45 lines
1.5 KiB
PHP
45 lines
1.5 KiB
PHP
<?php
|
|
|
|
/* Computes the lightness of a pixel. Lightness is used in HSL notation, as well
|
|
* as in various operations on images. This function returns a normalized value
|
|
* for lightness between 0 and 1, inclusive (0.0 being black, 1.0 being white).
|
|
*
|
|
* double imagelightnessat(resource img, int x, int y)
|
|
* img An open image resource to operate on (true color or palette)
|
|
* x, y The coordinates of the pixel to work on
|
|
*
|
|
* by Jed Smith <?php $u = "jed"; $d = "bz"; printf("<%s@%s.%s>", $u, $u, $d) ?>
|
|
*/
|
|
function imagelightnessat($img, $x, $y) {
|
|
if (!$img instanceof \GdImage && !is_resource($img)) {
|
|
trigger_error('imagelightnessat(): supplied argument is not a valid '
|
|
. 'Image resource', E_USER_WARNING);
|
|
|
|
return 0.0;
|
|
}
|
|
$c = @imagecolorat($img, $x, $y);
|
|
if ($c === false) {
|
|
return false;
|
|
}
|
|
if (imageistruecolor($img)) {
|
|
$red = ($c >> 16) & 0xFF;
|
|
$green = ($c >> 8) & 0xFF;
|
|
$blue = $c & 0xFF;
|
|
} else {
|
|
$i = imagecolorsforindex($img, $c);
|
|
$red = $i['red'];
|
|
$green = $i['green'];
|
|
$blue = $i['blue'];
|
|
}
|
|
$m = min($red, $green, $blue);
|
|
$n = max($red, $green, $blue);
|
|
/* Because RGB isn't normalized in GD, we divide by 510 here.
|
|
* Lightness = (Max(RGB) + Min(RGB)) / 2
|
|
* But that's assuming red, green, and blue are 0 through 1 inclusive.
|
|
* Red, green, and blue are actually 0-255 (255 + 255 = 510).
|
|
*/
|
|
$lightness = (float) (($m + $n) / 510.0);
|
|
|
|
return $lightness;
|
|
}
|