Greyscale Image Conversion with PHP  

by Matt Hawkins, 13/02/2012
Categories : PHP & MySQL

If you need to convert an image to greyscale (or grayscale) using PHP then code is for you! It can handle Truecolor and indexed images and I have tested with JPG and PNG images.

Lots of examples out there assume your input image is truecolor. If you are trying to convert an indexed PNG these examples result in a completely black image.

The example below tests the input image and handles the colour conversion correctly. This code can easily be used to create a function that accepts an image resource and returns the greyscale version.

// Source image to open
$src_file = "test.jpg";

// Load image
$img_src = ImageCreateFromjpeg($src_file);

// Get dimensions of source image
$img_srcw = imagesx($img_src);
$img_srch = imagesy($img_src);

// Create a blank truecolor image with the same dimensions
$img_dst = imagecreatetruecolor($img_srcw,$img_srch);

// Loop through every pixel of source image
for ($i=0; $i<$img_srcw; $i++) {
  for ($j=0; $j<$img_srch; $j++) {
    // Get the colour of pixel
    $colour = ImageColorAt($img_src, $i, $j);

    // Get RGB values
    if (imageistruecolor($img_src)) {
      // Image is true colour so $colour
      // contains RGB value
      $rr = ($colour >> 16) & 0xFF;
      $gg = ($colour >> 8) & 0xFF;
      $bb = $colour & 0xFF;                  
    } else {
      // Image is not true colour so $colour
      // contains index to a colour
      $colour_tran = imagecolorsforindex($img_src, $colour);
      $rr = $colour_tran['red'];
      $gg = $colour_tran['green'];
      $bb = $colour_tran['blue'];                  

    // Calculate grey value
    $g = round(($rr + $gg + $bb) / 3);

    // Create that colour in the images palette                
    $colour_grey = imagecolorallocate($img_dst, $g, $g, $g);

    // Set the pixel in output image to grey colour
    imagesetpixel ($img_dst, $i, $j, $colour_grey);

// Generate and output JPG image
header("Content-type: image/jpeg");
imagejpg ($img_dst);

// Destroy images and free memory


This example takes a JPEG image as an input but can be easily modified to accept a PNG image instead. The following lines would need to be adjusted :
$img_src = ImageCreateFromjpeg($src_file);
header("Content-type: image/jpeg");
imagejpg ($img_dst);

Author : Matt Hawkins  Last Edit By : Matt Hawkins
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