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imagecolorallocatealpha — 为一幅图像分配颜色 + alpha
$image
, int $red
, int $green
, int $blue
, int $alpha
) 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
。
Example #1 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() 。
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 between 0 and 127. 0 indicates completely opaque while 127 indicates completely transparent.
A color identifier or FALSE
if the allocation failed.
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。
版本 | 说明 |
---|---|
Prior to 5.1.3 | Returns -1 if the allocation failed. |
Example #2 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:
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)。
[#1] Brett G [2015-09-22 22:30:41]
When working with transparency, avoid imagecreate() and use imagecreatetruecolor() instead. Transparency effects may not work as expected within a palette-based image.
[#2] Ben Zemm [2011-11-23 18:40:49]
"This was stored as 8bit where 0 is completely transparant, and 255 is complete opaque."
This would be a lot more efficient:
<?php
$alpha7 = ((~((int)$alpha8)) & 0xff) >> 1;
?>
where alpha8==255, alpha7==0
where alpha8=127, alpha7== 64
where alpha8==0, alpha7==127
[#3] hosh [underscore] 90 [at] live [dot] com [2011-03-28 17:57:40]
I have had a case where I got the alpha from a raw RGBA pallete. This was stored as 8bit where 0 is completely transparant, and 255 is complete opaque.
This is impossible to use with imagecolorallocatealpha() as it requires a 7bit int where 0 is completely opaque and 127 is completely transparant (otherway around)
You can solve this by subtracting 255, removing the negative sign (either by converting to string and substr($alpha, 1) or some other way) and then bitshifting the answer to the right by one 1 bit.
Example:
<?php
$alpha = 0; // equivalent to alpha 127 needed for imagecolorallocatealpha()
$alpha = $alpha - 255; // subtract 255, this will give a negative number
$alpha = substr($alpha, 1); // remove negative/minus sign
$alpha = (int)$alpha; // convert back to integer
$alpha = $alpha >> 1; // bitshift to the right once.
echo $alpha; // output: 127
// This can be done in one line of course:
$alpha = 255; // equivalent to 0 for imagecolorallocatealpha()
$alpha = ((int)(substr($alpha - 255, 1))) >> 1;
echo $alpha; // outputs 0
?>
[#4] fjoggen at gmail dot com [2007-11-29 07:19:55]
If you need to calculate the integer representation of a color with an alpha channel, without initialising an image and using the imagecolorallocatealpha function. Then this function might be of some help:
<?php
function alphaColor($hexColor,$alpha)
{
return bindec(decbin($alpha).decbin(hexdec($hexColor));
}
echo alphaColor("FFFFFF",127);
?>
[#5] eric (at) junioronline.us [2006-01-25 01:41:16]
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)
[#6] fchristl at hotmail dot com [2003-09-19 12:55:46]
Check out this Source, it's a little funny feature showing you, for what imagecolorallocatealpha() is used:
<?php
$im=imagecreatetruecolor(300,300);
$white=imagecolorallocate($im,255,255,255);
imagefilledrectangle($im,0,0,imagesx($im),imagesy($im),$white);
for($i=0;$i<256;$i=$i+10)
{
$col=imagecolorallocatealpha($im,$i,$i,$i,ceil(rand(0,127)));
imagefilledellipse($im,$i,$i,$i,$i,$col);
}
header("content-type: image/png");
imagepng($im);
?>