How do you find a region of a certain color within an image on iOS?

Question

I'm doing an image processing iOS app, where we have a large image (eg:size will be 2000x2000). Assume that image is completely black, except one part of the image is a different color (lets say the size of that region is 200x200).

SI want to calculate the start and end position of that differently coloured region. How can I achieve this?

Maybe this might help you. Using OpenCV. http://stackoverflow.com/questions/8667818/opencv-c-obj-c-detecting-a-sheet-of-paper-square-detection — Pushpak Narasimhan, Apr 29 '14 at 06:52
You may check [this](https://github.com/BradLarson/GPUImage) out. — mownier, Apr 29 '14 at 07:11

score 0 · Accepted Answer · answered Apr 29 '14 at 07:39

Here's a simple way to allow the CPU to get pixel values from a UIImage. The steps are

allocate a buffer for the pixels
create a bitmap memory context using the buffer as the backing store
draw the image into the context (writes the pixels into the buffer)
examine the pixels in the buffer
free the buffer and associated resources

- (void)processImage:(UIImage *)input
{
    int width  = input.size.width;
    int height = input.size.height;

    // allocate the pixel buffer
    uint32_t *pixelBuffer = calloc( width * height, sizeof(uint32_t) );

    // create a context with RGBA pixels
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate( pixelBuffer, width, height, 8, width * sizeof(uint32_t), colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedLast );

    // invert the y-axis, so that increasing y is down
    CGContextScaleCTM( context, 1.0, -1.0 );
    CGContextTranslateCTM( context, 0, -height );

    // draw the image into the pixel buffer
    UIGraphicsPushContext( context );
    [input drawAtPoint:CGPointZero];
    UIGraphicsPopContext();

    // scan the image
    int x, y;
    uint8_t r, g, b, a;
    uint8_t *pixel = (uint8_t *)pixelBuffer;

    for ( y = 0; y < height; y++ )
        for ( x = 0; x < height; x++ )
        {
            r = pixel[0];
            g = pixel[1];
            b = pixel[2];
            a = pixel[3];

            // do something with the pixel value here

            pixel += 4;
        }

    // release the resources
    CGContextRelease( context );
    CGColorSpaceRelease( colorSpace );
    free( pixelBuffer );
}

How do you find a region of a certain color within an image on iOS?

1 Answers1