This Image Processing Reference section displays the code for an example program that demonstrates how to calculate a Haar wavelet transform for the color channels of an image.

HaarChannels.php

<?php
  function Clamp($iGray) {
    if ($iGray < 0) {
      return 0;
    } elseif ($iGray > 255) {
      return 255;
    } else {
      return $iGray;
    }
  }

  function CreateHaarWaveletTransformImage($qColorImage) {
    $iW   = ImageSX($qColorImage);
    $iH   = ImageSY($qColorImage);
    // Allocate the color channel wavelet images
    $qImageR   = ImageCreate(256, 256);
    $qImageG   = ImageCreate(256, 256);
    $qImageB   = ImageCreate(256, 256);
    $qExtra    = ImageCreate(256, 256);
    for ($iC = 0; $iC < 256; ++$iC) {
      ImageColorAllocate($qImageR, $iC,   0,   0);
      ImageColorAllocate($qImageG,   0, $iC,   0);
      ImageColorAllocate($qImageB,   0,   0, $iC);
      ImageColorAllocate($qExtra, $iC, $iC, $iC);
    }
    // Create the Red, Green, and Blue Channel images
    for ($j = 0; $j < $iH; ++$j) {
      for ($i = 0; $i < $iW; ++$i) {
        $qColor  = ImageColorAt($qColorImage, $i, $j);
        $iRed    = (($qColor >> 16) & 0xFF);
        $iGreen  = (($qColor >> 8) & 0xFF);
        $iBlue   = ($qColor & 0xFF);
        ImageSetPixel($qImageR, $i, $j, $iRed);
        ImageSetPixel($qImageG, $i, $j, $iGreen);
        ImageSetPixel($qImageB, $i, $j, $iBlue);
      }
    }
    $qaChannelImages = array();
    $qaChannelImages[0] = $qImageR;
    $qaChannelImages[1] = $qImageG;
    $qaChannelImages[2] = $qImageB;
    $iaaIn   = array(array(0,0), array(0,0));
    $iaaOut  = array(array(0,0), array(0,0));
    for ($iCH = 0; $iCH < 3; ++$iCH) {
      $qCurrImage     = $qaChannelImages[$iCH];
      $iLowPassWidth  = $iW/2;
      $iaaT = array(array(0,0), array(0,0));
      while ($iLowPassWidth > 1) {
        $iDoubleWidth = 2*$iLowPassWidth;
        for ($j = 0; $j < $iDoubleWidth; $j += 2) {
          for ($i = 0; $i < $iDoubleWidth; $i += 2) {
            // First the red image
            $iaaIn[0][0] = ImageColorAt($qCurrImage, $i, $j);
            $iaaIn[0][1] = ImageColorAt($qCurrImage, $i + 1, $j);
            $iaaIn[1][0] = ImageColorAt($qCurrImage, $i, $j + 1);
            $iaaIn[1][1] = ImageColorAt($qCurrImage, $i + 1, $j + 1);
            $iaaOut[0][0] = Clamp(($iaaIn[0][0] + $iaaIn[0][1] + $iaaIn[1][0] + $iaaIn[1][1]) >> 2);
            $iaaOut[0][1] = Clamp((($iaaIn[0][0] - $iaaIn[0][1] + $iaaIn[1][0] - $iaaIn[1][1]))
              + 128);
            $iaaOut[1][0] = Clamp((($iaaIn[0][0] + $iaaIn[0][1] - $iaaIn[1][0] - $iaaIn[1][1]))
              + 128);
            $iaaOut[1][1] = Clamp((($iaaIn[0][0] - $iaaIn[0][1] - $iaaIn[1][0] + $iaaIn[1][1]))
              + 128);
            ImageSetPixel($qExtra, ($i >> 1), ($j >> 1), $iaaOut[0][0]);
            ImageSetPixel($qExtra, ($i >> 1) + $iLowPassWidth, ($j >> 1), $iaaOut[0][1]);
            ImageSetPixel($qExtra, ($i >> 1), ($j >> 1) + $iLowPassWidth, $iaaOut[1][0]);
            ImageSetPixel($qExtra, ($i >> 1) + $iLowPassWidth, ($j >> 1) + $iLowPassWidth,
              $iaaOut[1][1]);
            ImageSetPixel($qCurrImage, ($i >> 1), ($j >> 1), $iaaOut[0][0]);
          }
        }
        $iLowPassWidth >>= 1;
      }
      // Copy the transformed image back to the channel
      for ($j = 0; $j < $iH; ++$j) {
        for ($i = 0; $i < $iW; ++$i) {
          $iPixel = ImageColorAt($qExtra, $i, $j);
          ImageSetPixel($qCurrImage, $i, $j, $iPixel);
        }
      }
    }
    $qHaarChannelImage = ImageCreateTrueColor(2*$iW, 2*$iH);
    // Copy the transformed color channel images
    ImageCopyMerge($qHaarChannelImage, $qColorImage, 0, 0, 0, 0, $iW, $iH, 100);
    ImageCopyMerge($qHaarChannelImage, $qImageR, $iW, 0, 0, 0, $iW, $iH, 100);
    ImageCopyMerge($qHaarChannelImage, $qImageG, $iW, $iH, 0, 0, $iW, $iH, 100);
    ImageCopyMerge($qHaarChannelImage, $qImageB, 0, $iH, 0, 0, $iW, $iH, 100);

    ImageDestroy($qImageR);
    ImageDestroy($qImageG);
    ImageDestroy($qImageB);
    ImageDestroy($qExtra);
    return $qHaarChannelImage;
  }

  $qOriginalImage = ImageCreateFromPng("CrucifixionDali256x256.png");

  $qHaarChannelImage = CreateHaarWaveletTransformImage($qOriginalImage);

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

Output