How can I use Open GL ES to crop image? - ios

Question

So I face some problems when I deal with image cropping. I am awared of two possible ways: UIGraphicsBeginImageContextWithOptions combined with drawAtPoint:blendMode: methods and CGImageCreateWithImageInRect. They both work but have some serious disadvantages:the first way takes a lot of time(in my case 7 secs approx.) and memory(I receive memory warning)(suppose I crop an image taken with iPhone camera); the second ends up with rotated image so that you need to put a bunch code to defeat this behavior which I don't want. What I want to know is how, for instance, apple's built in edit function of "Photos" app works, or Aviary or any other photo editor. Consider apple's editor(iOS 8), you can rotate image,change cropping rectangle,they also have blurring(!) outside the cropping rect and so on, but when you apply cropping it takes max 8 mb of memory and it happens immediately. How do they do this?
The only thought I have is that they use the potential of GPU(Aviary, for instance). So,if we combine all this questions in one, how can I use Open GL to make cropping be less painful operation? I've never worked with it, so any tuts,links and sources are welcome.Thank you in advance.

Answer 1

As already mentioned this should most likely not be done with openGL but even if...
The problem most people have is getting the rectangle in which the image should be displayed and the solution looks something like this:
- (CGRect)fillSizeForSource:(CGRect)source target:(CGRect)target
{
if(source.size.width/source.size.height > target.size.width/target.size.height)
{
// keep target height and make the width larger
CGSize newSize = CGSizeMake(target.size.height * (source.size.width/source.size.height), target.size.height);
return CGRectMake((target.size.width-newSize.width)*.5f, .0f, newSize.width, newSize.height);
}
else
{
// keep target width and make the height larger
CGSize newSize = CGSizeMake(target.size.width, target.size.width * (source.size.height/source.size.width));
return CGRectMake(.0f, (target.size.height-newSize.height)*.5f, newSize.width, newSize.height);
}
}
- (CGRect)fitSizeForSource:(CGRect)source target:(CGRect)target
{
if(source.size.width/source.size.height < target.size.width/target.size.height)
{
// keep target height and make the width smaller
CGSize newSize = CGSizeMake(target.size.height * (source.size.width/source.size.height), target.size.height);
return CGRectMake((target.size.width-newSize.width)*.5f, .0f, newSize.width, newSize.height);
}
else
{
// keep target width and make the height smaller
CGSize newSize = CGSizeMake(target.size.width, target.size.width * (source.size.height/source.size.width));
return CGRectMake(.0f, (target.size.height-newSize.height)*.5f, newSize.width, newSize.height);
}
}
I did not test this.
Or since you are on iOS simply create an image view with the desired size, add an image to it and then get the screenshot of the image.