordinates from MTLTexture.
  4. You modified Cube so it contains 24 vertices, each with it’s own texture coordinates.
  5. You modified the shaders to receive texture coordinates of the fragments, and then you read corresponding texel using sample().
  6. You added a cool rotation UI effect with UIPanGestureRecognizer.
  7. You debugged the Metal frame and identified why it rendered a subpar image.
  8. You resized a drawable texture in viewDidLayoutSubviews to fix the rotation issue and improve the image’s quality.

Where To Go From Here?

Here is the final example project from this iOS 8 Metal Tutorial.

We hope to make more Metal tutorials in the future, but in the meantime, be sure to check out some of these great resources:

Also tune into the OpenGL ES video tutorials on this site, and learn as Ray explains — in depth — how many of these same concepts work in OpenGL ES.

Thank you for joining me for this tour through Metal. As you can see, it’s a powerful technology that’s relatively easy to implement once you understand how it works.

If you have questions, comments or Metal discoveries to share, please leave them in the comments below!

Andrew Kharchyshyn

I am an iOS developer. Mostly focused on OpenGL ES and SpriteKit. When not doing games, I'm most likely playing them or watching anime.

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