imagecolorallocatealpha

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

imagecolorallocatealphaAllocate a color for an image

Descrizione

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.

Elenco dei parametri

image

Una risorsa immagine, restituita da una delle funzioni di creazione immagine, come imagecreatetruecolor().

red

Valore della componente rossa.

green

Valore della componente verde.

blue

Valore della componente blu.

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.

Valori restituiti

A color identifier or false if the allocation failed.

Avviso

Questa funzione può restituire il Booleano false, ma può anche restituire un valore non-Booleano valutato come false. Fare riferimento alla sezione Booleans per maggiori informazioni. Usare l'operatore === per controllare il valore restituito da questa funzione.

Log delle modifiche

Versione Descrizione
8.0.0 image expects a GdImage instance now; previously, a resource was expected.

Esempi

Example #1 Example of using imagecolorallocatealpha()

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

// something to get a white background with black border
$back imagecolorallocate($image255255255);
$border imagecolorallocate($image000);
imagefilledrectangle($image00$size 1$size 1$back);
imagerectangle($image00$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
$yellow imagecolorallocatealpha($image255255075);
$red    imagecolorallocatealpha($image2550075);
$blue   imagecolorallocatealpha($image0025575);

// 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);
?>

Il precedente esempio visualizzerà qualcosa simile a:

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));
?>

Il precedente esempio visualizzerà:

int(127)
int(0)

Vedere anche:

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User Contributed Notes 3 notes

up
7
eric (at) junioronline.us
16 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)
up
3
Brett G
7 years ago
When working with transparency, avoid imagecreate() and use imagecreatetruecolor() instead. Transparency effects may not work as expected within a palette-based image.
up
3
fjoggen at gmail dot com
14 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);
?>
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