NAME

MPSCNNLocalContrastNormalization

SYNOPSIS

#import <MPSCNNNormalization.h>

Inherits MPSCNNKernel.

Instance Methods

(nonnull instancetype) - initWithDevice:kernelWidth:kernelHeight:
(nullable instancetype) - initWithCoder:device:
(nonnull instancetype) - initWithDevice:

Properties

float alpha
float beta
float delta
float p0
float pm
float ps

Detailed Description

This depends on Metal.framework Specifies the local contrast normalization filter. The local contrast normalization is quite similar to spatial normalization (see MPSCNNSpatialNormalization) in that it applies the filter over local regions which extend spatially, but are in separate feature channels (i.e., they have shape 1 x kernelWidth x kernelHeight), but instead of dividing by the local 'energy' of the feature, the denominator uses the local variance of the feature - effectively the mean value of the feature is subtracted from the signal. For each feature channel, the function computes the variance VAR(i,j) and mean M(i,j) of X(i,j) inside each rectangle around the spatial point (i,j).

Then the result is computed for each element of X as follows:

Y(i,j) = pm + ps * ( X(i,j) - p0 * M(i,j)) / (delta + alpha * VAR(i,j))^beta,

where kw and kh are the kernelWidth and the kernelHeight and pm, ps and p0 are parameters that can be used to offset and scale the result in various ways. For example setting pm=0, ps=1, p0=1, delta=0, alpha=1.0 and beta=0.5 scales input data so that the result has unit variance and zero mean, provided that input variance is positive. It is the end-users responsibility to ensure that the combination of the parameters delta and alpha does not result in a situation where the denominator becomes zero - in such situations the resulting pixel-value is undefined. A good way to guard against tiny variances is to regulate the expression with a small value for delta, for example delta = 1/1024 = 0.0009765625.

Method Documentation

- (nullable instancetype) initWithCoder: (NSCoder *__nonnull) aDecoder(nonnull id< MTLDevice >) device

NSSecureCoding compatability While the standard NSSecureCoding/NSCoding method -initWithCoder: should work, since the file can't know which device your data is allocated on, we have to guess and may guess incorrectly. To avoid that problem, use initWithCoder:device instead.

Parameters:

aDecoder The NSCoder subclass with your serialized MPSKernel
device The MTLDevice on which to make the MPSKernel

Returns:

A new MPSKernel object, or nil if failure.

Reimplemented from MPSCNNKernel.

- (nonnull instancetype) initWithDevice: (nonnull id< MTLDevice >) device

Standard init with default properties per filter type

Parameters:

device The device that the filter will be used on. May not be NULL.

Returns:

A pointer to the newly initialized object. This will fail, returning nil if the device is not supported. Devices must be MTLFeatureSet_iOS_GPUFamily2_v1 or later.

Reimplemented from MPSCNNKernel.

- (nonnull instancetype) initWithDevice: (nonnull id< MTLDevice >) device(NSUInteger) kernelWidth(NSUInteger) kernelHeight

Initialize a local contrast normalization filter

Parameters:

device The device the filter will run on
kernelWidth The width of the kernel
kernelHeight The height of the kernel

Returns:

A valid MPSCNNLocalContrastNormalization object or nil, if failure.

NOTE: For now, kernelWidth must be equal to kernelHeight

Property Documentation

- alpha [read], [write], [nonatomic], [assign]

The value of alpha. Default is 0.0 The default value 0.0 is not recommended and is preserved for backwards compatibility. With alpha 0, it performs a local mean subtraction. The MPSCNNLocalContrastNormalizationNode used with the MPSNNGraph uses 1.0 as a default.

- beta [read], [write], [nonatomic], [assign]

The value of beta. Default is 0.5

- delta [read], [write], [nonatomic], [assign]

The value of delta. Default is 1/1024

- p0 [read], [write], [nonatomic], [assign]

The value of p0. Default is 1.0

- pm [read], [write], [nonatomic], [assign]

The value of pm. Default is 0.0

- ps [read], [write], [nonatomic], [assign]

The value of ps. Default is 1.0

Author

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