vImageScaledImage

vImageScaledImage

// Method: vImageScaledImage:(UIImage*) sourceImage withSize:(CGSize) destSize
// Returns even better scaling than drawing to a context with kCGInterpolationHigh.
// This employs the vImage routines in Accelerate.framework.
// For more information about vImage, see https://developer.apple.com/library/mac/#documentation/performance/Conceptual/vImage/Introduction/Introduction.html#//apple_ref/doc/uid/TP30001001-CH201-TPXREF101
// Large quantities of memory are manually allocated and (hopefully) freed here.  Test your application for leaks before and after using this method.
+ (UIImage*) vImageScaledImage:(UIImage*) sourceImage withSize:(CGSize) destSize;
{
    UIImage *destImage = nil;
    
    if (sourceImage)
    {
        // First, convert the UIImage to an array of bytes, in the format expected by vImage.
        // Thanks: http://stackoverflow.com/a/1262893/1318452
        CGImageRef sourceRef = [sourceImage CGImage];
        NSUInteger sourceWidth = CGImageGetWidth(sourceRef);
        NSUInteger sourceHeight = CGImageGetHeight(sourceRef);
        CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
        unsigned char *sourceData = (unsigned char*) calloc(sourceHeight * sourceWidth * 4, sizeof(unsigned char));
        NSUInteger bytesPerPixel = 4;
        NSUInteger sourceBytesPerRow = bytesPerPixel * sourceWidth;
        NSUInteger bitsPerComponent = 8;
        CGContextRef context = CGBitmapContextCreate(sourceData, sourceWidth, sourceHeight,
                                                     bitsPerComponent, sourceBytesPerRow, colorSpace,
                                                     kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Big);
        CGContextDrawImage(context, CGRectMake(0, 0, sourceWidth, sourceHeight), sourceRef);
        CGContextRelease(context);
        
        // We now have the source data.  Construct a pixel array
        NSUInteger destWidth = (NSUInteger) destSize.width;
        NSUInteger destHeight = (NSUInteger) destSize.height;
        NSUInteger destBytesPerRow = bytesPerPixel * destWidth;
        unsigned char *destData = (unsigned char*) calloc(destHeight * destWidth * 4, sizeof(unsigned char));
        
        // Now create vImage structures for the two pixel arrays.
        // Thanks: https://github.com/dhoerl/PhotoScrollerNetwork
        vImage_Buffer src = {
            .data = sourceData,
            .height = sourceHeight,
            .width = sourceWidth,
            .rowBytes = sourceBytesPerRow
        };
        
        vImage_Buffer dest = {
            .data = destData,
            .height = destHeight,
            .width = destWidth,
            .rowBytes = destBytesPerRow
        };
        
        // Carry out the scaling.
        vImage_Error err = vImageScale_ARGB8888 (
                                                 &src,
                                                 &dest,
                                                 NULL,
                                                 kvImageHighQualityResampling
                                                 );
        
        // The source bytes are no longer needed.
        free(sourceData);
        
        // Convert the destination bytes to a UIImage.
        CGContextRef destContext = CGBitmapContextCreate(destData, destWidth, destHeight,
                                                         bitsPerComponent, destBytesPerRow, colorSpace,
                                                         kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Big);
        CGImageRef destRef = CGBitmapContextCreateImage(destContext);
        
        // Store the result.
        destImage = [UIImage imageWithCGImage:destRef];
        
        // Free up the remaining memory.
        CGImageRelease(destRef);
        
        CGColorSpaceRelease(colorSpace);
        CGContextRelease(destContext);
        
        // The destination bytes are no longer needed.
        free(destData);
        
        if (err != kvImageNoError)
        {
            NSString *errorReason = [NSString stringWithFormat:@"vImageScale returned error code %d", err];
            NSDictionary *errorInfo = [NSDictionary dictionaryWithObjectsAndKeys:
                                       sourceImage, @"sourceImage",
                                       [NSValue valueWithCGSize:destSize], @"destSize",
                                       nil];
            
            NSException *exception = [NSException exceptionWithName:@"HighQualityImageScalingFailureException" reason:errorReason userInfo:errorInfo];
            
            @throw exception;
        }
    }
    return destImage;
}