destinationImageDescriptorForSourceImagesSourceStatesForKernelSuggestedDescriptor
default MPSImageDescriptor destinationImageDescriptorForSourceImagesSourceStatesForKernelSuggestedDescriptor(NSArray<? extends MPSImage> sourceImages,
NSArray<? extends MPSState> sourceStates,
MPSKernel kernel,
MPSImageDescriptor inDescriptor)
Determine padding and sizing of result images
A MPSNNPaddingMethod must both return a valid MPSImageDescriptor
and set the MPSKernel.offset to the correct value. This is a
required feature if the MPSNNPaddingMethodCustom bit is set in
the paddingMethod.
Some code that may prove helpful:
[@code]
const int centeringPolicy = 0; // When kernelSize is even: 0 pad bottom right. 1 pad top left. Centers the kernel for even sized kernels.
typedef enum Style{
StyleValidOnly = -1,
StyleSame = 0,
StyleFull = 1
}Style;
// Typical destination size in one dimension for forward filters (most filters)
static int DestSize( int sourceSize, int stride, int filterWindowSize, Style style ){
sourceSize += style * (filterWindowSize - 1); // adjust how many pixels we are allowed to read
return (sourceSize + stride - 1) / stride; // sourceSize / stride, round up
}
// Typical destination size in one dimension for reverse filters (e.g. convolution transpose)
static int DestSizeReverse( int sourceSize, int stride, int filterWindowSize, Style style ){
return (sourceSize-1) * stride + // center tap for the last N-1 results. Take stride into account
1 + // center tap for the first result
style * (filterWindowSize-1); // add or subtract (or ignore) the filter extent
}
// Find the MPSOffset in one dimension
static int Offset( int sourceSize, int stride, int filterWindowSize, Style style ){
// The correction needed to adjust from position of left edge to center per MPSOffset definition
int correction = filterWindowSize / 2;
// exit if all we want is to start consuming pixels at the left edge of the image.
if( 0 )
return correction;
// Center the area consumed in the source image:
// Calculate the size of the destination image
int destSize = DestSize( sourceSize, stride, filterWindowSize, style ); // use DestSizeReverse here instead as appropriate
// calculate extent of pixels we need to read in source to populate the destination
int readSize = (destSize-1) * stride + filterWindowSize;
// calculate number of missing pixels in source
int extraSize = readSize - sourceSize;
// number of missing pixels on left side
int leftExtraPixels = (extraSize + centeringPolicy) / 2;
// account for the fact that the offset is based on the center pixel, not the left edge
return correction - leftExtraPixels;
}
[@endcode]
- Parameters:
sourceImages - The list of source images to be usedsourceStates - The list of source states to be usedkernel - The MPSKernel the padding method will be applied to. Set the kernel.offsetinDescriptor - MPS will prepare a starting guess based on the padding policy (exclusive of
MPSNNPaddingMethodCustom) set for the object. You should adjust the offset
and image size accordingly. It is on an autoreleasepool.- Returns:
- The MPSImageDescriptor to use to make a MPSImage to capture the results from the filter.
The MPSImageDescriptor is assumed to be on an autoreleasepool. Your method must also set the
kernel.offset property.