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 boun

Answer 1

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;
}