imagecolorallocatealpha

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

imagecolorallocatealpha — 为一幅图像分配颜色 + alpha

说明

int imagecolorallocatealpha ( resource $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() 。

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 between 0 and 127. 0 indicates completely opaque while 127 indicates completely transparent.

The colors parameters are integers between 0 and 255 or hexadecimals between 0x00 and 0xFF.

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

版本	说明
Prior to 5.1.3	Returns -1 if the allocation failed.

范例

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：

Output of example : Example of using imagecolorallocatealpha()

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

用户评论:

[#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); ?>