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divfilter

Apply a spatial high pass filter to a cube (divide)

This program divides the pixel being processed by a low pass filter. This has the general effect of highlighting the edges of grouped contrasted DN values in the image, and therefore, feature changes will be emphasized. Predominant changes will be emphasized with larger boxcars, and smaller changes can be searched for using smaller boxcars.

This is done by convolving an NxM boxcar through the data, where N and M are odd integers. In other words, the average DN value of the boxcar is divided into its middle pixel DN value. Please refer to the following diagram:

Thus, P(output) = P(input) / lowPassFilter, or P(s,l) = P(s,l) / average(s,l,N,M) where P is the target pixel, s and l are the sample and line position in the boxcar, N and M are the size of the boxcar (in this case 3x3) and average(s,l,N,M) is the average of the NxM centered box.

For details on the box filtering see the following references:

      M. J. McDonnell, Box-Filtering Techniques, Computer Graphics
      and Image Processing, Vol. 17, 1981, pages 65-70

      E. M. Eliason and L. A. Soderblom, An Array Processing System
      for Lunar Geochemical and Geophysical Data,
      Porc. Lunar Sci. Conf. (8th) , 1977, pages 1163-1170.

Related Applications to Previous Versions of ISIS

This program replaces the following application existing in previous versions of ISIS:

boxfilter

History

Eric Eliason	1988-05-20	Original version
Tracie Sucharski	2002-08-12	Ported to Isis 3.0
K Teal Thompson	2003-04-10	Add example.
Kim Sides	2003-05-13	Added application test
Stuart Sides	2003-05-16	Modified schema location from astogeology... to isis.astrogeology..."
Jeff Anderson	2003-06-03	Updated application test to fix errors
Stuart Sides	2003-07-29	Modified filename parameters to be cube parameters where necessary
Brendan George	2006-03-20	Added Minopt parameter
Jac Shinaman	2007-02-22	Updated xml documentation with examples and new GUI configuration

Parameters

Files

Input cube to filter

Type	cube
File Mode	input
Filter	*.cub

The resultant filtered cube

Type	cube
File Mode	output
Pixel Type	real

Boxcar Size

This is the total number of samples in the boxcar. It must be odd and can not exceed twice the number of samples in the cube. In general, due to the computational algorithm, the size of the boxcar does not cause the program to operate significantly slower. (See Eliason/Soderblom document)

Type	integer
Minimum	1 (inclusive)
Odd	This value must be an odd number

This is the total number of lines in the boxcar. It must be odd and can not exceed twice the number of lines in the cube. In general, due to the computational algorithm, the size of the boxcar does not cause the program to operate significantly slower. (See Eliason/Soderblom document)

Type	integer
Minimum	1 (inclusive)
Odd	This value must be an odd number

Boxcar Restrictions

Valid minimum pixel value that will be used in boxcar computation. If a pixel value is less than LOW then it will not be used when computing boxcar statistics.

Type	double
Internal Default	Use all pixels
Less Than	HIGH

Valid maximum pixel value that will be used in boxcar computation. If a pixel value is greater than HIGH then it will not be used when computing boxcar statistics.

Type	double
Internal Default	Use all pixels
Greater Than	LOW

This parameter determines whether the minimum parameter is to be interpreted as a count of pixels, or a percentage of all pixels in the boxcar.

Type

string

Default

COUNT

Option List:

Option	Brief	Description
COUNT	Minimum valid pixel count	This option indicates that the minimum is a minimum count of pixels required for filtering.
PERCENTAGE	Minimum valid pixel percentage	This option indicates the the minimum parameter is to be interpreted as a percentage of the total pixels in the boxcar.

This is the minimum number of valid pixels which must occur inside the NxM boxcar for filtering to occur. For example, 3x5 boxcar has 15 pixels inside. If MINIMUM=10 then the filter will occur if there are 10 or greater valid pixels. A valid pixel is one that is not special (NULL, LIS, etc) and is in the range defined by LOW to HIGH.

Type	integer
Default	1
Minimum	1 (inclusive)

Special Pixels

This option is used to define how special pixels are handled. If the center pixel of the boxcar(input) is a special pixel, the output pixel will contain the same special pixel, or set to NULL depending on the value of this parameter.

Type	boolean
Default	TRUE

Example 1

101x101

Use a 101x101 boxcar

Command Line

In this example the boxcar size is 101 by 101.

divfilter  f=peaks.cub:4 t=divfilter s=101 li=101

GUI Screenshot

Example Gui

Divfilter Gui

Screenshot of GUI with parameters filled in to perform a divide filter of the input image with a boxcar of 101x101.

Input Image

Input image for divfilter

Input image

Parameter Name: FROM

This is the input image for the divfilter example.

Output Image

Output image for divfilter

Output image showing results of the divfilter application.

Parameter Name: TO

This is the output image for this example of divfilter. It is the same as running lowpass and dividing the input image by the lowpass output using the ratio application.