How to get a screenshot of a view containing GPUIm

Question

I'm writing an image editor for an iPhone App. I use GPUImage library to present an image (on GPUImageView) of the sprite on the canvas. And I use the following method to get the screenshot of the canvas:

@implementation UIView (SnapshotImage)
// Return a snapshot image of this view
- (UIImage*)snapshot{

    UIGraphicsBeginImageContextWithOptions(self.bounds.size,YES,0.0f);
    CGContextRef context = UIGraphicsGetCurrentContext();

    // In iOS 7, you can use the following instead of `-renderInContext:`. It should be faster.
    // [self drawViewHierarchyInRect:self.bounds afterScreenUpdates:YES];
    [self.layer renderInContext:context];

    UIImage *capturedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return capturedImage;
}
@end

After I used this method to get the whole screenshot of key window, the field of the canvas was empty (no image of any sprite).

UIView *keyWindow = [[UIApplication sharedApplication] keyWindow];
UIImage *snapshotIamge = [keyWindow snapshot];
// Present the image on the view...

I think there is a problem to get a screenshot of a view containing GPUImageView(s), which use OpenGLES to render images not Quartz. How to get a screenshot of a view containing GPUImageView?

And is it possible to get a cropped image with a specific CGRect when taking a screenshot (like cmd + shift + 4 on Mac)?

Answer 1

As you found, -renderInContext: doesn't work with OpenGL ES content. What you're going to need is to extract the processed image from GPUImage itself.

To do this, call -imageFromCurrentlyProcessedOutput on the filter that feeds directly into your GPUImageView. This will give you a UIImage with the final filtered image within it. You can then either use this directly, or you can composite it with the remaining content from your surrounding views.