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    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

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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
    17 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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