Image Colourising In PHP

Colourising images is fairly simple to accomplish, especially using PHP's GD library. All we need to do is load an image, create a blank image of the same size in a particular colour and then merge the two images together.

In fact, we can do this entirely with the imagecopymerge() function, but creating a function to wrap all of this makes sense as well.

The following function takes an image resource (as created by imagecreatefrompng()), the red, green, and blue values of the colour, and the percentage to overlay the colour on top of the image. The percentage can be set to 0 for no effect and 100 to fully replace the image with the given colour.

/**
 * Colourise an image.
 *
 * @param resource $image
 *   The image resource.
 * @param int $r
 *   The red part of the colour (0 to 255).
 * @param int $g
*   The green part of the colour (0 to 255).
 * @param int $b
*   The blue part of the colour (0 to 255).
 * @param int $percent
 *   The percentage of colourisation that should occur.
 */
function imagecolourise(&$image, $r, $g, $b, $percent) {
  // Ensure the percentage is restricted to a given value.
  if ($percent < 0) {
    $percent = 0;
  } elseif ($percent > 100) {
    $percent = 100;
  }

  // Get the image's width.
  $imageWidth = imagesx($image);

  // Get the image's height.
  $imageHeight = imagesy($image);

  // Create the layover image.
  $layover = imagecreate($imageWidth, $imageHeight);

  // Allocate the colour on this image.
  $colour = imagecolourallocate($layover, $r, $g, $b);

  // Merge the layover image on top of the other image.
  imagecopymerge($image, $layover, 0, 0, 0, 0, $imageWidth, $imageHeight, $percent);

  // Destroy the layover image.
  imagedestroy($layover);
}

We can use this function in the following way.

// Load an image.
$image = imagecreatefromjpeg('my_image.jpeg');

// Colourise the image with a blue colour at 75% saturation.
imagecolourise($image, 10, 10, 200, 75);

// Save the new image.
imagepng($image, 'colourised.png');

// Destroy the loaded image.
imagedestroy($image);

Taking the following image as an example.

TRΛVELER .

If we process this through the above code we can apply a very deep blue to the image so that it looks like this.

Using this function it is possible to quickly apply a mask to an image to help it blend in the background of a web page. It is also possible to use the imagecopymerge() function to create a watermark for an image by layering one image on top of another.