Instance Methods

(MPSNNPaddingMethod) - paddingMethod
(NSString *__nonnull) - label
(MPSImageDescriptor *__nonnull) - destinationImageDescriptorForSourceImages:sourceStates:forKernel:suggestedDescriptor:
(instancetype __nullable) - inverse

- (MPSImageDescriptor * __nonnull MPSNNPadding) destinationImageDescriptorForSourceImages: (NSArray< MPSImage * > *__nonnull) sourceImages(NSArray< MPSState * > *__nullable) sourceStates(MPSKernel *__nonnull) kernel(MPSImageDescriptor *__nonnull) inDescriptor [optional]

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:

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;
}

Parameters:

Returns:

- (instancetype __nullable MPSNNPadding) inverse [optional]

Make a 'inverted' padding policy suitable for a training gradient pass.

- (NSString*__nonnull MPSNNPadding) label [optional]

A human readable string that describes the padding policy. Useful for verbose debugging support.

- (MPSNNPaddingMethod MPSNNPadding) paddingMethod [required]

Get the preferred padding method for the node