Problem solved - I just didn't understand how to perform the gesture. It's a two handed gesture, so you need to hold down the option key in the simulator to get two points.

I'm reading the texture data like this: let region = MTLRegionMake2D(0, 0, w, h) var a = Array<UInt8>(repeating:0, count: 4*w*h) texture.getBytes(&a, bytesPerRow: (4 * MemoryLayout<UInt8>.size * w), from: region, mipmapLevel: 0) I create textures like this let descriptor = MTLTextureDescriptor.texture2DDescriptor(pixelFormat:.bgra8Unorm_srgb , width: width, height: height, mipmapped: false) descriptor.usage = MTLTextureUsage(rawValue: MTLTextureUsage.renderTarget.rawValue | MTLTextureUsage.shaderRead.rawValue | MTLTextureUsage.shaderWrite.rawValue) let texture = device.makeTexture(descriptor: descriptor)! And for testing I'm filling the textures with known pixel data like this: var a:[UInt8] = ... texture.replace(region: region, mipmapLevel: 0, withBytes: &a, bytesPerRow: (4 * MemoryLayout<UInt8>.size * w)) Then I load the textures from the tiff data with MTKTextureLoader: func loadTexture(data:Data) throws -> MTLTexture { let usage = MTLTextureUsage(rawValue: MTLTextureUsage.renderTarget.rawValue | MTLTextureUsage.shaderRead.rawValue | MTLTextureUsage.shaderWrite.rawValue) return try loader.newTexture(data: data,options:[MTKTextureLoader.Option.textureUsage:usage.rawValue,MTKTextureLoader.Option.origin:MTKTextureLoader.Origin.flippedVertically.rawValue]) }

ImageMagick give this: (233,174,145,170) #E9AE91AA srgba(233,174,145,0.666667) So it seems the problem is the saving rather than the loading.

I can't see a direct way to read the bytes in the tiff. I've tried: func printTiffBytes(url:URL) { if let imageSource = CGImageSourceCreateWithURL(url as CFURL, nil), let image = CGImageSourceCreateImageAtIndex(imageSource, 0, nil) { let width = image.width let height = image.height let bitsPerComponent = image.bitsPerComponent let bytesPerRow = image.bytesPerRow let totalBytes = height * bytesPerRow let colorSpace = CGColorSpaceCreateDeviceRGB() // let colorSpace = CGColorSpace(name: CGColorSpace.linearSRGB)! //let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.last.rawValue) // Error! unsupported parameter combination let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.premultipliedLast.rawValue) let data = UnsafeMutableRawPointer.allocate(byteCount: totalBytes, alignment: MemoryLayout<UInt8>.alignment) defer { data.deallocate() } if let context = CGContext(data: data, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo.rawValue) { context.draw(image, in: CGRect(x: 0.0, y: 0.0, width: CGFloat(width), height: CGFloat(height))) let buffer = data.assumingMemoryBound(to: UInt8.self) for i in 0..<totalBytes { print(buffer[i]) } } } } but I cannot get it to wrk with CGImageAlphaInfo.last so I'm not all that confident this is really what's in the tiff. But for the test pixel [120, 145, 195, 170] I get [155,116 ,97,170] (I guess RGB order is reversed?)

The flag seems to be when you create the CIContext CIContext(options: [CIContextOption.outputPremultiplied: false]) After making that change, Alpha info no longer say its premultiplied, but I'm still getting a more or less the same result: [120, 145, 195, 170] -> [145, 174, 233, 170]

Thanks. Yes, the alpha info is indeed "premultipliedLast" but I can't see how I can change that. Any idea where to set that flag?