This Image Processing Reference section displays the code for an example program that demonstrates how to separate the red, green, and blue color channels in a color image with GD.

SeparateColorChannels.php

<?php
  function CreateColorChannelsImage($qOriginalImage) {
    $iWidth        = ImageSX($qOriginalImage);
    $iHeight       = ImageSY($qOriginalImage);
    $qRedImage     = ImageCreate($iWidth, $iHeight);
    $qGreenImage   = ImageCreate($iWidth, $iHeight);
    $qBlueImage    = ImageCreate($iWidth, $iHeight);
    for ($iC = 0; $iC < 256; ++$iC) {
      ImageColorAllocate($qRedImage, $iC, 0, 0);
      ImageColorAllocate($qGreenImage, 0, $iC, 0);
      ImageColorAllocate($qBlueImage, 0, 0, $iC);
    }
    for ($j = 0; $j < $iHeight; ++$j) {
      for ($i = 0; $i < $iWidth; ++$i) {
        $qColor  = ImageColorAt($qOriginalImage, $i, $j);
        $iRed    = (($qColor >> 16) & 0xFF);
        $iGreen  = (($qColor >> 8) & 0xFF);
        $iBlue   = ($qColor & 0xFF);
        ImageSetPixel($qRedImage, $i, $j, $iRed);
        ImageSetPixel($qGreenImage, $i, $j, $iGreen);
        ImageSetPixel($qBlueImage, $i, $j, $iBlue);
      }
    }
    $qChannelsImage = ImageCreateTrueColor(2*$iWidth, 2*$iHeight);
    // Copy the color channel images to the main image
    ImageCopyMerge($qChannelsImage, $qOriginalImage, 0, 0, 0, 0, $iWidth, $iHeight, 100);
    ImageCopyMerge($qChannelsImage, $qRedImage, $iWidth, 0, 0, 0, $iWidth, $iHeight, 100);
    ImageCopyMerge($qChannelsImage, $qGreenImage, 0, $iHeight, 0, 0, $iWidth, $iHeight, 100);
    ImageCopyMerge($qChannelsImage, $qBlueImage, $iWidth, $iHeight, 0, 0, $iWidth, $iHeight, 100);
    ImageDestroy($qRedImage);
    ImageDestroy($qGreenImage);
    ImageDestroy($qBlueImage);
    return $qChannelsImage;
  }

  $qOriginalImage = ImageCreateFromPng("SupperAtEmmaus.png");

  $qChannelsImage = CreateColorChannelsImage($qOriginalImage);

  Header('Content-type: image/png');
  ImagePng($qChannelsImage);
  ImageDestroy($qOriginalImage);
  ImageDestroy($qChannelsImage);
?>

Output