Question about UIImage, scaling, and UIGraphicsBeginImageContextWithOptions
Question about UIImage, scaling, and UIGraphicsBeginImageContextWithOptions
- Subject: Question about UIImage, scaling, and UIGraphicsBeginImageContextWithOptions
- From: Ray <email@hidden>
- Date: Tue, 27 Mar 2012 09:38:26 +0200
OK, so I have this iOS 3.x code base that I need to update. I have the following code, to make a thumbnail image:
...
// let's say we have a UIImage *selectedImage with a size of 1000 x 1000
UIImage *newImage;
NSData *imageData;
UIGraphicsBeginImageContext(CGSizeMake(44.0, 44.0));
[selectedImage drawInRect:CGRectMake(0.0, 0.0, 44.0, 44.0)];
newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
imageData = UIImagePNGRepresentation(newImage);
...
Code is simplified, just for illustrative purposes. I store imageData as a binary property (called "listImage") in a Core Data entity and use it as a thumbnail image in a UITableView, like so:
cell.imageView.image = [UIImage imageWithData:item.listImage];
It's small, so no problems. On both a normal and retina screen iPhone running iOS 4 this works OK, although we don't make use of the increased resolution, yet. In order to do that, I update the code with:
UIGraphicsBeginImageContextWithOptions(CGSizeMake(44.0, 44.0), YES, 0);
Now, when I generate a new image on an iPhone with a retina screen, in which case the above UIGraphicsBeginImageContextWithOptions uses a scale of 2.0, the image in the table view shows too big. This is because [UIImage imageWithData:item.listImage] always returns a 1.0 scaled image. Now I can solve this by using something like:
UIImage *theImage = [UIImage imageWithData:listImage];
[UIImage imageWithCGImage:theImage.CGImage scale:theImage.size.width > 44.0 ? 2.0 : 1.0 orientation:theImage.imageOrientation];
But this seems kludgy and it's using programmer's knowledge, so to speak. I watched WWDC2010 session 134 "Optimize your iPhone App for the Retina Display" again, searched stackoverflow etc. but I can't find a more elegant solution. What would be a more thorough approach? Maybe it's staring me in the face but I don't see it...
Thanks for your answers,
- Ray.
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