Question

I m the beginner in iphone software development.
I developing the application on skin cancer in which i want to calculate or count red color pixel from UIImage which is captured by iphone camera.It is possible to count red pixel from UIImage?

Answer 1

Since this is a question that is asked almost weekly, I decided to make a little example project that shows how to do this. You can look at the code at:

http://github.com/st3fan/iphone-experiments/tree/master/Miscellaneous/PixelAccess/

The important bit is the following code, which takes a UIImage and then counts the number of pure red pixels. It is an example and you can use it and modify it for your own algorithms:

/**
 * Structure to keep one pixel in RRRRRRRRGGGGGGGGBBBBBBBBAAAAAAAA format
 */

struct pixel {
    unsigned char r, g, b, a;
};

/**
 * Process the image and return the number of pure red pixels in it.
 */

- (NSUInteger) processImage: (UIImage*) image
{
    NSUInteger numberOfRedPixels = 0;

    // Allocate a buffer big enough to hold all the pixels

    struct pixel* pixels = (struct pixel*) calloc(1, image.size.width * image.size.height * sizeof(struct pixel));
    if (pixels != nil)
    {
        // Create a new bitmap

        CGContextRef context = CGBitmapContextCreate(
            (void*) pixels,
            image.size.width,
            image.size.height,
            8,
            image.size.width * 4,
            CGImageGetColorSpace(image.CGImage),
            kCGImageAlphaPremultipliedLast
        );

        if (context != NULL)
        {
            // Draw the image in the bitmap

            CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, image.size.width, image.size.height), image.CGImage);

            // Now that we have the image drawn in our own buffer, we can loop over the pixels to
            // process it. This simple case simply counts all pixels that have a pure red component.

            // There are probably more efficient and interesting ways to do this. But the important
            // part is that the pixels buffer can be read directly.

            NSUInteger numberOfPixels = image.size.width * image.size.height;

            while (numberOfPixels > 0) {
                if (pixels->r == 255) {
                    numberOfRedPixels++;
                }
                pixels++;
                numberOfPixels--;
            }

            CGContextRelease(context);
        }

        free(pixels);
    }

    return numberOfRedPixels;
}

A simple example on how to call this:

- (IBAction) processImage
{
    NSUInteger numberOfRedPixels = [self processImage: [UIImage imageNamed: @"DutchFlag.png"]];
    label_.text = [NSString stringWithFormat: @"There are %d red pixels in the image", numberOfRedPixels];
}

The example project on Github contains a complete working example.