Question

I need to scale down an image, but in a sharp way. In Photoshop for example there are the image size reduction options “Bicubic Smoother” (blurry) and “Bicubic Sharper”.

Is this image downscaling algorithm open sourced or documented somewhere or does the SDK offer methods to do this?

Answer 1

Merely using imageWithCGImage is not sufficient. It will scale, but the result will be blurry and suboptimal whether scaling up or down.

If you want to get the aliasing right and get rid of the “jaggies” you need something like this: http://vocaro.com/trevor/blog/2009/10/12/resize-a-uiimage-the-right-way/.

My working test code looks something like this, which is Trevor’s solution with one small adjustment to work with my transparent PNGs:

- (UIImage *)resizeImage:(UIImage*)image newSize:(CGSize)newSize {
    CGRect newRect = CGRectIntegral(CGRectMake(0, 0, newSize.width, newSize.height));
    CGImageRef imageRef = image.CGImage;

    UIGraphicsBeginImageContextWithOptions(newSize, NO, 0);
    CGContextRef context = UIGraphicsGetCurrentContext();

    // Set the quality level to use when rescaling
    CGContextSetInterpolationQuality(context, kCGInterpolationHigh);
    CGAffineTransform flipVertical = CGAffineTransformMake(1, 0, 0, -1, 0, newSize.height);

    CGContextConcatCTM(context, flipVertical);  
    // Draw into the context; this scales the image
    CGContextDrawImage(context, newRect, imageRef);

    // Get the resized image from the context and a UIImage
    CGImageRef newImageRef = CGBitmapContextCreateImage(context);
    UIImage *newImage = [UIImage imageWithCGImage:newImageRef];

    CGImageRelease(newImageRef);
    UIGraphicsEndImageContext();    

    return newImage;
}