imagecolorallocate
"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
imagecolorallocatealpha
(PHP 4 >= 4.3.2, PHP 5)
imagecolorallocatealpha — Allocate a color for an image
설명
int imagecolorallocatealpha
( resource $image
, int $red
, int $green
, int $blue
, int $alpha
)
imagecolorallocatealpha() behaves identically to imagecolorallocate() with the addition of the transparency parameter alpha.
인수
- image
-
imagecreatetruecolor() 등의 이미지 생성 함수에서 반환한 이미지 자원.
- 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.
반환값
A color identifier or FALSE if the allocation failed.
변경점
| 버전 | 설명 |
|---|---|
| Prior to 5.1.3 | Returns -1 if the allocation failed. |
예제
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($image);
?>
위 예제의 출력 예시:
주의
Note: 이 함수는 GD 2.0.1 이상이 필요합니다. (2.0.28 이상을 권장합니다)
참고
- imagecolorallocate() - Allocate a color for an image
- imagecolordeallocate() - De-allocate a color for an image
add a note
User Contributed Notes
imagecolorallocatealpha
Ben Zemm
23-Nov-2011 06:40
hosh [underscore] 90 [at] live [dot] com
28-Mar-2011 05:57
I have had a case where I got the alpha from a raw RGBA pallete. This was stored as 8bit where 0 is completely transparant, and 255 is complete opaque.
This is impossible to use with imagecolorallocatealpha() as it requires a 7bit int where 0 is completely opaque and 127 is completely transparant (otherway around)
You can solve this by subtracting 255, removing the negative sign (either by converting to string and substr($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 $alpha; // outputs 0
?>
fjoggen at gmail dot com
29-Nov-2007 07:19
If you need to calculate the integer representation of a color with an alpha channel, without initialising an image and using the imagecolorallocatealpha function. Then this function might be of some help:
<?php
function alphaColor($hexColor,$alpha)
{
return bindec(decbin($alpha).decbin(hexdec($hexColor));
}
echo alphaColor("FFFFFF",127);
?>
eric (at) junioronline.us
25-Jan-2006 01:41
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)
fchristl at hotmail dot com
19-Sep-2003 12:55
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);
?>