Generate mipmaped MTLTextures from AVFoundation camera stream - ios

Question

I'm using AVFoundation to get camera stream.
I'm using this code to get MTLTextures from:
- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection
{
CVPixelBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
id<MTLTexture> texture = nil;
{
size_t width = CVPixelBufferGetWidth(pixelBuffer);
size_t height = CVPixelBufferGetHeight(pixelBuffer);
MTLPixelFormat pixelFormat = MTLPixelFormatBGRA8Unorm;
CVMetalTextureRef metalTextureRef = NULL;
CVReturn status = CVMetalTextureCacheCreateTextureFromImage(NULL, _textureCache, pixelBuffer, NULL, pixelFormat, width, height, 0, &metalTextureRef);
if(status == kCVReturnSuccess)
{
texture = CVMetalTextureGetTexture(metalTextureRef);
if (self.delegate){
[self.delegate textureUpdated:texture];
}
CFRelease(metalTextureRef);
}
}
}
It works fine, except for that generated MTLTexture object is not mipmaped (has only one mip level).
In this call:
CVMetalTextureCacheCreateTextureFromImage(NULL, _textureCache, pixelBuffer, NULL, pixelFormat, width, height, 0, &metalTextureRef);
There is a third parameter called "textureAtributes", I think it's possible to specify that I want mipmaped texture, but I haven't found any word in documentation what exactly goes there. Neither had I find a source code in which something else is passed instead of NULL.
In OpenGLES for iOS there is similar method, with same parameter, and also no words in documentation .

Answer 1

Just received an answer from Metal engineer here. Here's a quote:
No, it is not possible to generate a mipmapped texture from a
CVPixelBuffer directly.
CVPixelBuffer images typically have a linear/stride layout, as non-GPU hardware blocks might be interacting with those, and most GPU
hardware only supports mipmapping from tiled textures. You'll need to
issue a blit to copy from the linear MTLTexture to a private
MTLTexture of your own creation, then generate mipmaps.
As for textureAttributes, there is only one key supported: kCVMetalTextureCacheMaximumTextureAgeKey

Answer 2

There isn't a method to get a mipmapped texture directly, but you can generate one yourself easily enough.
First use your Metal device to create an empty Metal texture that is the same size and format as your existing texture, but has a full mipmap chain. See newTexture documentation
Use your MTLCommandBuffer object to create a blitEncoder object. See blitCommandEncoder documentation
Use the blitEncoder to copy from your camera texture to your empty texture. destinationLevel should be zero as you are only copying the top level mipmap. See copyFromTexture documentation
Finally use the blitEncoder to generate all the mip levels by calling generateMipmapsForTexture. See generateMipMapsForTexture documentation
At the end of this you have a metal texture from the camera with a full mip chain.