Capture a Screen Shot of UIView - Slow Performance - ios

Question

I have a Drawing App of sorts, I would like to create a Snapshot of the Canvas UIView (both on and off screen) and then scale it down. The code I have for doing that take bloody for ever on an iPad 3. Simulator there is no delay. The Canvas is 2048x2048.
Is there another way I should be doing this? Or something I have a miss in the code?
Thank you!
-(UIImage *) createScreenShotThumbnailWithWidth:(CGFloat)width{
// Size of our View
CGSize size = editorContentView.bounds.size;
//First Grab our Screen Shot at Full Resolution
UIGraphicsBeginImageContext(size);
[editorContentView.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *screenShot = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
//Calculate the scal ratio of the image with the width supplied.
CGFloat ratio = 0;
if (size.width > size.height) {
ratio = width / size.width;
} else {
ratio = width / size.height;
}
//Setup our rect to draw the Screen shot into
CGSize newSize = CGSizeMake(ratio * size.width, ratio * size.height);
//Send back our screen shot
return [self imageWithImage:screenShot scaledToSize:newSize];
}

Answer 1

Did you use the "Time Profiler" Instrument ("Product" Menu -> "Profile") to check where in your code you spend the most of your time? (use it with your Device of course, not the Simulator, to have realistic profiling). I'd guess it is not in the image capture portion you quoted in your question, but in your rescaling method imageWithImage:scaledToSize: method.
Instead of rendering the image at its whole size in a context, then rescaling the image to the final size, you should render the layer in the context directly at the expected size by applying some affine transform to the context.
So simply use CGContextConcatCTM(someScalingAffineTransform); on UIGraphicsGetCurrentContext() right after your line UIGraphicsBeginImageContext(size);, to apply an scaling affine transform that will make the layer be rendered at a different scale/size.
This way it will be directly rendered as the expected size which will be much faster, instead of being rendered at 100% and then having you to rescale it afterwards in a time-consuming way

Answer 2

Thank you AliSoftware, Here is the Code I ended up using:
-(UIImage *) createScreenShotThumbnailWithWidth:(CGFloat)width{
if (IoUIDebug & IoUIDebugSelectorNames) {
NSLog(#"%# - %#", INTERFACENAME, NSStringFromSelector(_cmd) );
}
// Size of our View
CGSize size = editorContentView.bounds.size;
//Calculate the scal ratio of the image with the width supplied.
CGFloat ratio = 0;
if (size.width > size.height) {
ratio = width / size.width;
} else {
ratio = width / size.height;
}
CGSize newSize = CGSizeMake(ratio * size.width, ratio * size.height);
//Create GraphicsContext with our new size
UIGraphicsBeginImageContext(newSize);
//Create Transform to scale down the Context
CGAffineTransform transform = CGAffineTransformIdentity;
transform = CGAffineTransformScale(transform, ratio, ratio);
//Apply the Transform to the Context
CGContextConcatCTM(UIGraphicsGetCurrentContext(),transform);
//Render our Image into the the Scaled Graphic Context
[editorContentView.layer renderInContext:UIGraphicsGetCurrentContext()];
//Save a copy of the Image of the Graphic Context
UIImage* screenShot = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return screenShot;
}