Crop UIImage to alpha

Question

I have a rather large, almost full screen image that I'm going to be displaying on an iPad. The image is about 80% transparent. I need to, on the client, determine the bounding box of the opaque pixels, and then crop to that bounding box.

Scanning other questions here on StackOverflow and reading some of the CoreGraphics docs, I think I could accomplish this by:

CGBitmapContextCreate(...) // Use this to render the image to a byte array

 ..
   - iterate through this byte array to find the bounding box
 ..

CGImageCreateWithImageInRect(image, boundingRect);

That just seems very inefficient and clunky. Is there something clever I can do with CGImage masks or something which makes use of the device's graphics acceleration to do this?

Answer 1

Thanks to user404709 for making all the hard work. Below code also handles retina images and frees the CFDataRef.

- (UIImage *)trimmedImage {

    CGImageRef inImage = self.CGImage;
    CFDataRef m_DataRef;
    m_DataRef = CGDataProviderCopyData(CGImageGetDataProvider(inImage));

    UInt8 * m_PixelBuf = (UInt8 *) CFDataGetBytePtr(m_DataRef);

    size_t width = CGImageGetWidth(inImage);
    size_t height = CGImageGetHeight(inImage);

    CGPoint top,left,right,bottom;

    BOOL breakOut = NO;
    for (int x = 0;breakOut==NO && x < width; x++) {
        for (int y = 0; y < height; y++) {
            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                left = CGPointMake(x, y);
                breakOut = YES;
                break;
            }
        }
    }

    breakOut = NO;
    for (int y = 0;breakOut==NO && y < height; y++) {

        for (int x = 0; x < width; x++) {

            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                top = CGPointMake(x, y);
                breakOut = YES;
                break;
            }

        }
    }

    breakOut = NO;
    for (int y = height-1;breakOut==NO && y >= 0; y--) {

        for (int x = width-1; x >= 0; x--) {

            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                bottom = CGPointMake(x, y);
                breakOut = YES;
                break;
            }

        }
    }

    breakOut = NO;
    for (int x = width-1;breakOut==NO && x >= 0; x--) {

        for (int y = height-1; y >= 0; y--) {

            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                right = CGPointMake(x, y);
                breakOut = YES;
                break;
            }

        }
    }


    CGFloat scale = self.scale;

    CGRect cropRect = CGRectMake(left.x / scale, top.y/scale, (right.x - left.x)/scale, (bottom.y - top.y) / scale);
    UIGraphicsBeginImageContextWithOptions( cropRect.size,
                                           NO,
                                           scale);
    [self drawAtPoint:CGPointMake(-cropRect.origin.x, -cropRect.origin.y)
           blendMode:kCGBlendModeCopy
               alpha:1.];
    UIImage *croppedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    CFRelease(m_DataRef);
    return croppedImage;
}

Answer 2

I created a category on UImage which does this if any one needs it...

+ (UIImage *)cropTransparencyFromImage:(UIImage *)img {

    CGImageRef inImage = img.CGImage;           
    CFDataRef m_DataRef;  
    m_DataRef = CGDataProviderCopyData(CGImageGetDataProvider(inImage));  
    UInt8 * m_PixelBuf = (UInt8 *) CFDataGetBytePtr(m_DataRef);  

    int width = img.size.width;
    int height = img.size.height;

    CGPoint top,left,right,bottom;

    BOOL breakOut = NO;
    for (int x = 0;breakOut==NO && x < width; x++) {
        for (int y = 0; y < height; y++) {
            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                left = CGPointMake(x, y);
                breakOut = YES;
                break;
            }
        }
    }

    breakOut = NO;
    for (int y = 0;breakOut==NO && y < height; y++) {

        for (int x = 0; x < width; x++) {

            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                top = CGPointMake(x, y);
                breakOut = YES;
                break;
            }

        }
    }

    breakOut = NO;
    for (int y = height-1;breakOut==NO && y >= 0; y--) {

        for (int x = width-1; x >= 0; x--) {

            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                bottom = CGPointMake(x, y);
                breakOut = YES;
                break;
            }

        }
    }

    breakOut = NO;
    for (int x = width-1;breakOut==NO && x >= 0; x--) {

        for (int y = height-1; y >= 0; y--) {

            int loc = x + (y * width);
            loc *= 4;
            if (m_PixelBuf[loc + 3] != 0) {
                right = CGPointMake(x, y);
                breakOut = YES;
                break;
            }

        }
    }


    CGRect cropRect = CGRectMake(left.x, top.y, right.x - left.x, bottom.y - top.y);

    UIGraphicsBeginImageContextWithOptions( cropRect.size,
                                           NO,
                                           0.);
    [img drawAtPoint:CGPointMake(-cropRect.origin.x, -cropRect.origin.y)
           blendMode:kCGBlendModeCopy
               alpha:1.];
    UIImage *croppedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return croppedImage;
}