Histogram of Image in iPhone

Question

I am looking for a way to get a histogram of an image on the iPhone. The OpenCV library is way too big to be included in my app (OpenCV is about 70MB compiled), but I can use OpenGL. However, I have no idea on how to do either of these.

I have found how to get the pixels of the image, but cannot form a histogram. This seems like it should be simple, but I don't know how to store uint8_t into an array.

Here is the relevant question/answer for finding pixels:

Getting RGB pixel data from CGImage

Answer 1

The uint8_t* is just a pointer to a c array containing the bytes of the given color, i.e. {r, g, b, a} or whatever the color byte layout is for your image buffer.

So, referencing the link you provided, and the definition of histogram:

//Say we're in the inner loop and we have a given pixel in rgba format
const uint8_t* pixel = &bytes[row * bpr + col * bytes_per_pixel];
//Now save to histogram_counts uint32_t[4] planes r,g,b,a
//or you could just do one for brightness
//If you want to do data besides rgba, use bytes_per_pixel instead of 4
for (int i=0; i<4; i++) {
    //Increment count of pixels with this value
    histogram_counts[i][pixel[i]]++;
}

Answer 2

You can take RGB color of your image with CGRef. Look at the below method which I used for this.

- (UIImage *)processUsingPixels:(UIImage*)inputImage {

// 1. Get the raw pixels of the image
UInt32 * inputPixels;

CGImageRef inputCGImage = [inputImage CGImage];
NSUInteger inputWidth = CGImageGetWidth(inputCGImage);
NSUInteger inputHeight = CGImageGetHeight(inputCGImage);

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

NSUInteger bytesPerPixel = 4;
NSUInteger bitsPerComponent = 8;

NSUInteger inputBytesPerRow = bytesPerPixel * inputWidth;

inputPixels = (UInt32 *)calloc(inputHeight * inputWidth, sizeof(UInt32));

CGContextRef context = CGBitmapContextCreate(inputPixels, inputWidth, inputHeight,
                                             bitsPerComponent, inputBytesPerRow, colorSpace,
                                             kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);

// 3. Convert the image to Black & White
for (NSUInteger j = 0; j < inputHeight; j++) {
    for (NSUInteger i = 0; i < inputWidth; i++) {
        UInt32 * currentPixel = inputPixels + (j * inputWidth) + i;
        UInt32 color = *currentPixel;

        // Average of RGB = greyscale
        UInt32 averageColor = (R(color) + G(color) + B(color)) / 3.0;

        *currentPixel = RGBAMake(averageColor, averageColor, averageColor, A(color));
    }
}

// 4. Create a new UIImage
CGImageRef newCGImage = CGBitmapContextCreateImage(context);
UIImage * processedImage = [UIImage imageWithCGImage:newCGImage];

// 5. Cleanup!
CGColorSpaceRelease(colorSpace);
CGContextRelease(context);

   return processedImage;
}