gistfile1.m

- (void)start
{
    NSData *imageData = [self dataFromImage:self.image];
    
    //NSInteger maxBytes = 3145728;
    NSInteger maxBytes = 30000;
    NSInteger compressedSize = imageData.length;
    NSLog(@"Compressed size: %i", compressedSize);
    
    NSInteger originalWidth = CGImageGetWidth(self.image.CGImage);
    NSInteger originalHeight = CGImageGetHeight(self.image.CGImage);
    
    NSInteger bitsPerPixel = CGImageGetBitsPerPixel(self.image.CGImage);
    NSInteger bytesPerPixel = bitsPerPixel / 8;
    
    if (compressedSize > maxBytes)
    {
        //NSInteger uncompressedSize = (bytesPerPixel * originalWidth) * originalHeight;
        NSInteger uncompressedSize = UIImagePNGRepresentation(self.image).length;
        float compressieFactor = (float)uncompressedSize / (float)compressedSize;
        maxBytes = maxBytes * compressieFactor;
        
        NSInteger tooMuch = uncompressedSize - maxBytes;
        float ratio = ((float)originalWidth / (float)originalHeight);
        float heightFactor = (float)(ratio * bytesPerPixel);
        float newTooMuch = (float)tooMuch / heightFactor;
        NSInteger heightToDelete = sqrtf(newTooMuch);
        NSInteger widthToDelete = ratio * heightToDelete;
        
        self.image = [self.image photoByScalingForSize:CGSizeMake(originalWidth - widthToDelete, originalHeight - heightToDelete)];
        imageData = [self dataFromImage:self.image];
    }
}

- (NSData *)dataFromImage:(UIImage *)image
{
    CFMutableDataRef destinationData = CFDataCreateMutable(kCFAllocatorDefault, 0);
    CGImageDestinationRef destinationRef = CGImageDestinationCreateWithData(destinationData, kUTTypeJPEG, 1, NULL);
    CFDictionaryRef properties = (__bridge CFDictionaryRef)@{ (NSString *)kCGImageDestinationLossyCompressionQuality : [NSNumber numberWithFloat:1.0f]};
    CGImageDestinationSetProperties(destinationRef, properties);
    
    if (destinationRef != nil) {
        CGImageDestinationAddImage(destinationRef, image.CGImage, nil);
        if (CGImageDestinationFinalize(destinationRef))
        {
            NSData *imageData = [NSData dataWithData:(__bridge NSData *)destinationData];
            CFRelease(destinationData);
            CFRelease(destinationRef);
            return imageData;
        }
        CFRelease(destinationData);
        CFRelease(destinationRef);
    }
    CFRelease(destinationData);
    return nil;
}

//Method in UIImage category
static inline double radians (double degrees) {return degrees * M_PI/180;}
- (UIImage *)photoByScalingForSize:(CGSize)targetSize
{
    UIImage *sourceImage = self;
    CGFloat targetWidth = targetSize.width;
    CGFloat targetHeight = targetSize.height;
    
    CGImageRef imageRef = [sourceImage CGImage];
    CGBitmapInfo bitmapInfo = CGImageGetBitmapInfo(imageRef);
    CGColorSpaceRef colorSpaceInfo = CGImageGetColorSpace(imageRef);
    
    if (bitmapInfo == kCGImageAlphaNone) {
        bitmapInfo = kCGImageAlphaNoneSkipLast;
    }
    
    CGContextRef bitmap;
    
    if (sourceImage.imageOrientation == UIImageOrientationUp || sourceImage.imageOrientation == UIImageOrientationDown) {
        bitmap = CGBitmapContextCreate(NULL, targetWidth, targetHeight, CGImageGetBitsPerComponent(imageRef), CGImageGetBytesPerRow(imageRef), colorSpaceInfo, bitmapInfo);
        
    } else {
        bitmap = CGBitmapContextCreate(NULL, targetHeight, targetWidth, CGImageGetBitsPerComponent(imageRef), CGImageGetBytesPerRow(imageRef), colorSpaceInfo, bitmapInfo);
        
    }
    
    if (sourceImage.imageOrientation == UIImageOrientationLeft) {
        CGContextRotateCTM (bitmap, radians(90));
        CGContextTranslateCTM (bitmap, 0, -targetHeight);
        
    } else if (sourceImage.imageOrientation == UIImageOrientationRight) {
        CGContextRotateCTM (bitmap, radians(-90));
        CGContextTranslateCTM (bitmap, -targetWidth, 0);
    } else if (sourceImage.imageOrientation == UIImageOrientationDown) {
        CGContextTranslateCTM (bitmap, targetWidth, targetHeight);
        CGContextRotateCTM (bitmap, radians(-180.));
    }
    
    CGContextDrawImage(bitmap, CGRectMake(0, 0, targetWidth, targetHeight), imageRef);
    CGImageRef ref = CGBitmapContextCreateImage(bitmap);
    UIImage *newImage = [UIImage imageWithCGImage:ref];
    
    CGContextRelease(bitmap);
    CGImageRelease(ref);
    
    return newImage;
}