What is the range of alpha value? (0 to 127)

M66 2025-05-29

In PHP, the imagecolorallocatealpha() function is a function used to assign colors on an image, which allows setting the transparency of an image. The value of transparency is controlled by the alpha parameter, where the value range of alpha does have certain limitations. Understanding this limitation is very important for the correct use of transparency.

Function introduction

The basic syntax of the imagecolorallocatealpha() function is as follows:

 int imagecolorallocatealpha ( resource $image , int $red , int $green , int $blue , int $alpha )

$image : Target image resource.
$red : The red component, ranging from 0 to 255.
$green : Green component, ranging from 0 to 255.
$blue : Blue component, ranging from 0 to 255.
$alpha : Transparency component, ranging from 0 to 127.

Among them, the value of alpha is used to control the transparency of the color. The transparency setting is directly related to the alpha value, 0 means completely opaque, and 127 means completely transparent.

Alpha value range

For the imagecolorallocatealpha() function, the value range of the alpha parameter is not from 0 to 255, but from 0 to 127.

0 : Completely opaque, indicating that the color is opaque.
127 : Completely transparent, indicating that the color is completely transparent, and is usually used in transparent areas of the image.

By adjusting the alpha value, we can control the transparency effect of the color. For example, setting alpha to 64 will make the color translucent.

Why 0 to 127?

The imagecolorallocatealpha() function in PHP is implemented based on the GD library, which uses a 7-digit transparency value. That is, the maximum alpha value is 127, which is different from the 8-bit transparency value (0-255). This design takes into account performance and compatibility, and in many cases the maximum value of 127 is sufficient to meet most transparency needs.

Code Example

Suppose we want to create an image with a translucent background, the code can be like this:

 <?php
// Create a true color image
$image = imagecreatetruecolor(200, 200);

// Assign transparent colors to images
$transparent = imagecolorallocatealpha($image, 255, 0, 0, 64);  // Translucent red

// Fill the image background with transparent color
imagefill($image, 0, 0, $transparent);

// Send header information，Tell the browser to outputPNGFormat image
header('Content-Type: image/png');
imagepng($image);

// Destroy image resources
imagedestroy($image);
?>

In this example, we assign a translucent red color (alpha value is set to 64) and fill it onto the image background. The generated image will show a translucent red background.

Things to note

When using transparency, it is recommended to use PNG format to output images, because PNG supports transparency, while JPEG format does not.
When the alpha value is set to 127, the color in the image will be completely transparent. In image synthesis, this is often used to create transparent effects.

By mastering the use of imagecolorallocatealpha() function, you can easily create images with transparent effects in PHP, enhancing the user experience, especially in web development.

imagecolorallocatealpha