CGImage to MPSTexture or MPSImage - swift

Question

I have an CGImage which is constructed out of a CVPixelbuffer (ARGB). I want to convert that CGImage into a MTLTexture. I use:
let texture: MTLTexture = try m_textureLoader.newTexture(with: cgImage, options: [MTKTextureLoaderOptionSRGB : NSNumber(value: true)] )
Later I want to use the texture in an MPSImage having 3 channels:
let sid = MPSImageDescriptor(channelFormat: MPSImageFeatureChannelFormat.float16, width: 40, height: 40, featureChannels: 3)
preImage = MPSTemporaryImage(commandBuffer: commandBuffer, imageDescriptor: sid)
lanczos.encode(commandBuffer: commandBuffer, sourceTexture: texture!, destinationTexture: preImage.texture)
scale.encode (commandBuffer: commandBuffer, sourceImage: preImage, destinationImage: srcImage)
Now my questions:
How does textureLoader.newTexture(...) map the four ARGB channels to the 3 channels specified in the MPSImageDescriptor ?
How can I ensure that the RGB components are used and not e.g. ARG ?
Is there a way to specify that channel mapping ?
Thanks, Chris

Answer 1

Why not construct the MTLTexture from the CVPixelBuffer directly? Is much quicker!
Do this once at the beginning of your program:
// declare this somewhere, so we can re-use it
var textureCache: CVMetalTextureCache?
// create the texture cache object
guard CVMetalTextureCacheCreate(kCFAllocatorDefault, nil, device, nil, &textureCache) == kCVReturnSuccess else {
print("Error: could not create a texture cache")
return false
}
Do this once your have your CVPixelBuffer:
let width = CVPixelBufferGetWidth(pixelBuffer)
let height = CVPixelBufferGetHeight(pixelBuffer)
var texture: CVMetalTexture?
CVMetalTextureCacheCreateTextureFromImage(kCFAllocatorDefault, textureCache,
pixelBuffer, nil, .bgra8Unorm, width, height, 0, &texture)
if let texture = texture {
metalTexture = CVMetalTextureGetTexture(texture)
}
Now metalTexture contains an MTLTexture object with the contents of the CVPixelBuffer.