Histogram Equalisation creating pixelated images

Hi there,

I am using histeq to transform a set of images. However, while some of the images are transformed fine, a fair few are looking too pixelated. I will input an example below. First, here is my code:

filenames = dir(fullfile(Stimuli, '*.jpg')); 

for k = 1:numel(filenames)
  images   = filenames(k).name;
  equalHist     = imread(images);
  gsImg      = histeq(equalHist, 255);
  [~,name,~] = fileparts(filename);
  gsFilename = sprintf('%s_histeq.jpg', name);
  imwrite(gsImg,gsFilename);
end

This is how some of the images are looking. E.g. edges are pixelated.

I have tried adjusting the intensity availibility , e.g. 255, 127, etc, but the problem persists.

Any ideas welcomed!

Thanks

[DGM]

Two things:

• your images are integer data
• your images are lossy-compressed JPG

What happens when you take a roughly-quantized shallow gradient and stretch its range?

A = imread('shallowgrad.jpg'); % a shallow JPG gradient
A = rgb2gray(A);
imshow(A)

B = imadjust(A); % stretch the range
imshow(B)

IA is right about not using histeq(), but the problem is more fundamental. Images have limited information. They look grossly quantized because they are. Regardless of the method, extreme contrast changes will emphasize the deficiencies of the source image.

[jlouise2022]

Thanks for the explanation!

Answer 1

[Image Analyst]

Yes, you've learned that histogram equalized images look crappy. It's not surprising. So just don't do it. There is no need to. There are ways to get it to look less posterized (by adding a half gray level of noise) but they will still look bad so don't bother.

[jlouise2022]

Haha... thanks, well noted.