Write a program to change brightness and change the contrast of the color image (keep the average brightness of the image). I don't understand how to keep the average brightness of the image. thank you!!!

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"Write a program to change brightness and change the contrast of the color image (keep the average brightness of the image)."
I don't understand how to keep the average brightness of the image. thank you!!!

Accepted Answer

Walter Roberson
Walter Roberson on 24 Sep 2017
Edited: Walter Roberson on 24 Sep 2017
The way to keep the average brightness of an image is:
  1. calculate the average brightness of the original image; Call this BRorig
  2. make whatever local changes to color and contrast;
  3. calculate the average brightness of the revised image; Call this BRnew
  4. multiply the revised image by the ratio BRorig/BRnew
  5 Comments
Walter Roberson
Walter Roberson on 24 Sep 2017
You could also calculate the total brightness of the original image (instead of the average) and the total brightness of the modified image, and then pick random locations to add increments to, taking into account that brightness = 0.2989 * R + 0.5870 * G + 0.1140 * B

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More Answers (2)

Image Analyst
Image Analyst on 24 Sep 2017
Edited: Image Analyst on 24 Sep 2017
I'd use a different approach. You can use rgb2hsv to convert to HSV color space. Now, it really depends on what your teacher means by contrast. One way is to increase the brightness contrast and to do that you'd subtract the mean from the V channel, multiply by some value, add back in the original mean, and then transform back:
hsvImage = rgb2hsv(rgbImage);
hChannel = hsvImage(:, :, 1);
sChannel = hsvImage(:, :, 2);
vChannel = hsvImage(:, :, 3);
meanV = mean2(vChannel);
newV = meanV + 1.5 * (vChannel - meanV); % Increase contrast by factor of 1.5
newHSVImage = cat(3, hChannel, sChannel, newV);
newRGBImage = hsv2rgb(newHSVImage);
The other thing they might mean is to increase the saturation so the colors look more vivid. In that case, you'd just multiply the S channel by some factor.
hsvImage = rgb2hsv(rgbImage);
hChannel = hsvImage(:, :, 1);
sChannel = hsvImage(:, :, 2);
vChannel = hsvImage(:, :, 3);
newS = 1.5 * sChannel; % Increase contrast by factor of 1.5
newHSVImage = cat(3, hChannel, newS, vChannel);
newRGBImage = hsv2rgb(newHSVImage);
Of course, clipping may occur if you go back to uint8 (as John said).
Or you could change both the V and S channels.
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Rodrigo Souza
Rodrigo Souza on 28 Jan 2019
Hi all,
I have put together three scripts/functions for calculating (mean and SD), matching or normalizing luminance of colored images (using HSV and CIE Lab color spaces).
They may be specially useful for pupillometry measures of infant research.
The scripts and their descriptions are available at my OSF page: https://osf.io/auzjy/
Hope it helps.

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