imagecolorallocatealpha

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

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 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
5.1.3	Returns `FALSE` if the allocation failed. Previously -1 was returned.

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()

Notes

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

User Contributed Notes

eric (at) junioronline.us

10 years ago


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)

hosh [underscore] 90 [at] live [dot] com

5 years ago


I have had a case where I got the alpha from a raw RGBA pallete. This was stored as 8bit where 0 is completely transparant, and 255 is complete opaque.
This is impossible to use with imagecolorallocatealpha() as it requires a 7bit int where 0 is completely opaque and 127 is completely transparant (otherway around)
You can solve this by subtracting 255, removing the negative sign (either by converting to string and substr($alpha, 1) or some other way) and then bitshifting the answer to the right by one 1 bit.

Example:
<?php
$alpha = 0; // equivalent to alpha 127 needed for imagecolorallocatealpha()
$alpha = $alpha - 255; // subtract 255, this will give a negative number
$alpha = substr($alpha, 1); // remove negative/minus sign
$alpha = (int)$alpha; // convert back to integer
$alpha = $alpha >> 1; // bitshift to the right once.
echo $alpha; // output: 127

// This can be done in one line of course:
$alpha = 255; // equivalent to 0 for imagecolorallocatealpha()
$alpha = ((int)(substr($alpha - 255, 1))) >> 1;
echo $alpha; // outputs 0
?>

Ben Zemm

4 years ago


"This was stored as 8bit where 0 is completely transparant, and 255 is complete opaque."

This would be a lot more efficient:

<?php
$alpha7 = ((~((int)$alpha8)) & 0xff) >> 1;
?>

where alpha8==255, alpha7==0
where alpha8=127, alpha7== 64
where alpha8==0, alpha7==127

fjoggen at gmail dot com

8 years ago


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);
?>

Brett G

7 months ago


When working with transparency, avoid imagecreate() and use imagecreatetruecolor() instead. Transparency effects may not work as expected within a palette-based image.

fchristl at hotmail dot com

12 years ago


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);
?>