Last updated: Fri, 22 Aug 2008

imagecolorallocatealpha

(PHP 4 >= 4.3.2, PHP 5)

imagecolorallocatealpha — Allocate a color for an image

Description

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

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

Parameters

image: An image resource, 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 colors parameters are integers between 0 and 255 or hexadecimals between 0x00 and 0xFF.

Return Values

A color identifier or FALSE if the allocation failed.

ChangeLog

Version	Description
Prior to 5.1.3	Returns -1 if the allocation failed.

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:

Notes

Note: This function requires GD 2.0.1 or later (2.0.28 or later is recommended).


If you need to calculate the integer representation of a color with an alpha channel, without initialising an image and using the imagecolorallocatealpha function. Then this function might be of some help:



<?php

function alphaColor($hexColor,$alpha)

{

        return bindec(decbin($alpha).decbin(hexdec($hexColor));

}



echo alphaColor("FFFFFF",127);

?>

eric (at) junioronline.us
25-Jan-2006 11:41


If you only wish to extract the alpha value for a color, you can simply extract it like so:



<?php



    $color = imagecolorat($im, 50, 50);

    $alpha = $color >> 24;



?>



It actually shifts off the first 24 bits (where 8x3 are used for each color), and returns the remaining 7 allocated bits (commonly used for alpha)

fchristl at hotmail dot com
19-Sep-2003 10:55


Check out this Source, it's a little funny feature showing you, for what imagecolorallocatealpha() is used:



<?php

$im=imagecreatetruecolor(300,300);

$white=imagecolorallocate($im,255,255,255);

imagefilledrectangle($im,0,0,imagesx($im),imagesy($im),$white);

for($i=0;$i<256;$i=$i+10)

{

$col=imagecolorallocatealpha($im,$i,$i,$i,ceil(rand(0,127)));

imagefilledellipse($im,$i,$i,$i,$i,$col);

}

header("content-type: image/png");

imagepng($im);

?>

add a note

imagecolorat

imagecolorallocate