iOS Objective-C中的自动白平衡

Question

I'm trying to fix the white balance of a picture on iOS. In my app, people can take a picture and get 2 things : an OCR and an "improved" version of the image.

To do the OCR, I improve the image with GPUImage (using GPUImageAdaptiveThresholdFilter ). But that image is made only of black and white pixels. For my "improved" version, I want to have : - the right color balance (meaning my white is really white and not yellow when I take the picture inside) - good contrast.

I tried with GPUImageContrastFilter and GPUImageWhiteBalanceFilter . GPUImageWhiteBalanceFilter works well but GPUImageWhiteBalanceFilter takes parameters (like temperature ) and that really depends on the image input.

So is there a way to "calculate" these parameters or are there objective c algorithms to fix white balance ? Either ready to use, or simple using GPUImage ?

Thanks !

Answer 1

You can use CoreImage to auto enhance images. It will do the analysis for you.

This is done by acquiring a set of CIFilter with the necessary modifications that you can just apply to your image.

See more info here .

Answer 2

It may take time, but you can use your OCR function as a kind of scoring algorithm for a hill-climbing (or other) optimization algorithm.

1. Take a portion of the image (to speed it up)
2. Pick some parameters and improve the image
3. Run it through OCR -- hopefully your OCR can give some confidence or you have some way of scoring the output (number of words?)
4. Change the parameters -- use an optimization algorithm to decide how to do this.
5. Go to 3 -- stop when you run out of time or it's good enough

If OCR is too slow -- you may have to pick something else to score with. Perhaps there's a way to compare your output images with known good ones and get a diff.