FED

Description

Performs edge detection filtering on image data. The edge detection filter creates an image where edges (sharp changes in gray-level values) are emphasized.

Parameters

Name	Type	Length	Value range
Input: Input raster channel *	Raster port	1 - 1
Output: Output raster channel *	Raster port	1 - 1
Filter X Size	Integer	1 - 1	1 - 33 Default: 3
Filter Y Size	Integer	1 - 1	1 - 33 Default: 3
Mask	Integer	0 - 4	Xoffset, Yoffset, Xsize, Ysize
Gain Factor	Float	0 - 1	0.000001 - Default: 1.0

* Required parameter

Parameter descriptions

Input: Input raster channel

Specifies the input image channel to be filtered.

Output: Output raster channel

Specifies the output channel to receive the filtered result.

The output channel can be the same as the input channel (DBIC). If an area mask is specified, only the area under the bitmap mask (MASK) is written to the output channel.

Filter X Size

Specifies the width (X) of the filter, in pixels.

The filter need not be square; Pixel X Size can be different from Pixel Y Size. Both values must be odd integers between 1 and 33, inclusively. The default filter size is 3x3.

Filter Y Size

Specifies the width (X) of the filter, in pixels.

The filter need not be square; Pixel X Size can be different from Pixel Y Size. Both values must be odd integers between 1 and 33, inclusively. The default filter size is 3x3.

Mask

Specifies the bitmap that defines the area to be processed within the input raster.

If no value is specified, the entire channel is processed.

Gain Factor

Specifies a gain multiplication factor to be applied to the input channel.

Values of 0.0 or negative real numbers are not allowed. The default value is 1.0.

Details

FED performs edge detection filtering on image data. The edge detection filter creates an image where edges (sharp changes in gray-level values) are emphasized. FED computes the mean of the absolute difference between the gray-level value at the central pixel and at each of its neighbors within the filter window. The dimensions of the filter window must be odd. The minimum filter size is 1 x 3, with a maximum allowable size of 33 x 33. The filter window does not need to be square.

Edge detection filtering produces an image where higher gray-level values indicate the presence of an edge between two objects. The user has the option of applying a factor to each input channel before averaging. A gain factor of 1.0 (default) implies no gain factor.

The MASK parameter specifies the area within the input channel to be processed. Only the area under mask will be filtered; the rest of the image will be unchanged.

If no mask is specified, the entire image is processed by default.

Algorithm

The edge detection filter computes the mean of the absolute difference between the gray-level value at the central pixel and at each of its neighbors within the filter window.

All pixels are filtered. To filter pixels located near the edges of the image, edge pixel values are replicated to provide sufficient data.

         +---------+
         | a1 a2 a3|
         | a4 CT a5|     <---   Filter window 3 X 3
         | a6 a7 a8|
         +---------+

The edge detection filter finds edge pixel values in the filter window. The filtered pixel is the sum of the absolute differences between the center pixel (CT) and the surrounding pixels in the filter window, divided by the number of pixels surrounding the central pixel (8).

filtered pixel CT = SUM (ABS(CT-a1) +...+ ABS(CT-a8)) / 8

The following is an example of using a 5x5 filter on an 8x8 image, with a gain factor of 1.0:

   Image before filtering           Image after filtering

   8 8  8 9 9 9 7 6 6 6   6 6
   8 8  8 9 9 9 7 6 6 6   6 6
       +----------------+           +----------------+
   8 8 |8 9 9 9 7 6 6 6 | 6 6       |1 2 3 4 4 2 0 0 |
   8 8 |8 9 9 7 6 6 6 6 | 6 6       |2 2 4 4 3 1 0 0 |   
   9 9 |9 8 8 6 6 6 6 6 | 6 6       |3 3 3 4 2 1 0 0 |
   9 9 |9 8 7 7 6 5 6 6 | 6 6       |4 3 3 3 1 3 0 0 |
   7 7 |7 7 7 6 6 6 6 6 | 6 6       |3 3 3 1 1 0 0 0 |
   6 6 |6 6 6 6 6 6 6 6 | 6 6       |2 2 1 1 1 0 0 0 |   
   6 6 |6 6 6 6 6 6 6 5 | 5 5       |1 1 0 0 0 0 0 3 |
   6 6 |6 6 6 6 6 6 6 6 | 6 6       |0 0 0 0 0 0 0 0 |
       +----------------+           +----------------+        
   6 6  6 6 6 6 6 6 6 6   6 6      
   6 6  6 6 6 6 6 6 6 6   6 6

In the "Image before filtering" on the left, columns and scanlines outside the box represent the last image column or scanline being reused, for the case where the filter does not have a full window.