Infinite loop in enhanceContrastLinearly

[LukeDeWaal]

Line 96 in imgproc.cpp there is a loop:

for (first = 0; first < src_hist->range; first++)

which will run forever for some images while not for others. Why it fails only with certain images is unknown.
It fails with the attached image as an example.

My best guess is that this is caused by the fact that the variable first is an unsigned char, and src_hist->range is set to 256, so when updating the variable in the for loop the unsigned char loops back to 0, causing an infinite loop.

A possible fix would be to set the looping condition to first < src_hist->range-1 instead.

[matthijsr]

This appears to be caused by using

  // Determine the threshold from the histogram, by taking 10% of the maximum value in the histogram.
  auto threshold = (int) (hist->max(0) * 0.1);

In water.cpp, which claims to get the max value of the histogram, but actually only obtains the max value of the 0 (red) channel.
A quick solution is:

//auto threshold = (int) (hist->max(0) * 0.1);
  auto threshold0 = (int) (hist->max(0) * 0.1);
  auto threshold1 = (int) (hist->max(1) * 0.1);
  auto threshold2 = (int) (hist->max(2) * 0.1);
...
//enhanceContrastLinearly(previous, hist, img_enhanced.get(), threshold, threshold, 0);
//enhanceContrastLinearly(previous, hist, img_enhanced.get(), threshold, threshold, 1);
//enhanceContrastLinearly(previous, hist, img_enhanced.get(), threshold, threshold, 2);
  enhanceContrastLinearly(previous, hist, img_enhanced.get(), threshold0, threshold0, 0);
  enhanceContrastLinearly(previous, hist, img_enhanced.get(), threshold1, threshold1, 1);
  enhanceContrastLinearly(previous, hist, img_enhanced.get(), threshold2, threshold2, 2);