Keeping only the red/green/blue part of the image

Question

I have made a very basic algorithm which extracts only the red / green / blue pixels of the image and displays them. However, it works well on some images and produces unexpected results in some. Like when I want to keep only green , it also keeps turquoise.

Turquoise is a shade of green but it is not what I want to display. I only want things that are 'visually' green.

Here is a sample output that shows what has gone wrong:
enter image description here
The algorithm picked up the turquoiose color of the flower pot on which the dog sits. The original image is here.

My algorithm is below (for the green one.) All the algorithms are akin to each other.

void keepGreen() {
  for (int i = 0;  // iterate over the pixels of the image
  i < img.pixels.length;
  i++) {
    float inputRed = red(img.pixels[i]); // extract red
    float inputGreen = green(img.pixels[i]); // extract green
    float inputBlue = blue(img.pixels[i]); // extract blue

    int pixel = -1;

    float outputRed = -1;
    float outputGreen = -1;
    float outputBlue = -1;

    if(inputRed <= inputGreen*0.9 && inputBlue <= inputGreen*0.9){ // check if the pixel is visually green
      outputRed = inputRed; // yes, let it stay
      outputGreen = inputGreen;
      outputBlue = inputBlue;
    }else{ // no, make it gray
      int mostProminent =(int) max(inputRed, inputGreen, inputBlue);
      int leastProminent =(int) min(inputRed, inputGreen, inputBlue);
      int avg = (int) ((mostProminent + leastProminent) / 2);
      outputRed = avg;
      outputGreen = avg;
      outputBlue = avg;
      pixel = color(avg, avg, avg);
    }
    img.pixels[i] = color(outputRed, outputGreen, outputBlue); // set the pixel to the new value
  }
  img.updatePixels(); // update the image
  image(img, WIDTH/2, HEIGHT/2, calculatedWidth, calculatedHeight); // display
}

How can I avoid those errors ?

Jiminion Jiminion · Accepted Answer · 2013-07-18T19:10:10

Just because Blue is less than Green doesn't mean the pixel doesn't look green. For example, turquoise might be red=50, blue=200, green=150. Perhaps you need to (also) gray out pixels that have substantial green in their own right, regardless of red/blue.

Keeping only the red/green/blue part of the image

2 Answers