Generating tiles from a large PNG

Question

I'm looking to use CATiledLayer to display a huge PNG image on an iOS device. For this to work, I need to split the larger image into tiles (at 100%, 50%, 25% and 12.5%) on the client (creating tiles at the server side is not an option).

I can see that there are libraries such as libjpeg-turbo that may work, however these are for JPEGs and I need to work with PNGs.

Does anyone know of a way that I can take a large PNG (~20Mb) and generate tiles from it on the device?

Any pointers or suggestions would be appreciated!

Thank you!

Answer 1

You can use the built-in Core Graphics CGDataProviderCreateWithFilename and CGImageCreateWithPNGDataProvider APIs to open the image, then create each of the tiles by doing something like:

const CGSize tileSize = CGSizeMake(256, 256);
const CGPoint tileOrigin = ...; // Calculate using current column, row, and tile size.
const CGRect tileFrame = CGRectMake(-tileOrigin.x, -tileOrigin.y, imageSize.width, imageSize.height);

UIGraphicsBeginImageContextWithOptions(tileSize, YES, 1);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextDrawImage(context, tileFrame, image.CGImage);

UIImage *tileImage = UIGraphicsGetImageFromCurrentImageContext();
[UIImagePNGRepresentation(tileImage) writeToFile:tilePath atomically:YES];
UIGraphicsEndImageContext();

You may also want to look at the related sample projects (Large Image Downsizing, and PhotoScroller) referenced under the UIScrollView Class Reference.