How to to remove certain colors from an image with PHP or Ruby?

Question

Say there are 3 circles: red, blue, black.

I only want the black circle to remain. How can I remove the red and blue circles?

Answer 1

Since you have asked for a PHP solution:

• First load your picture with imagecreatefrompng or the similar functions for other image formats
• Afterwards, use imagesx and imagesy to get the size of the image.

• Now, what you can loop over all pixels via

  for ($i = 0; $i < $imageWidth; $i++) {
      for ($j = 0; $j < $imageHeight; $j++) {
          // check color and replace
      }
  }
  

• Finally, use imagecolorat to get the color (check if it is in a specific range, don't take only black as a good color, but also all colors that have >= 250 at each value of red, green and blue for example)

• ... and imagecolorset to set the color
• Now you can save the image using imagepng for example.