EDGE

Description

EDGE applies an edge detection filter for image data. The output is an image showing the magnitude of edge pixels. An optional gradient direction image can also be created.

Parameters

Name	Type	Length	Value range
Input: Input raster channel *	Raster port	1 - 1024
OutputMag: Gradient magnitude output channel	Raster port	0 - 1024	1 -
Filter Radius	Integer	0 - 1	1 - Default: 5
Report	String	0 - 192	See parameter description

* Required parameter

Parameter descriptions

Input: Input raster channel

Contains the input unfiltered channel(s).

OutputMag: Gradient magnitude output channel

Specifies the output channel(s) to receive the output magnitude channel(s).

If the output file (FILO) is an existing file, the output magnitude channel (DBOC_MAG) type must be CHN_8U and the number of output channels must be the same as the number of input channels (DBIC). If the output file is created, this parameter is ignored.

Filter Radius

Specifies the size of the Gaussian filter's radius, in pixels. The filter is applied as the first step of edge detection. The filter size determines the smallest feature to be detected. The default value is 5.

Report

Specifies where to direct the generated report.

Available options are:

• LOG: generates a report on the terminal (default)
• <modulename.RPT>: appends a report to the module's RPT file
• DISK: generates a report on file "IMPRPT.LST"

Details

EDGE applies a Canny Edge Detector filter. This process produces an output image where higher gray-level values indicate the presence of an edge between two objects. An optional gradient direction image can also be created.

For more information about the method used, see the Algorithm section.

Algorithm

In the PCI implementation, the Gaussian filter and gradient operator are decomposed into two 1-dimensional filters: a 1D Gaussian filter and a 1D DOG (difference of Gaussian) filter. One of these two 1D filters is applied to rows; the other is applied to the columns. The choice of which one to apply to the rows/columns provides either the X gradient or Y gradient.

When the gradients are computed, the magnitude at each pixel is compared to those of its neighbors. Along the gradient direction, if the gradients of neighbor pixels (computed via interpolation) are larger then the gradient of the current pixel (thus it is non-maximum), the gradient of the current pixel is suppressed (set to zero).

The output gradient magnitude represents edge strength. The output is scaled based on the dynamic range of the input image.

An optional output of gradient direction image can be created. Each pixel is in the range of [0,255], scaled from [0, 2*PI].