MTLTexture from CMSampleBuffer has 0 bytesPerRow - ios

Question

I am converting the CMSampleBuffer argument in the captureOutput function of my AVCaptureVideoDataOuput delegate into a MTLTexture like so (side note, I have set the pixel format of the video output to kCVPixelFormatType_32BGRA):
func captureOutput(_ captureOutput: AVCaptureOutput!, didOutputSampleBuffer sampleBuffer: CMSampleBuffer!, from connection: AVCaptureConnection!) {
let imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer)!
let width = CVPixelBufferGetWidth(imageBuffer)
let height = CVPixelBufferGetHeight(imageBuffer)
var outTexture: CVMetalTexture? = nil
var textCache : CVMetalTextureCache?
CVMetalTextureCacheCreate(kCFAllocatorDefault, nil, metalDevice, nil, &textCache)
var textureRef : CVMetalTexture?
CVMetalTextureCacheCreateTextureFromImage(kCFAllocatorDefault, textCache!, imageBuffer, nil, MTLPixelFormat.bgra8Unorm, width, height, 0, &textureRef)
let texture = CVMetalTextureGetTexture(textureRef!)!
print(texture.bufferBytesPerRow)
}
The issue is when I print the bytes per row of the texture, it always prints 0, which is problematic because I later try to convert the texture back into a UIImage using the methodology in this article: https://www.invasivecode.com/weblog/metal-image-processing. Why is the texture I receive seemingly empty? I know the CMSampleBuffer property is fine because I can convert it into a UIIMage and draw it like so:
let myPixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer)
let myCIimage = CIImage(cvPixelBuffer: myPixelBuffer!)
let image = UIImage(ciImage: myCIimage)
self.imageView.image = image

Answer 1

The bufferBytesPerRow property is only meaningful for a texture that was created using the makeTexture(descriptor:offset:bytesPerRow:) method of a MTLBuffer. As you can see, the bytes-per-row is an input to that method to tell Metal how to interpret the data in the buffer. (The texture descriptor provides additional information, too, of course.) This method is only a means to get that back out.
Note that textures created from buffers can also report which buffer they were created from and the offset supplied to the above method.
Textures created in other ways don't have that information. These textures have no intrinsic bytes-per-row. Their data is not necessarily organized internally in a simple raster buffer.
If/when you want to get the data from a texture to either a Metal buffer or a plain old byte array, you have the freedom to choose a bytes-per-row value that's useful for your purposes, so long as it's at least the bytes-per-pixel of the texture pixel format times the texture's width. (It's more complicated for compressed formats.) The docs for getBytes(_:bytesPerRow:from:mipmapLevel:) and copy(from:sourceSlice:sourceLevel:sourceOrigin:sourceSize:to:destinationOffset:destinationBytesPerRow:destinationBytesPerImage:) explain further.