Adjust contrast of image

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Hello!

I have been trying to make a piece of code, that will allow me to adjust the contrast of an image.

Earlier I found a fast and easy way using matrix to adjust brightness, and I really need a similar system for the contrast.

What I have found until now is either working way too slow (adjusting a pixel at a time) or seem to be a variation of brightness adjustment(?) like this:

C#

float[][] wk_colormatrix ={
         new float[] {wk_float, 0, 0, 0, 0},
         new float[] {0, wk_float, 0, 0, 0},
         new float[] {0, 0, wk_float, 0, 0},
         new float[] {0, 0, 0, 1, 0},
         new float[] {wk_float_x, wk_float_x, wk_float_x, 0, 1}
     };

My experience tell me, that if I can't find what I need on the internet, it's either impossible, or so easy and simple that no one ever needed to ask about it ;-)

However, it is obviously possible, since it works great int i.e. Paint.net, where I can adjust contrast of an image "on the fly" so to speak!

So, I've decided to throw in the towel: Anything that can help me here will be more than welcome!

Sincerely/Best regards

Mads Kristian

Posted 25-Mar-13 21:35pm

Mads1967

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Prasad Khandekar 26-Mar-13 10:10am

Might want to have alook at this (http://stackoverflow.com/questions/3115076/adjust-the-contrast-of-an-image-in-c-sharp-efficiently). And this fantastic CP article (http://www.codeproject.com/Articles/33838/Image-Processing-using-C)

Mads1967 27-Mar-13 3:11am

Thank you for the links.
I had already found them, but now I did take a closer look especially on that last one.
Actually, when adjusting the contrast it does approximately the same as I found earlier, and thought it was - well - not the right way to do it: The image somehow turns *very* "yellowish" on places where it should be white.
However, when giving the result in Paint.net a second inspection, it looks like they actually make it black and white when contrast is taken close to the limit. This way an eventual yellow shade vanishes into a white.
I'm not an expert into image manipulation, and there is a lot of parameters I don't know, like saturation, hue, etc.
What I need is something that would turn dark grey letters on a light grey surface into something as close to "black letters on a white surface" as possible.
Maybe the solution is somewhere in the CP article about Image-Processing-using-C after all.

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