Locate first transparent pixel on a UIImage

gistfile1.matlab

+ (CGPoint)getFirstTransparentPixelOfImage:(UIImage*)image
{

    int xx = 0;
    int yy = 0;
    int count = image.size.width * image.size.height;
    
    
    // First get the image into your data buffer
    CGImageRef imageRef = [image CGImage];
    NSUInteger width = CGImageGetWidth(imageRef);
    NSUInteger height = CGImageGetHeight(imageRef);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *rawData = (unsigned char*) calloc(height * width * 4, sizeof(unsigned char));
    NSUInteger bytesPerPixel = 4;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    CGContextRef context = CGBitmapContextCreate(rawData, width, height,
                                                 bitsPerComponent, bytesPerRow, colorSpace,
                                                 kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    CGColorSpaceRelease(colorSpace);
    
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    CGContextRelease(context);
    
    CGPoint p = CGPointMake(width-1, height-1);
    
    // Now your rawData contains the image data in the RGBA8888 pixel format.
    int byteIndex = (bytesPerRow * yy) + xx * bytesPerPixel;
    for (int ii = 0 ; ii < count ; ++ii)
    {
        CGFloat alpha = (rawData[byteIndex + 3] * 1.0) / 255.0;

        if (alpha>0.0)
        {

            p.y = ii/width;
            p.x = ii - p.y * width;
            
            free(rawData);
            
            return p;
            
        }

        byteIndex += 4;

    }
    
    free(rawData);
    
    return p;
}