dual_threshold

dual_threshold ( Image : RegionCrossings : MinSize, MinGray, Threshold : )

Threshold operator for signed images.

dual_threshold segments the input image into a region with gray values >= Threshold (``positive'' regions) and a region with gray values <= -Threshold (``negative'' regions). ``Positive'' or ``negative'' regions having a size of less than MinSize are suppressed, as well as regions whose maximum gray value is less than MinGray in absolute value.

The segmentation performed is not complete, i.e., the ``positive'' and ``negative'' regions together do not necessarily cover the entire image: Areas with a gray value between -Threshold and Threshold, -MinGray and MinGray, respectively, are not taken into account.

dual_threshold is usually called after applying a Laplace operator (laplace, laplace_of_gauss, derivate_gauss or diff_of_gauss) to an image or with the difference of two images (sub_image).

The zero crossings of a Laplace image correspond to edges in an image, and are the separating regions of the ``positive'' and ``negative'' regions in the Laplace image. They can be determined by calling dual_threshold with Threshold = 1 and then creating the complement regions with complement. The parameter MinGray determines the noise invariance, while MinSize determines the resolution of the edge detection.

Using byte images, only the positive part of the operator is applied. Therefore dual_threshold behaves like a standard threshold operator (threshold) with successive connection and select_gray.

Parameters

`Image` (input_object)	image(-array) `->` object : byte / int2 / int4 / real
Input image.

`RegionCrossings` (output_object)	region-array `->` object
Positive and negative regions.

`MinSize` (input_control)	integer `->` integer
Regions smaller than MinSize are suppressed.
Default value: 20
Suggested values: 0, 10, 20, 50, 100, 200, 500, 1000
Typical range of values: `0 <= MinSize <= 10000` (lin)
Minimum increment: 1
Recommended increment: 10

`MinGray` (input_control)	real `->` real
Regions whose maximum absolute gray value is smaller than MinGray are suppressed.
Default value: 5.0
Suggested values: 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 9.0, 11.0, 15.0, 20.0
Typical range of values: `0.001 <= MinGray <= 10000.0` (lin)
Minimum increment: 1.0
Recommended increment: 10.0
Restriction: `MinGray > 0`

`Threshold` (input_control)	real `->` real
Regions that have a gray value smallerr than Threshold (or larger than -Threshold) are suppressed.
Default value: 2.0
Suggested values: 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 9.0, 11.0, 15.0, 20.0
Typical range of values: `0.001 <= Threshold <= 10000.0` (lin)
Minimum increment: 1.0
Recommended increment: 10.0
Restriction: `(Threshold >= 1) && (Threshold <= MinGray)`

Example

/* Edge detection with the Laplace operator (and edge thinning) */
diff_of_gauss(Image,Laplace,2.0,1.6) 
/* find "`positive"' and "`negative"' regions: */
dual_threshold(Laplace,Region,20,2,1) 
/*The zero runnings are the complement to these image section: */
complement('full',Region,Nulldurchgaenge).

/* Simulation of \OpRef{dual_threshold} */
dual_threshold(Laplace,Result,MinS,MinG,Threshold):
  threshold(Laplace,Tmp1,Threshold,999999) 
  connection(Tmp1,Tmp2) 
  select_shape(Tmp2,Tmp3,'area','and',MinS,999999) 
  select_gray(Laplace,Tmp3,Tmp4,'max','and',MinG,999999) 
  threshold(Laplace,Tmp5,-999999,-Threshold) 
  connection(Tmp5,Tmp6) 
  select_shape(Tmp6,Tmp7,'area','and',MinS,999999) 
  select_gray(Laplace,Tmp7,Tmp8,'min','and',-999999,-MinG) 
  concat_obj(Tmp4,Tmp8,Result).

Result

dual_threshold returns 2 (H_MSG_TRUE) if all parameters are correct. The behavior 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.

Parallelization Information

dual_threshold is reentrant and automatically parallelized (on tuple level).

Possible Predecessors

min_max_gray, sobel_amp, gauss_image, reduce_domain, diff_of_gauss, sub_image, derivate_gauss, laplace_of_gauss, laplace, expand_region

Possible Successors

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

Alternatives

threshold, dyn_threshold, check_difference

Module

Foundation