fast_threshold ( Image : Region : MinGrey, MaxGrey, MinHeight : )

Fast selection of greyvalues within a given grey intervall.

fast_threshold selects the pixels from the input image whose gray values g fulfill the following condition:

             MinGrey <= g <=  MaxGrey .
To reduce procesing time, the selection is done in two steps: At first all pixels along rows with distances MinHeight are processed. In the next step the neighborhood (size MinHeight @x MinHeight) of all previously selected points are processed.


Parameters

Image (input_object)
image(-array) -> object : byte / direction / cyclic
Image to be thresholded.

Region (output_object)
region(-array) -> object
Regions with gray values lying in the specified interval.

MinGrey (input_control)
number -> real / integer
Lower threshold for the gray values.
Default value: 128
Suggested values: 0.0, 10.0, 30.0, 64.0, 128.0, 200.0, 220.0, 255.0
Range of values: 0.0 <= MinGrey <= 255.0 (lin)
Minimum increment: 1
Recommended increment: 5.0

MaxGrey (input_control)
number -> real / integer
Upper threshold for the gray values.
Default value: 255.0
Suggested values: 0.0, 10.0, 30.0, 64.0, 128.0, 200.0, 220.0, 255.0
Range of values: 0.0 <= MaxGrey <= 255.0 (lin)
Minimum increment: 1
Recommended increment: 5.0

MinHeight (input_control)
number -> integer
Minimum height of objects to be extracted.
Default value: 20
Suggested values: 5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 100
Range of values: 2 <= MinHeight <= 200 (lin)
Minimum increment: 1
Recommended increment: 2


Complexity

Let F be the area of the ouput region and height the height of Image. Then the runtime complexity is O(F + height / MinHeight).


Result

fast_threshold returns TRUE if all parameters are correct. The behaviour with respect to the input images and output regions can be determined by setting the values of the flags 'no_object_result', 'empty_region_result', and 'store_empty_region' with set_system. If necessary, an exception is raised.


Possible Predecessors

histo_to_thresh, min_max__, sobel_amp, gauss__, reduce_domain, fill_interlace__


Possible Successors

connection, dilation1, erosion1, opening, closing, count, shape_trans, skeleton


Alternatives

threshold__


See also

class_2dim__, hysteresis_threshold__, dyn_threshold__



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