imagecolorallocatealpha

(PHP 4 >= 4.3.2, PHP 5, PHP 7, PHP 8)

imagecolorallocatealpha — Allocate a color for an image

Description

imagecolorallocatealpha(
    GdImage $image,
    int $red,
    int $green,
    int $blue,
    int $alpha
): int|false

imagecolorallocatealpha() behaves identically to imagecolorallocate() with the addition of the transparency parameter alpha.

Parameters

image

A GdImage object, returned by one of the image creation functions, such as imagecreatetruecolor().

red

Value of red component.

green

Value of green component.

blue

Value of blue component.

alpha

A value between 0 and 127. 0 indicates completely opaque while 127 indicates completely transparent.

The red, green and blue parameters are integers between 0 and 255 or hexadecimals between 0x00 and 0xFF.

Return Values

A color identifier or false if the allocation failed.

Warning

This function may return Boolean false, but may also return a non-Boolean value which evaluates to false. Please read the section on Booleans for more information. Use the === operator for testing the return value of this function.

Changelog

Version Description
8.0.0 image expects a GdImage instance now; previously, a valid gd resource was expected.

Examples

Example #1 Example of using imagecolorallocatealpha()

<?php
$size = 300;
$image=imagecreatetruecolor($size, $size);

// something to get a white background with black border
$back = imagecolorallocate($image, 255, 255, 255);
$border = imagecolorallocate($image, 0, 0, 0);
imagefilledrectangle($image, 0, 0, $size - 1, $size - 1, $back);
imagerectangle($image, 0, 0, $size - 1, $size - 1, $border);

$yellow_x = 100;
$yellow_y = 75;
$red_x = 120;
$red_y = 165;
$blue_x = 187;
$blue_y = 125;
$radius = 150;

// allocate colors with alpha values
$yellow = imagecolorallocatealpha($image, 255, 255, 0, 75);
$red = imagecolorallocatealpha($image, 255, 0, 0, 75);
$blue = imagecolorallocatealpha($image, 0, 0, 255, 75);

// drawing 3 overlapped circle
imagefilledellipse($image, $yellow_x, $yellow_y, $radius, $radius, $yellow);
imagefilledellipse($image, $red_x, $red_y, $radius, $radius, $red);
imagefilledellipse($image, $blue_x, $blue_y, $radius, $radius, $blue);

// don't forget to output a correct header!
header('Content-Type: image/png');

// and finally, output the result
imagepng($image);
imagedestroy($image);
?>

The above example will output something similar to:

Output of example : Example of using imagecolorallocatealpha()

Example #2 Convert typical alpha values for use with imagecolorallocatealpha()

Usually alpha values of 0 designate fully transparent pixels, and the alpha channel has 8 bits. To convert such alpha values to be compatible with imagecolorallocatealpha(), some simple arithmetic is sufficient:

<?php
$alpha8 = 0; // fully transparent
var_dump(127 - ($alpha8 >> 1));
$alpha8 = 255; // fully opaque
var_dump(127 - ($alpha8 >> 1));
?>

The above example will output:

int(127)
int(0)

See Also