Filters

User-defined rank order and median filters

Standard rank order filters are MED3x3 and MED5x5; these filters sort 9 resp. 25 pixel values and assign the 5th resp. the 13th value (median value) to the center pixel in the output map. In the Filter dialog box as well as on the command line, you may define other rank order and median filters of any user-defined size and assigning any user-defined rank order value to the central pixel.

Using the Filter dialog box:

To use one of the standard rank order filters:

Open the Filter dialog box,
for Filter Type, select: Rank Order,
select the Predefined check box,
for Filter Name, select: Med3x3 or Med5x5.

To define your own rank order filter:

Open the Filter dialog box,
for Filter Type, select: Rank Order,
clear the Predefined check box,
specify the size of the filter,
specify the rank order value which should be assigned to the central pixel,
optionally specify a threshold: if the absolute difference between the result and the original value is larger than the threshold, the original value is kept; if the difference is smaller than or equal to the threshold, the ranked value is assigned.

This ensures that larger differences between neighbouring pixels remain in the output, while small variations are removed.

To define your own median filter:

Open the Filter dialog box,
for Filter Type, select: Median,
specify the size of the filter,
optionally specify a threshold.

See also the example below.

Using the command line (advanced):

To use a standard Rank order filter, type one of the following expressions on the command line of the Main window:

	OUTMAP	=	MapFilter(InputMapName, Med3x3)
	OUTMAP	=	MapFilter(InputMapName, Med5x5)

where:

	OUTMAP	is the name of your output map.
	MapFilter	is the command to start the Filter operation.
	InputMapName	is the name of your input map.
	Med3x3	is the name of the standard Median 3x3 filter.
	Med5x5	is the name of the standard Median 5x5 filter.

To define your own Rank order filter, type one of the following expressions on the command line of the Main window:

	OUTMAP	=	MapFilter(InputMapName, Rankorder(rows,cols,rank))
	OUTMAP	=	MapFilter(InputMapName, Rankorder(rows,cols,rank,threshold))

where:

	OUTMAP	is the name of your output map.
	MapFilter	is the command to start the Filter operation.
	InputMapName	is the name of your input map.
	Rankorder	is the command to define a rank order filter.
	rows	are the number of rows of your rank order filter.
	cols	are the number of columns of your rank order filter.
	rank	is the rank order number that determines the output value.
	threshold	when a threshold is set, the value of the center pixel will only be replaced by the new value if the difference between the original and new pixel value is smaller than or equal to the threshold.

To define your own Median filter, type one of the following expressions on the command line of the Main window:

	OUTMAP	=	MapFilter(InputMapName,Median(rows,cols))
	OUTMAP	=	MapFilter(InputMapName,Median(rows,cols,threshold))

where:

	OUTMAP	is the name of your output map.
	MapFilter	is the command to start the Filter operation.
	InputMapName	is the name of your input map.
	Median	is the command to define a median filter.
	rows	are the number of rows of your median filter.
	cols	are the number of columns of your median filter.
	rank	is the rank order number that determines the output value.
	threshold	when a threshold is set, the value of the center pixel will only be replaced by the new value if the difference between the original and new pixel value is smaller than or equal to the threshold.

Examples:

To obtain the minimum of each 3x3 neighbouring pixels:

in the Filter dialog box, select for the Filter type Rank Order,
clear the Predefined check box,
specify 3 for the number of rows and 3 for the number of columns,
specify 1 for the rank, and
optionally specify a threshold.
on the command line, use a filter definition of: Rankorder(3,3,1)

Rank 1 means the smallest value of each 9 (3 by 3) pixel values examined will be used for the output pixel.

To obtain the maximum of each 7 horizontally neighbouring pixels:

in the Filter dialog box,
select for the Filter type Rank Order
clear the Predefined check box,
specify 1 for the number of rows and 7 for the number of columns,
specify 7 for the rank, and
optionally specify a threshold.
on the command line, use a filter definition of: Rankorder(1,7,7)

Rank 7 means that the largest value of each 7 (1 by 7) pixel values examined is used as output value.

To obtain the median value of each 5 horizontally neighbouring pixels:

in the Filter dialog box,
select for the Filter type Median,
clear the Predefined check box,
specify 1 for the number of rows and 5 for the number of columns, and
optionally specify a threshold.
on the command line, use a filter definition of: Median(1,5).

Tips:

Both the number of rows and the number of columns defining the filter size must be odd values.
With Neighbourhood operators similar operations can be performed as with rank order filters: for example, the minimum, maximum, sum and predominant pixel value of each 9 pixels considered can be assigned as output value to the central pixel. For more information, refer to MapCalc special : neighbourhood operations.
With the Aggregate Map operation, similar results can be obtained. Aggregate Map however moves from block to block in the map, while the Filter operation moves pixel by pixel.