ios - Convert RGB image to 1 channel image (black/white)

Question

How can I convert RGB image to 1 channel image (black/white) using ios5?

Input image is usually a photo of a book page. Goal is to reduce the size of a photocopy by converting it to the 1 channel image.

Answer 1

If I understand your question, you want to apply a black and white thresholding to the image based on a pixel's luminance. For a fast way of doing this, you could use my open source GPUImage project (supporting back to iOS 4.x) and a couple of the image processing operations it provides.

In particular, the GPUImageLuminanceThresholdFilter and GPUImageAdaptiveThresholdFilter might be what you're looking for here. The former turns a pixel to black or white based on a luminance threshold you set (the default is 50%). The latter takes the local average luminance into account when applying this threshold, which can produce better results for text on pages of a book.

Usage of these filters on a UIImage is fairly simple:

UIImage *inputImage = [UIImage imageNamed:@"book.jpg"];
GPUImageLuminanceThresholdFilter *thresholdFilter = [[GPUImageLuminanceThresholdFilter alloc] init];
UIImage *quickFilteredImage = [thresholdFilter imageByFilteringImage:inputImage];

These can be applied to a live camera feed and photos taken by the camera, as well.