Operator Reference

char_thresholdchar_thresholdCharThresholdCharThresholdchar_threshold (Operator)

char_thresholdchar_thresholdCharThresholdCharThresholdchar_threshold — Perform a threshold segmentation for extracting characters.

Signature

char_threshold(Image, HistoRegion : Characters : Sigma, Percent : Threshold)

Description

The main application of char_thresholdchar_thresholdCharThresholdCharThresholdchar_threshold is to segment single-channel images of dark characters on bright paper. The operator works as follows: First, a histogram of the gray values in the image ImageImageImageimageimage is computed for the points in the region HistoRegionHistoRegionHistoRegionhistoRegionhisto_region{}. To eliminate noise, the histogram is smoothed with the given SigmaSigmaSigmasigmasigma (Gaussian smoothing). In the histogram, the background (white paper) corresponds to a large peak at high gray values, while the characters form a small peak at low gray values. In contrast to the operator binary_thresholdbinary_thresholdBinaryThresholdBinaryThresholdbinary_threshold (with MethodMethodMethodmethodmethod='smooth_histo'"smooth_histo""smooth_histo""smooth_histo""smooth_histo"), which locates the minimum between the two peaks, here the threshold for the segmentation is determined in relation to the maximum of the histogram, i.e., the background, with the following condition: histogram[threshold] * 100.0 < histogram[maximum] * (100.0 - PercentPercentPercentpercentpercent)

For example, if you choose PercentPercentPercentpercentpercent = 95 the operator locates the gray value whose frequency is at most 5 percent of the maximum frequency. Because char_thresholdchar_thresholdCharThresholdCharThresholdchar_threshold assumes that the characters are darker than the background, the threshold is searched for “to the left” of the maximum.

In comparison to binary_thresholdbinary_thresholdBinaryThresholdBinaryThresholdbinary_threshold, this operator should be used if there is no clear minimum between the histogram peaks corresponding to the characters and the background, respectively, or if there is no peak corresponding to the characters at all. This may happen, e.g., if the image contains only few characters or in the case of a non-uniform illumination.

Execution Information

Multithreading type: reentrant (runs in parallel with non-exclusive operators).
Multithreading scope: global (may be called from any thread).
Automatically parallelized on tuple level.
Automatically parallelized on internal data level.

Parameters

ImageImageImageimageimage (input_object) singlechannelimage(-array) → object (byte)

Input image.

HistoRegionHistoRegionHistoRegionhistoRegionhisto_region (input_object) region → object

Region in which the histogram is computed.

CharactersCharactersCharacterscharacterscharacters (output_object) region(-array) → object

Dark regions (characters).

SigmaSigmaSigmasigmasigma (input_control) number → (real)

Sigma for the Gaussian smoothing of the histogram.

Default: 2.0

Suggested values: 0.0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0

Value range: 0.0 ≤ Sigma Sigma Sigma sigma sigma ≤ 50.0 (lin)

Minimum increment: 0.01

Recommended increment: 0.2

PercentPercentPercentpercentpercent (input_control) number → (real / integer)

Percentage for the gray value difference.

Default: 95

Suggested values: 90, 92, 95, 96, 97, 98, 99, 99.5, 100

Value range: 0.0 ≤ Percent Percent Percent percent percent ≤ 100.0 (lin)

Minimum increment: 0.1

Recommended increment: 0.5

ThresholdThresholdThresholdthresholdthreshold (output_control) integer(-array) → (integer)

Calculated threshold.

Example (HDevelop)

read_image (Image, 'letters')
char_threshold (Image, Image, Seg, 0.0, 5.0, Threshold)
connection (Seg, Connected)

Example (C)

read_image(&Image,"letters");
char_threshold(Image,Image,&Seg,0.0,5.0,&Threshold);
connection(Seg,&Connected);
set_colored(WindowHandle,12);
set_shape(WindowHandle,"rectangle1");
disp_region(Connected,WindowHandle);

Example (C++)

#include "HIOStream.h"
#if !defined(USE_IOSTREAM_H)
using namespace std;
#endif
#include "HalconCpp.h"
using namespace Halcon;

int main (int argc, char *argv[])
{
  if (argc != 2)
  {
    cout << "Usage : " << argv[0] << " 'image' " << endl;
    return -1;
  }

  HImage       image(argv[1]);
  HWindow      w;
  HTuple       threshold;

  w.SetDraw("margin");
  w.SetShape("rectangle1");
  w.SetColored(12);

  image.Display(w);

  HRegionArray reg = image.CharThreshold(image,0,5,&threshold);
  HRegionArray con = reg.Connection();

  cout << "Display image after CharThreshold segmentation " << endl;
  con.Display(w);
  w.Click();

  return 0;
}

Example (HDevelop)

read_image (Image, 'letters')
char_threshold (Image, Image, Seg, 0.0, 5.0, Threshold)
connection (Seg, Connected)

Possible Predecessors

anisotropic_diffusionanisotropic_diffusionAnisotropicDiffusionAnisotropicDiffusionanisotropic_diffusion, median_imagemedian_imageMedianImageMedianImagemedian_image, illuminateilluminateIlluminateIlluminateilluminate

Possible Successors

connectionconnectionConnectionConnectionconnection, select_shapeselect_shapeSelectShapeSelectShapeselect_shape, select_grayselect_graySelectGraySelectGrayselect_gray

Alternatives

binary_thresholdbinary_thresholdBinaryThresholdBinaryThresholdbinary_threshold, auto_thresholdauto_thresholdAutoThresholdAutoThresholdauto_threshold, gray_histogray_histoGrayHistoGrayHistogray_histo, smooth_funct_1d_gausssmooth_funct_1d_gaussSmoothFunct1dGaussSmoothFunct1dGausssmooth_funct_1d_gauss, thresholdthresholdThresholdThresholdthreshold

Module

Foundation

Operators