Problem

I'm trying out the performance shaders for the first time and encountered a runtime problem. The MTLTexture that MTKTextureLoader returns seems to be uncompatible with Metal Performance Shaders' MPSImageFindKeypoints encoder.

The only hint so far that I found is from @warrenm's sample code on MPS that specifies MTKTextureLoaderOptions just like I did. I did not find any other mentions in the docs.

Any help is highly appreciated.

Error

/BuildRoot/Library/Caches/com.apple.xbs/Sources/MetalImage/MetalImage-121.0.2/MPSImage/Filters/MPSKeypoint.mm:166: failed assertion `Source 0x282ce8fc0 texture type (80) is unsupported

where 0x282ce8fc0 is the MTLTexture from the texture loader. As far as I could see there is no MTLTexture type 80, the enum ranges up to 8 or so (not hex).

Code

CGFloat w = CGImageGetWidth(_image);
CGFloat h = CGImageGetHeight(_image);
id<MTLDevice> device = MTLCreateSystemDefaultDevice();
id<MTLCommandQueue> commandQueue = [device newCommandQueue];

NSDictionary* textureOptions = @{ MTKTextureLoaderOptionSRGB: [[NSNumber alloc] initWithBool:NO] };
id<MTLTexture> texture = [[[MTKTextureLoader alloc] initWithDevice:device] newTextureWithCGImage:_image
                                                                                         options:textureOptions
                                                                                           error:nil];
id<MTLBuffer> keypointDataBuffer;
id<MTLBuffer> keypointCountBuffer;

MTLRegion region = MTLRegionMake2D(0, 0, w, h);

id<MTLCommandBuffer> commandBuffer = [commandQueue commandBuffer];
MPSImageKeypointRangeInfo rangeInfo = {100,0.5};
MPSImageFindKeypoints* imageFindKeypoints = [[MPSImageFindKeypoints alloc] initWithDevice:device
                                                                                     info:&rangeInfo];
[imageFindKeypoints encodeToCommandBuffer:commandBuffer
                            sourceTexture:texture
                                  regions:&region
                          numberOfRegions:1
                      keypointCountBuffer:keypointCountBuffer
                keypointCountBufferOffset:0
                       keypointDataBuffer:keypointDataBuffer
                 keypointDataBufferOffset:0];

[commandBuffer commit];

NSLog(keypointCountBuffer);
NSLog(keypointDataBuffer);

Edit

After converting my image to the correct pixel format I am now initialising the buffers like so:

id<MTLBuffer> keypointDataBuffer = [device newBufferWithLength:maxKeypoints*(sizeof(MPSImageKeypointData)) options:MTLResourceOptionCPUCacheModeDefault];
id<MTLBuffer> keypointCountBuffer = [device newBufferWithLength:sizeof(int) options:MTLResourceOptionCPUCacheModeDefault];

There is no error anymore. But how can I reading the contents now?

((MPSImageKeypointData*)[keypointDataBuffer contents])[0].keypointCoordinate returns (0,0) for all indexes. Also I don't know how to read the keypointsCountBuffer. The buffer contents converted to an int value show a higher value than the defined maxKeypoints. I don't see where the docs say what kind of format the count buffer has.

Finally the code is running and just for completeness sake I thought I should post the whole code as an answer

code

id<MTLDevice> device = MTLCreateSystemDefaultDevice();
id<MTLCommandQueue> commandQueue = [device newCommandQueue];

// init textures
NSDictionary* textureOptions = @{ MTKTextureLoaderOptionSRGB: [[NSNumber alloc] initWithBool:NO] };
id<MTLTexture> texture = [[[MTKTextureLoader alloc] initWithDevice:device] newTextureWithCGImage:_lopoImage
                                                                                         options:textureOptions
                                                                                           error:nil];
MTLTextureDescriptor *descriptor = [MTLTextureDescriptor texture2DDescriptorWithPixelFormat:(MTLPixelFormatR8Unorm) width:w height:h mipmapped:NO];
descriptor.usage = (MTLTextureUsageShaderRead | MTLTextureUsageShaderWrite);
id<MTLTexture> unormTexture = [device newTextureWithDescriptor:descriptor];

// init arrays and buffers for keypoint finder
int maxKeypoints = w*h;
id<MTLBuffer> keypointDataBuffer = [device newBufferWithLength:sizeof(MPSImageKeypointData)*maxKeypoints options:MTLResourceOptionCPUCacheModeWriteCombined];
id<MTLBuffer> keypointCountBuffer = [device newBufferWithLength:sizeof(int) options:MTLResourceOptionCPUCacheModeWriteCombined];

MTLRegion region = MTLRegionMake2D(0, 0, w, h);

// init colorspace converter
CGColorSpaceRef srcColorSpace = CGColorSpaceCreateWithName(kCGColorSpaceSRGB);
CGColorSpaceRef dstColorSpace = CGColorSpaceCreateWithName(kCGColorSpaceLinearGray);

CGColorConversionInfoRef conversionInfo = CGColorConversionInfoCreate(srcColorSpace, dstColorSpace);
MPSImageConversion *conversion = [[MPSImageConversion alloc] initWithDevice:device
                                  srcAlpha:(MPSAlphaTypeAlphaIsOne)
                                 destAlpha:(MPSAlphaTypeNonPremultiplied)
                           backgroundColor:nil
                            conversionInfo:conversionInfo];

// init keypoint finder
MPSImageKeypointRangeInfo rangeInfo = {maxKeypoints,0.75};
MPSImageFindKeypoints* imageFindKeypoints = [[MPSImageFindKeypoints alloc] initWithDevice:device
                                                                                     info:&rangeInfo];

// encode command buffer
id<MTLCommandBuffer> commandBuffer = [commandQueue commandBuffer];
[conversion encodeToCommandBuffer:commandBuffer sourceTexture:texture destinationTexture:unormTexture];
[imageFindKeypoints encodeToCommandBuffer:commandBuffer
                            sourceTexture:unormTexture
                                  regions:&region
                          numberOfRegions:1
                      keypointCountBuffer:keypointCountBuffer
                keypointCountBufferOffset:0
                       keypointDataBuffer:keypointDataBuffer
                 keypointDataBufferOffset:0];

// run command buffer
[commandBuffer commit];
[commandBuffer waitUntilCompleted];

// read keypoints
int count = ((int*)[keypointCountBuffer contents])[0];
MPSImageKeypointData* keypointDataArray = ((MPSImageKeypointData*)[keypointDataBuffer contents]);
for (int i = 0 ; i<count;i++) {
    simd_ushort2 coordinate = keypointDataArray[i].keypointCoordinate;
    NSLog(@"color:%f | at:(%u,%u)", keypointDataArray[i].keypointColorValue, coordinate[0], coordinate[1] );
}

I guess there should be a more clever way to allocate the keypoint buffers with [device newBufferWithBytesNoCopy] so then you would not need to copy the contents back into your allocated arrays. It just didn't figure out to correctly align the buffer.

Also I should mention that I guess usually you will have a grayscale texture after any kind of feature detection so that the image converting part will not be necessary.