Today I wrote a useful Objective-C method which uses Quartz 2D from the iPhone SDK to dynamically generate square thumbnails from any portfolio or landscape UIImage. I failed to find helpful results on Google for the various CGContextClipToRect/CGContextTranslateCTM/CGContextScaleCTM Quartz functions so after a lot of trial and error I managed to get the results I was after with a fairly efficient script – shown below.
So, if for example you want to get a 32×32 pixel square thumbnail, you can pass 32 as the ‘length’ parameter in the function below. For portfolio images the code will take a full square from the original image from the centre using the full width of your original portfolio image. Likewise, for a landscape image the script will use all the height available, clipping the right and left sides.
I think the most import thing I learned when writing this script is that when you call UIGraphicsBeginImageContext(CGSize) make sure that the size you pass in is the size you want your final image to be. Otherwise you end up having to call UIGraphicsEndImageContext() and UIGraphicsBeginImageContext() multiple times with different CGSize instances which I’m sure isn’t an optimised way to achieve the same results.
– (UIImage *)thumbWithSideOfLength:(float)length {
NSString *subdir = @”my/images/directory”;
NSString *filename = @”myOriginalImage.png”;
NSString *fullPathToThumbImage = [subdir stringByAppendingPathComponent:[NSString stringWithFormat:@”%dx%d%@”,(int) length, (int) length,filename];
NSString *fullPathToMainImage = [subdir stringByAppendingPathComponent:filename];UIImage *thumbnail;
NSFileManager *fileManager = [NSFileManager defaultManager];
if ([fileManager fileExistsAtPath:fullPathToThumbImage] == YES) {
thumbnail = [UIImage imageWithContentsOfFile:fullPathToThumbImage];}
else {
//couldn’t find a previously created thumb image so create one first…
UIImage *mainImage = [UIImage imageWithContentsOfFile:fullPathToMainImage];UIImageView *mainImageView = [[UIImageView alloc] initWithImage:mainImage];
BOOL widthGreaterThanHeight = (mainImage.size.width > mainImage.size.height);
float sideFull = (widthGreaterThanHeight) ? mainImage.size.height : mainImage.size.width;CGRect clippedRect = CGRectMake(0, 0, sideFull, sideFull);
//creating a square context the size of the final image which we will then
// manipulate and transform before drawing in the original image
UIGraphicsBeginImageContext(CGSizeMake(length, length));
CGContextRef currentContext = UIGraphicsGetCurrentContext();CGContextClipToRect( currentContext, clippedRect);
CGFloat scaleFactor = length/sideFull;
if (widthGreaterThanHeight) {
//a landscape image – make context shift the original image to the left when drawn into the context
CGContextTranslateCTM(currentContext, -((mainImage.size.width – sideFull) / 2) * scaleFactor, 0);}
else {
//a portfolio image – make context shift the original image upwards when drawn into the context
CGContextTranslateCTM(currentContext, 0, -((mainImage.size.height – sideFull) / 2) * scaleFactor);}
//this will automatically scale any CGImage down/up to the required thumbnail side (length) when the CGImage gets drawn into the context on the next line of code
CGContextScaleCTM(currentContext, scaleFactor, scaleFactor);[mainImageView.layer renderInContext:currentContext];
thumbnail = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
NSData *imageData = UIImagePNGRepresentation(thumbnail);[imageData writeToFile:fullPathToThumbImage atomically:YES];
thumbnail = [UIImage imageWithContentsOfFile:fullPathToThumbImage];
}
return thumbnail;
}
So if, for example your original image looked like this:
Using the method above you could request a 100×100 pixel thumbnail which would result in this:
