CGContext slow in iOS5

Question

I'm using CGContext in two steps: First create context, draw background image, draw with UIBezierPaths, then get the image & release the context. Secondly combine this image with another one like this:

UIGraphicsBeginImageContextWithOptions(self.anchorImage.size, NO, 1); [self.anchorImage drawInRect:CGRectMake(0, 0, self.anchorImage.size.width, self.anchorImage.size.height)]; [tempImage drawInRect:CGRectMake(0, 0, self.anchorImage.size.width, self.anchorImage.size.height)]; UIImage *image = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext();

This worked well in iOS4, however it's very very slow in iOS5 (I'm testing on a 3GS). Am I doing something wrong? Or is there a more optimal way of doing this? Or is there a specific iOS5 way of doing it?

Is there a reason you are closing the first context, and not just drawing the second image in the first context over / under the UIBezierPaths? — Joris Kluivers
– Joris Kluivers, Commented Feb 6, 2012 at 14:35

Joris Kluivers · Accepted Answer · 2012-02-06 14:15:55Z

It's not clear to me if you are just generating images, or drawing into a view. Anyway, to improve speed you could offload the generation of these images to a different dispatch queue, so your current thread (probably the main thread?) will not block.

Instead of UIGraphics* I would use CGBitmapContextCreate in combination with CGBitmapContextCreateImage. When your final image has been generated update the image view (or do whatever else you want to do with the image).

dispatch_async(your_async_queue, ^() { CGContextRef ctx = CGBitmapContextCreate(/* params */); // your drawing CGImageRef imageRef = CGBitmapContextCreateImage(context); UIImage * image = [[UIImage alloc] initWithCGImage:imageRef]; CGImageRelease(imageRef); CGContextRelease(context); dispatch_async(dispatch_get_main_queue(), ^() { // do something with image on the main thread }); }

See the CGBitmapContext documentation for the full method signatures.

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