MPSCNNPoolingAverageGradient(3) 			 MetalPerformanceShaders.framework			   MPSCNNPoolingAverageGradient(3)

NAME

       MPSCNNPoolingAverageGradient

SYNOPSIS

       #import <MPSCNNPooling.h>

       Inherits MPSCNNPoolingGradient.

   Instance Methods
       (nonnull instancetype) - initWithDevice:kernelWidth:kernelHeight:strideInPixelsX:strideInPixelsY:
       (nullable instancetype) - initWithCoder:device:

   Properties
       NSUInteger zeroPadSizeX
       NSUInteger zeroPadSizeY

   Additional Inherited Members
Detailed Description
       This depends on Metal.framework	Specifies the filter for computing the gradient of the average pooling filter. The operation
       backpropagates a gradient vector using chain rule.

       Average pooling forward pass is defined as:

       out(x) = sum_{dx in Window(x)} in(s*x+dx) / N(x), where

       the pooling window definition 'Window(x)' follows MPSCNNPooling specification, 'N(x)' is effective pooling window size in pixels as
       specified in MPSCNNPoolingAverage, 's' is the pixel stride and in() is the source input image.

       Hence the partial derivative of the output value wrt. to the input value needed in the gradient backpropagation in MPSCNNPoolingGradient
       is:

       d out(x)/d in(y) = sum_{dx in Window(x)} delta_{s*x+dx, y} / N(x), where

       delta_{x,y} is the Kronecker delta symbol for which

       delta_{x,y} =  {  1, when x == y
		      {  0, otherwise.

       In practice this means that the gradient value for the destination image at pixel 'x' is the sum over these contributions coming from all
       pooling windows that contribute to the average pooling computation in the forward pass, multiplied by the input gradient value in the
       source area of the corresponding pooling window.

       Note: As average pooling is a linear operation of its inputs, the gradient does not depend at all on the original input values, but the
       original input image size is needed so that we know the limits where the input values seize to exist to inhibit accumulation of gradient
       values for those pixels. Therefore, as secondary input, any correctly sized image will produce correct results for the gradient
       backpropagation and hence it is recommended to use a temporary image of correct size (see MPSTemporaryImage) for the secondary source image
       parameter.

Method Documentation
   - (nullable instancetype) initWithCoder: (NSCoder *__nonnull) aDecoder(nonnull id< MTLDevice >) device
       NSSecureCoding compatability  See MPSKernel::initWithCoder.

       Parameters:
	   aDecoder The NSCoder subclass with your serialized MPSCNNPoolingAverageGradient
	   device The MTLDevice on which to make the MPSCNNPoolingAverageGradient

       Returns:
	   A new MPSCNNPoolingAverageGradient object, or nil if failure.

       Reimplemented from MPSCNNPoolingGradient.

   - (nonnull instancetype) initWithDevice: (nonnull id< MTLDevice >) device(NSUInteger) kernelWidth(NSUInteger) kernelHeight(NSUInteger)
       strideInPixelsX(NSUInteger) strideInPixelsY
       Initialize a gradient average pooling filter

       Parameters:
	   device The device the filter will run on
	   kernelWidth The width of the kernel. Can be an odd or even value.
	   kernelHeight The height of the kernel. Can be an odd or even value.
	   strideInPixelsX The input stride (upsampling factor) in the x dimension.
	   strideInPixelsY The input stride (upsampling factor) in the y dimension.

       Returns:
	   A valid MPSCNNPoolingGradient object or nil, if failure.

       Reimplemented from MPSCNNPoolingGradient.

Property Documentation
   - zeroPadSizeX [read],  [write],  [nonatomic],  [assign]
       How much zero padding to apply to both left and right borders of the input image for average pooling, when using

       See also:
	   edgeMode MPSImageEdgeModeClamp. For

	   edgeMode MPSImageEdgeModeZero this property is ignored and the area outside the image is interpreted to contain zeros. The zero padding
	   size is used to shrink the pooling window to fit inside the area bound by the source image and its padding region, but the effect is
	   that the normalization factor of the average computation is computed also for the zeros in the padding region.

   - zeroPadSizeY [read],  [write],  [nonatomic],  [assign]
       How much zero padding to apply to both top and bottom borders of the input image for average pooling, when using

       See also:
	   edgeMode MPSImageEdgeModeClamp. For

	   edgeMode MPSImageEdgeModeZero this property is ignored and the area outside the image is interpreted to contain zeros. The zero padding
	   size is used to shrink the pooling window to fit inside the area bound by the source image and its padding region, but the effect is
	   that the normalization factor of the average computation is computed also for the zeros in the padding region.

Author
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Version MetalPerformanceShaders-100				  Thu Feb 8 2018				   MPSCNNPoolingAverageGradient(3)