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

ColorHistograms.php

<?php
  function CreateColorHistogramsImage($qOriginalImage) {
    $iWidth        = ImageSX($qOriginalImage);
    $iHeight       = ImageSY($qOriginalImage);
    // Allocate the color histogram images
    $qHistImageR   = ImageCreate(256, 256);
    $qHistImageG   = ImageCreate(256, 256);
    $qHistImageB   = ImageCreate(256, 256);
    $iaHistR       = array();
    $iaHistG       = array();
    $iaHistB       = array();
    // Initialize the entries 0 to 255 to zero
    $iaHistR       = array_fill(0, 256, 0);
    $iaHistG       = array_fill(0, 256, 0);
    $iaHistB       = array_fill(0, 256, 0);
    // Color the histogram backgrounds
    ImageColorAllocate($qHistImageR, 255, 196, 196);
    ImageColorAllocate($qHistImageG, 196, 255, 196);
    ImageColorAllocate($qHistImageB, 196, 196, 255);
    $qRed   = ImageColorAllocate($qHistImageR, 255, 0, 0);
    $qGreen = ImageColorAllocate($qHistImageG, 0, 255, 0);
    $qBlue  = ImageColorAllocate($qHistImageB, 0, 0, 255);
    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);
        ++$iaHistR[$iRed];
        ++$iaHistG[$iGreen];
        ++$iaHistB[$iBlue];
      }
    }
    $iMax = 1;
    // Find the largest value in histograms for scaling
    for ($iC = 0; $iC < 256; ++$iC) {
      $iMax = (($iaHistR[$iC] > $iMax) ? $iaHistR[$iC] : $iMax);
      $iMax = (($iaHistG[$iC] > $iMax) ? $iaHistG[$iC] : $iMax);
      $iMax = (($iaHistB[$iC] > $iMax) ? $iaHistB[$iC] : $iMax);
    }
    // Draw the histogram lines in each image
    for ($iC = 0; $iC < 256; ++$iC) {
      $iHeightR = IntVal(Round(255*$iaHistR[$iC]/$iMax));
      $iHeightG = IntVal(Round(255*$iaHistG[$iC]/$iMax));
      $iHeightB = IntVal(Round(255*$iaHistB[$iC]/$iMax));
      ImageLine($qHistImageR, $iC, 255, $iC, 255-$iHeightR, $qRed);
      ImageLine($qHistImageG, $iC, 255, $iC, 255-$iHeightG, $qGreen);
      ImageLine($qHistImageB, $iC, 255, $iC, 255-$iHeightB, $qBlue);
    }
	$iHistImageWidth = ((768 > $iWidth) ? 768 : $iWidth);
    $qHistogramsImage = ImageCreateTrueColor($iHistImageWidth, 256+$iHeight);
    // Copy the histogram images to the main image
    ImageCopyMerge($qHistogramsImage, $qHistImageR, 0, 0, 0, 0, 256, 256, 100);
    ImageCopyMerge($qHistogramsImage, $qHistImageG, 256, 0, 0, 0, 256, 256, 100);
    ImageCopyMerge($qHistogramsImage, $qHistImageB, 512, 0, 0, 0, 256, 256, 100);
    ImageCopyMerge($qHistogramsImage, $qOriginalImage, 0, 256, 0, 0, $iWidth, $iHeight, 100);
    ImageDestroy($qHistImageR);
    ImageDestroy($qHistImageG);
    ImageDestroy($qHistImageB);
    return $qHistogramsImage;
  }

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

  $qHistogramsImage = CreateColorHistogramsImage($qOriginalImage);

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

Output