imagecolorallocatealpha

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

    imagecolorallocatealpha — Allocate 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 valid gd 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
    $back = imagecolorallocate($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);
    ?>

    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
    6
    eric (at) junioronline.us
    18 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
    9 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
    2
    fjoggen at gmail dot com
    16 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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