ios Resizing images for thumbs - Blurry although high interpolation - iphone

Question

I have written a category on UIImage to resize images like the iphone photos app.
Although the Interpolation is set to high, the image does not really look like the one in the photos app. Instead it looks unsharp or blurry.
Here is what I did:
- (UIImage *)resizeImageProportionallyIntoNewSize:(CGSize)newSize;
{
CGFloat scaleWidth = 1.0f;
CGFloat scaleHeight = 1.0f;
NSLog(#"Origin Size = %#", NSStringFromCGSize(self.size));
if (CGSizeEqualToSize(self.size, newSize) == NO) {
//calculate "the longer side"
if(self.size.width > self.size.height) {
scaleWidth = self.size.width / self.size.height;
} else {
scaleHeight = self.size.height / self.size.width;
}
}
// now draw the new image in a context with scaling proportionally
UIImage *sourceImage = self;
UIImage *newImage = nil;
//now we create a context in newSize and draw the image out of the bounds of the context to get
//an proportionally scaled image by cutting of the image overlay
if([[UIScreen mainScreen] scale] == 2.00) {
UIGraphicsBeginImageContextWithOptions(newSize, YES, 2.0);
}
UIGraphicsBeginImageContext(newSize);
// Set the quality level to use when rescaling
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetInterpolationQuality(context, kCGInterpolationHigh);
//Center image point so that on each egde is a little cutoff
CGRect thumbnailRect = CGRectZero;
thumbnailRect.size.width = (int) newSize.width * scaleWidth;
thumbnailRect.size.height = (int) newSize.height * scaleHeight;
thumbnailRect.origin.x = (int) (newSize.width - thumbnailRect.size.width) * 0.5;
thumbnailRect.origin.y = (int) (newSize.height - thumbnailRect.size.height) * 0.5;
[sourceImage drawInRect:thumbnailRect];
newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
if(newImage == nil) NSLog(#"could not scale image");
return newImage ;
}
BTW: There is no difference on using high interpolation or not. Maybe I am doing something wrong.
Thanks in advance for your help!

Answer 1

The best way to make a thumbnail is to ask the system to do it for you, using the Image I/O Framework. The only trick is that since you're using a CGImage you must take account of the screen resolution:
CGImageSourceRef src = CGImageSourceCreateWith... // whatever
CGFloat scale = [UIScreen mainScreen].scale;
CGFloat w = // maximum size, multiplied by the scale
NSDictionary* d =
[NSDictionary dictionaryWithObjectsAndKeys:
(id)kCFBooleanTrue, kCGImageSourceShouldAllowFloat,
(id)kCFBooleanTrue, kCGImageSourceCreateThumbnailWithTransform,
(id)kCFBooleanTrue, kCGImageSourceCreateThumbnailFromImageAlways,
[NSNumber numberWithInt:(int)w], kCGImageSourceThumbnailMaxPixelSize,
nil];
CGImageRef imref =
CGImageSourceCreateThumbnailAtIndex(src, 0, (__bridge CFDictionaryRef)d);
UIImage* im =
[UIImage imageWithCGImage:imref scale:scale orientation:UIImageOrientationUp];
CFRelease(imref); CFRelease(src);

Answer 2

if it is unsharp / blurry, you should try to turn off anti-aliasing by:
CGContextSetShouldAntialias(context, NO);
hope this helps.