Question

im trying to create an image filter in OpenGL CE. currently I am trying to create a series of 4×4 matrices and multiply them together. then use glColorMask and glColor4f to adjust the image accordingly. I’ve been able to integrate hue rotation, saturation, and brightness. but i am having trouble adding contrast. thus far google hasn’t been to helpful. I’ve found a few matrices but they don’t seem to work. do you guys have any ideas?

Answer 1

I have to say, I haven’t heard of using a 4×4 matrix to do brightness or contrast. I think of these operations as being done on the histogram of the image, rather than on a local per-pixel basis.

Say, for instance, that your image has values from 0 to 200, and you want to make it brighter. You can then add values to the image, and what is shown on the screen will be brighter. If you want to enhance the contrast on that image, you would do a multiplication like:

(image_value - original_min)/(original_max - original_min) * (new_max - new_min) + new_min

if you want your new min to be 0 and your new max to be 255, then that equation would stretch the contrast accordingly. The original_min and original_max do not have to be the actual min and max of the entire image, the could be the min and max of a subsection of the image, if you want to enhance a particular area and don’t mind clipping values above or below your new_min/new_max.

I suppose if you already know your range and so forth, you could incorporate that formula into a 4×4 matrix to achieve your goal, but only after you’ve done a pass to find the min and max of the original image.

I would also make sure to uncouple the display of your image from your image data; the above operations are destructive, in that you’ll lose information, so you want to keep the original and display a copy.