PHP 8.1.15 Released!

imagecolorallocatealpha

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

imagecolorallocatealpha为图像分配颜色

说明

imagecolorallocatealpha(
    GdImage $image,
    int $red,
    int $green,
    int $blue,
    int $alpha
): int|false

imagecolorallocatealpha() 的行为和 imagecolorallocate() 相同,但多了一个额外的透明度参数 alpha

参数

image

由图象创建函数(例如imagecreatetruecolor())返回的 GdImage 对象。

red

红色成分的值。

green

绿色成分的值。

blue

蓝色成分的值。

alpha

介于 0127 之间的值。0 表示完全不透明,而 127 表示完全透明。

redgreenblue 参数是 0 到 255 之间的整数或 0x00 到 0xFF 之间的十六进制数。

返回值

颜色标识符,如果分配失败,则为 false

警告

此函数可能返回布尔值 false,但也可能返回等同于 false 的非布尔值。请阅读 布尔类型章节以获取更多信息。应使用 === 运算符来测试此函数的返回值。

更新日志

版本 说明
8.0.0 image expects a GdImage instance now; previously, a resource was expected.

范例

示例 #1 使用 imagecolorallocatealpha() 的示例

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

// 白色背景加黑色边框
$back = imagecolorallocate($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;

// 用 alpha 值分配一些颜色
$yellow = imagecolorallocatealpha($image, 255, 255, 0, 75);
$red = imagecolorallocatealpha($image, 255, 0, 0, 75);
$blue = imagecolorallocatealpha($image, 0, 0, 255, 75);

// 绘制 3 个重叠的圆
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);

// 不要忘记输出正确的 header!
header('Content-Type: image/png');

// 最后,输出结果
imagepng($image);
imagedestroy($image);
?>

以上例程的输出类似于:

示例输出:使用 imagecolorallocatealpha() 的示例

示例 #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));
?>

以上例程会输出:

int(127)
int(0)

参见

add a note

User Contributed Notes 3 notes

up
6
eric (at) junioronline.us
17 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
15 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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