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GradientDirAndPresenceImage

Header:	FIL.h
Namespace:	fil
Module:	FoundationBasic

Computes an image of gradient directions mapped to the range from 1 to 255. Zero means "no edge".

Applications: For highly optimized analysis of gradient directions.

Syntax

void fil::GradientDirAndPresenceImage
(
	const fil::Image& inImage,
	ftl::Optional<const fil::Region&> inRoi,
	fil::GradientMaskOperator::Type inOperator,
	float inEdgeThreshold,
	fil::AngleRange::Type inAngleRange,
	fil::Image& outDirectionsImage
)

Parameters

Name	Type	Range	Default	Description
inImage	const Image&			Input image
inRoi	Optional<const Region&>		NIL	Range of output pixels to be computed
inOperator	GradientMaskOperator::Type			Defines how the gradient is computed
inEdgeThreshold	float	0.01 -	10.0f	Minimum edge magnitude (other pixels will be set to 0)
inAngleRange	AngleRange::Type		_0_360	Range of output angles
outDirectionsImage	Image&

Description

The operation computes the angle of the intensity change direction at each pixel of the inImage. Firstly the selected inOperator is used to obtain two-dimensional gradient vector at each pixel. When this vector length exceeds inEdgeThreshold the angle of the vector is calculated, scaled and stored in outDirectionsImage pixel.

This operation always generates a single-channel image with the uint8 pixel type on the output, regardless of the input image format, with following pixel values:

When gradient length does not exceed threshold, the value of 0 is stored in pixel.
When gradient length exceeds threshold, its angle is scaled to range 0..255 and stored in pixel value.

When a pixel value is non-zero, one can restore the original angle with the formula:

	double angle = value * 360.0 / 255.0;   // if inAngleRange == _0_360
	double angle = value * 180.0 / 255.0;   // if inAngleRange == _0_180
	double angle = value * 90.0 / 255.0;    // if inAngleRange == _0_90

To measure the angular distance between two directions described by two non-zero pixel values, use the following formulas:

    int valueDif = abs(inValue1 - inValue2);
	double angleDif = (valueDif < 128 ? (double)valueDif : (255.0 - valueDif)) * 360.0 / 255.0;  // if inAngleRange == _0_360
	double angleDif = ((double)valueDif) * 180.0 / 255.0;                                        // if inAngleRange == _0_180
	double angleDif = ((double)valueDif) * 90.0 / 255.0;                                         // if inAngleRange == _0_90

Hardware Acceleration

This operation is optimized for SSE41 technology for pixels of type: UINT8.

This operation is optimized for AVX2 technology for pixels of type: UINT8.

This operation is optimized for NEON technology for pixels of type: UINT8.

This operation supports automatic parallelization for multicore and multiprocessor systems.

Errors

List of possible exceptions: