Refactor imagegammacorrect()

We apply the law `(b**r)**s == b**(r*s)` which holds for all non-negative b
and positive r,s, so a single pow() suffices. Furthermore, we precompute the
gamma, so the refactored code is simpler and faster.

ext/gd/gd.c

@@ -3025,7 +3025,7 @@ PHP_FUNCTION(imagegammacorrect)
 	zval *IM;
 	gdImagePtr im;
 	int i;
-	double input, output;
+	double input, output, gamma;
 
 	if (zend_parse_parameters(ZEND_NUM_ARGS(), "rdd", &IM, &input, &output) == FAILURE) {
 		return;
@@ -3036,6 +3036,8 @@ PHP_FUNCTION(imagegammacorrect)
 		RETURN_FALSE;
 	}
 
+	gamma = input / output;
+
 	if ((im = (gdImagePtr)zend_fetch_resource(Z_RES_P(IM), "Image", le_gd)) == NULL) {
 		RETURN_FALSE;
 	}
@@ -3048,9 +3050,9 @@ PHP_FUNCTION(imagegammacorrect)
 				c = gdImageGetPixel(im, x, y);
 				gdImageSetPixel(im, x, y,
 					gdTrueColorAlpha(
-						(int) ((pow((pow((gdTrueColorGetRed(c)   / 255.0), input)), 1.0 / output) * 255) + .5),
-						(int) ((pow((pow((gdTrueColorGetGreen(c) / 255.0), input)), 1.0 / output) * 255) + .5),
-						(int) ((pow((pow((gdTrueColorGetBlue(c)  / 255.0), input)), 1.0 / output) * 255) + .5),
+						(int) ((pow((gdTrueColorGetRed(c)   / 255.0), gamma) * 255) + .5),
+						(int) ((pow((gdTrueColorGetGreen(c) / 255.0), gamma) * 255) + .5),
+						(int) ((pow((gdTrueColorGetBlue(c)  / 255.0), gamma) * 255) + .5),
 						gdTrueColorGetAlpha(c)
 					)
 				);
@@ -3060,9 +3062,9 @@ PHP_FUNCTION(imagegammacorrect)
 	}
 
 	for (i = 0; i < gdImageColorsTotal(im); i++) {
-		im->red[i]   = (int)((pow((pow((im->red[i]   / 255.0), input)), 1.0 / output) * 255) + .5);
-		im->green[i] = (int)((pow((pow((im->green[i] / 255.0), input)), 1.0 / output) * 255) + .5);
-		im->blue[i]  = (int)((pow((pow((im->blue[i]  / 255.0), input)), 1.0 / output) * 255) + .5);
+		im->red[i]   = (int)((pow((im->red[i]   / 255.0), gamma) * 255) + .5);
+		im->green[i] = (int)((pow((im->green[i] / 255.0), gamma) * 255) + .5);
+		im->blue[i]  = (int)((pow((im->blue[i]  / 255.0), gamma) * 255) + .5);
 	}
 
 	RETURN_TRUE;

ext/gd/tests/imagegammacorrect_variation2.phpt

@@ -0,0 +1,72 @@
+--TEST--
+Apply imagegammacorrect() to a step wedge
+--SKIPIF--
+<?php
+if (!extension_loaded('gd')) die('skip gd extension not available');
+?>
+--FILE--
+<?php
+require __DIR__ . DIRECTORY_SEPARATOR . 'func.inc';
+
+test_gamma_both(1, 2);
+test_gamma_both(1, 1);
+test_gamma_both(2, 1);
+
+function test_gamma_both($in, $out)
+{
+    test_gamma($in, $out, 'imagecreate');
+    test_gamma($in, $out, 'imagecreatetruecolor');
+}
+
+function test_gamma($in, $out, $constructor)
+{
+    $im = $constructor(640, 480);
+    for ($j = 0; $j < 4; $j++) {
+        for ($i = 0; $i < 32; $i++) {
+            draw_cell($im, $i, $j);
+        }
+    }
+    
+    imagegammacorrect($im, $in, $out);
+    
+    $filename = __DIR__ . DIRECTORY_SEPARATOR
+        . "imagegammacorrect_variation2_{$in}_{$out}.png";
+    $kind = $constructor === 'imagecreate' ? 'palette' : 'truecolor';
+    echo "$kind gamma ($in, $out): ";
+    test_image_equals_file($filename, $im);
+}
+
+function draw_cell($im, $x, $y)
+{
+    $x1 = 20 * $x;
+    $y1 = 120 * $y;
+    $x2 = $x1 + 19;
+    $y2 = $y1 + 119;
+    $color = cell_color($im, $x, $y);
+    imagefilledrectangle($im, $x1,$y1, $x2,$y2, $color);
+}
+
+function cell_color($im, $x, $y)
+{
+    $channel = 8 * $x + 4;
+    switch ($y) {
+        case 0:
+            return imagecolorallocate($im, $channel, $channel, $channel);
+        case 1:
+            return imagecolorallocate($im, $channel, 0, 0);
+        case 2:
+            return imagecolorallocate($im, 0, $channel, 0);
+        case 3:
+            return imagecolorallocate($im, 0, 0, $channel);
+    }
+}
+?>
+===DONE===
+--EXPECT--
+palette gamma (1, 2): The images are equal.
+truecolor gamma (1, 2): The images are equal.
+palette gamma (1, 1): The images are equal.
+truecolor gamma (1, 1): The images are equal.
+palette gamma (2, 1): The images are equal.
+truecolor gamma (2, 1): The images are equal.
+===DONE===