Take a snapshot of current screen with Metal in swift

Question

I tried:

let scale = UIScreen.mainScreen().scale        
UIGraphicsBeginImageContextWithOptions(metalLayer.bounds.size, false, scale)
            
// metalLayer.renderInContext(UIGraphicsGetCurrentContext()!)

// self.view.layer ...

metalLayer.presentationLayer()!.renderInContext(UIGraphicsGetCurrentContext()!)
            
let image = UIGraphicsGetImageFromCurrentImageContext()           
UIGraphicsEndImageContext()
UIImageWriteToSavedPhotosAlbum(image, nil, nil, nil)

But the result is an empty screenshot. Any help would be nice!

Please keep in mind that I want to take a snapshot of a CAMetalLayer.

Answer 1

To make a screenshot, you need to get MTLTexture of the frame buffer.

1. If you use MTKView:

let texture = view.currentDrawable!.texture

2. If you don't use MTKView

Here's what I would do - I would have a property which holds last drawable presented to the screen:

let lastDrawableDisplayed: CAMetalDrawable?

And then when you present drawable to the screen, I would update it:

let commandBuffer = commandQueue.commandBuffer()
commandBuffer.addCompletedHandler { buffer in
  self.lastDrawableDisplayed = drawable
}

Now you whenever you need to take a screenshot, you can get a texture like this:

let texture = lastDrawableDisplayed.texture

Ok, now when you have MTLTexture you can convert it to CGImage and then to UIImage or NSImage.

Here's the code for OS X playground (MetalKit.MTLTextureLoader is not available for iOS playgrounds), in which I convert MTLTexture to CGImage

I made a small extension over MTLTexture for this.

import Metal
import MetalKit
import Cocoa

let device = MTLCreateSystemDefaultDevice()!
let textureLoader = MTKTextureLoader(device: device)

let path = "path/to/your/image.jpg"
let data = NSData(contentsOfFile: path)!

let texture = try! textureLoader.newTextureWithData(data, options: nil)

extension MTLTexture {
  
  func bytes() -> UnsafeMutablePointer<Void> {
    let width = self.width
    let height = self.height
    let rowBytes = self.width * 4
    let p = malloc(width * height * 4)
    
    self.getBytes(p, bytesPerRow: rowBytes, fromRegion: MTLRegionMake2D(0, 0, width, height), mipmapLevel: 0)
    
    return p
  }
  
  func toImage() -> CGImage? {
    let p = bytes()
    
    let pColorSpace = CGColorSpaceCreateDeviceRGB()
    
    let rawBitmapInfo = CGImageAlphaInfo.NoneSkipFirst.rawValue | CGBitmapInfo.ByteOrder32Little.rawValue
    let bitmapInfo:CGBitmapInfo = CGBitmapInfo(rawValue: rawBitmapInfo)
    
    let selftureSize = self.width * self.height * 4
    let rowBytes = self.width * 4
    let provider = CGDataProviderCreateWithData(nil, p, selftureSize, nil)
    let cgImageRef = CGImageCreate(self.width, self.height, 8, 32, rowBytes, pColorSpace, bitmapInfo, provider, nil, true, CGColorRenderingIntent.RenderingIntentDefault)!
    
    return cgImageRef
  }
}

if let imageRef = texture.toImage() {
  let image = NSImage(CGImage: imageRef, size: NSSize(width: texture.width, height: texture.height))
}

Answer 2

For swift 4.0, Just converting code provided by haawa

let lastDrawableDisplayed = metalView?.currentDrawable?.texture

if let imageRef = lastDrawableDisplayed?.toImage() {
    let uiImage:UIImage = UIImage.init(cgImage: imageRef)
}

extension MTLTexture {

    func bytes() -> UnsafeMutableRawPointer {
        let width = self.width
        let height   = self.height
        let rowBytes = self.width * 4
        let p = malloc(width * height * 4)

        self.getBytes(p!, bytesPerRow: rowBytes, from: MTLRegionMake2D(0, 0, width, height), mipmapLevel: 0)

        return p!
    }

    func toImage() -> CGImage? {
        let p = bytes()

        let pColorSpace = CGColorSpaceCreateDeviceRGB()

        let rawBitmapInfo = CGImageAlphaInfo.noneSkipFirst.rawValue | CGBitmapInfo.byteOrder32Little.rawValue
        let bitmapInfo:CGBitmapInfo = CGBitmapInfo(rawValue: rawBitmapInfo)

        let selftureSize = self.width * self.height * 4
        let rowBytes = self.width * 4
        let releaseMaskImagePixelData: CGDataProviderReleaseDataCallback = { (info: UnsafeMutableRawPointer?, data: UnsafeRawPointer, size: Int) -> () in
            return
        }
        let provider = CGDataProvider(dataInfo: nil, data: p, size: selftureSize, releaseData: releaseMaskImagePixelData)
        let cgImageRef = CGImage(width: self.width, height: self.height, bitsPerComponent: 8, bitsPerPixel: 32, bytesPerRow: rowBytes, space: pColorSpace, bitmapInfo: bitmapInfo, provider: provider!, decode: nil, shouldInterpolate: true, intent: CGColorRenderingIntent.defaultIntent)!

        return cgImageRef
    }
}