Initial commit

imagelightnessat.func.php

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+<?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, $y)
+{
+    if(!is_resource($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).
+     */
+    $lightness = (double)(($m + $n) / 510.0);
+    return($lightness);
+}
+
+?>