imagecolortransparent - PHP code example

How to Use the PHP Function ImagecolorTransparent

PHP is not limited to creating HTML output; it can also be used to create and manipulate images in a wide range of formats, including GIF, PNG, JPEG, TGA, BMP, XPM, and WBMP. This is possible because of the GD library of image functions that are built into the PHP core and available for use by programs that call them.

One of the most useful of these functions is imagecolortransparent, which sets a transparent color in an image. It takes two arguments, an image resource (typically returned by one of the image creation functions such as imagecreatetruecolor()) and a color identifier created with the imagecolorallocate() function. On success, it returns the identifier of this new transparent color; if the image does not have transparency, or if the identifier does not match a transparent color in the image, it will return -1.

This article gives a few examples of how to use this function in real world applications. Please feel free to add to these examples by commenting below!

Using PHP to compress images is easy. Unlike many other web development languages, PHP supports resizing images and compressing them, without having to use external software. To do this you simply pass a destination image - which can be any file - to the imagejpeg() function, along with a source image, and imagecolortransparent() will create a new destination image that is completely transparent. This new image can then be used in place of the original, without any loss of quality.

imagecolortransparent example

  public function getTransparentColor() {
    if (!$this->getResource() || $this->getType() != IMAGETYPE_GIF) {
      return NULL;
    }
    // Find out if a transparent color is set, will return -1 if no     // transparent color has been defined in the image.     $transparent = imagecolortransparent($this->getResource());
    if ($transparent >= 0) {
      // Find out the number of colors in the image palette. It will be 0 for       // truecolor images.       $palette_size = imagecolorstotal($this->getResource());
      if ($palette_size == 0 || $transparent < $palette_size) {
        // Return the transparent color, either if it is a truecolor image         // or if the transparent color is part of the palette.         // Since the index of the transparent color is a property of the         // image rather than of the palette, it is possible that an image         // could be created with this index set outside the palette size.         // (see http://stackoverflow.com/a/3898007).

imagesavealpha($res, TRUE);
        break;

      case IMAGETYPE_GIF:
        if (empty($arguments['transparent_color'])) {
          // No transparency color specified, fill white transparent.           $fill_color = imagecolorallocatealpha($res, 255, 255, 255, 127);
        }
        else {
          $fill_rgb = Color::hexToRgb($arguments['transparent_color']);
          $fill_color = imagecolorallocatealpha($res, $fill_rgb['red'], $fill_rgb['green'], $fill_rgb['blue'], 127);
          imagecolortransparent($res, $fill_color);
        }
        imagefill($res, 0, 0, $fill_color);
        break;

      case IMAGETYPE_JPEG:
        imagefill($res, 0, 0, imagecolorallocate($res, 255, 255, 255));
        break;

    }

    // Update the toolkit properties.

$distance = pow(($actual[0] - $expected[0]), 2) + pow(($actual[1] - $expected[1]), 2) + pow(($actual[2] - $expected[2]), 2) + pow(($actual[3] - $expected[3]), 2);
    $this->assertLessThanOrEqual($tolerance, $distance, $message . " - Actual: {" . implode(',', $actual) . "}, Expected: {" . implode(',', $expected) . "}, Distance: " . $distance . ", Tolerance: " . $tolerance);
  }

  /** * Function for finding a pixel's RGBa values. */
  public function getPixelColor(ImageInterface $image, int $x, int $y): array {
    $toolkit = $image->getToolkit();
    $color_index = imagecolorat($toolkit->getResource(), $x, $y);

    $transparent_index = imagecolortransparent($toolkit->getResource());
    if ($color_index == $transparent_index) {
      return [0, 0, 0, 127];
    }

    return array_values(imagecolorsforindex($toolkit->getResource(), $color_index));
  }

  /** * Data provider for ::testManipulations(). */
  public function providerTestImageFiles(): array {

// Background color is not specified: use transparent white as background.       $background = ['red' => 255, 'green' => 255, 'blue' => 255, 'alpha' => 127];
    }
    // Store the color index for the background as that is what GD uses.     $arguments['background_idx'] = imagecolorallocatealpha($this->getToolkit()->getResource(), $background['red'], $background['green'], $background['blue'], $background['alpha']);

    if ($this->getToolkit()->getType() === IMAGETYPE_GIF) {
      // GIF does not work with a transparency channel, but can define 1 color       // in its palette to act as transparent.
      // Get the current transparent color, if any.       $gif_transparent_id = imagecolortransparent($this->getToolkit()->getResource());
      if ($gif_transparent_id !== -1) {
        // The gif already has a transparent color set: remember it to set it on         // the rotated image as well.         $arguments['gif_transparent_color'] = imagecolorsforindex($this->getToolkit()->getResource(), $gif_transparent_id);

        if ($background['alpha'] >= 127) {
          // We want a transparent background: use the color already set to act           // as transparent, as background.           $arguments['background_idx'] = $gif_transparent_id;
        }
      }