imagecolorallocatealpha()

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

为一幅图像分配颜色+ alpha

说明

imagecolorallocatealpha(resource $image,int $red,int $green,int $blue,int $alpha): int

imagecolorallocatealpha()的行为和imagecolorallocate()相同，但多了一个额外的透明度参数$alpha，其值从0到127。0表示完全不透明，127表示完全透明。

如果分配失败则返回FALSE。

使用imagecolorallocatealpha()的例子

<?php
$size = 300;
$image=imagecreatetruecolor($size, $size);
// 用白色背景加黑色边框画个方框
$back = imagecolorallocate($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);
// 画三个交迭的圆
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);
?>

Note:此函数需要 GD 2.0.1 或更高版本(推荐 2.0.28 及更高版本)。

参见imagecolorallocate()和imagecolordeallocate()。

参数

$image

由图象创建函数(例如imagecreatetruecolor())返回的图象资源。

$red

红色成分的值。

$green

绿色成分的值。

$blue

蓝色成分的值。

$alpha

A value between0and127.0indicates completely opaque while127indicates completely transparent.

返回值

A color identifier or FALSE if the allocation failed.

Warning

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

范例

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);
?>

以上例程的输出类似于：

注释

Note:此函数需要 GD 2.0.1 或更高版本(推荐 2.0.28 及更高版本)。

参见

• imagecolorallocate()为一幅图像分配颜色
• imagecolordeallocate()取消图像颜色的分配

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)

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);
?>

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