Fix libgd 223: gdImageRotateGeneric() does not properly interpolate

We port the respective upstream fix[1].  We only run the test against
bundled libgd, since external libgd may yield different results.

Cf. <libgd/libgd@2b26be8>.

Closes phpGH-17380.

Author: cmb69

NEWS

@@ -10,6 +10,8 @@ PHP                                                                        NEWS
     transparency). (cmb)
   . Fixed bug GH-17373 (imagefttext() ignores clipping rect for palette
     images). (cmb)
+  . Ported fix for libgd 223 (gdImageRotateGeneric() does not properly
+    interpolate). (cmb)
 
 - Intl:
   . Fixed bug GH-11874 (intl causing segfault in docker images). (nielsdos)

ext/gd/libgd/gd_interpolation.c

@@ -738,8 +738,8 @@ static int getPixelInterpolateWeight(gdImagePtr im, const double x, const double
  */
 int getPixelInterpolated(gdImagePtr im, const double x, const double y, const int bgColor)
 {
-	const int xi=(int)((x) < 0 ? x - 1: x);
-	const int yi=(int)((y) < 0 ? y - 1: y);
+	const int xi=(int)(x);
+	const int yi=(int)(y);
 	int yii;
 	int i;
 	double kernel, kernel_cache_y;
@@ -1703,13 +1703,6 @@ gdImagePtr gdImageRotateGeneric(gdImagePtr src, const float degrees, const int b
 	int new_width, new_height;
 	gdRect bbox;
 
-	const gdFixed f_slop_y = f_sin;
-	const gdFixed f_slop_x = f_cos;
-	const gdFixed f_slop = f_slop_x > 0 && f_slop_y > 0 ?
-							(f_slop_x > f_slop_y ? gd_divfx(f_slop_y, f_slop_x) : gd_divfx(f_slop_x, f_slop_y))
-						: 0;
-
-
 	if (bgColor < 0) {
 		return NULL;
 	}
@@ -1735,15 +1728,10 @@ gdImagePtr gdImageRotateGeneric(gdImagePtr src, const float degrees, const int b
 			long m = gd_fxtoi(f_m);
 			long n = gd_fxtoi(f_n);
 
-			if ((n <= 0) || (m <= 0) || (m >= src_h) || (n >= src_w)) {
+			if (m < -1 || n < -1 || m >= src_h || n >= src_w ) {
 				dst->tpixels[dst_offset_y][dst_offset_x++] = bgColor;
-			} else if ((n <= 1) || (m <= 1) || (m >= src_h - 1) || (n >= src_w - 1)) {
-				register int c = getPixelInterpolated(src, n, m, bgColor);
-				c = c | (( gdTrueColorGetAlpha(c) + ((int)(127* gd_fxtof(f_slop)))) << 24);
-
-				dst->tpixels[dst_offset_y][dst_offset_x++] = _color_blend(bgColor, c);
 			} else {
-				dst->tpixels[dst_offset_y][dst_offset_x++] = getPixelInterpolated(src, n, m, bgColor);
+				dst->tpixels[dst_offset_y][dst_offset_x++] = getPixelInterpolated(src, gd_fxtod(f_n), gd_fxtod(f_m), bgColor);
 			}
 		}
 		dst_offset_y++;

ext/gd/tests/gd223.phpt

@@ -0,0 +1,26 @@
+--TEST--
+libgd bug 223 (gdImageRotateGeneric() does not properly interpolate)
+--EXTENSIONS--
+gd
+--SKIPIF--
+<?php
+if (!GD_BUNDLED) die("skip only for bundled libgd");
+?>
+--FILE--
+<?php
+require_once __DIR__ . "/func.inc";
+
+$im = imagecreatetruecolor(64, 64);
+for ($j = 0; $j < 64; $j++) {
+    for ($i = 0; $i < 64; $i++) {
+        imagesetpixel($im, $i, $j, ($i % 2 || $j % 2) ? 0x000000 : 0xffffff);
+    }
+}
+
+imagesetinterpolation($im, IMG_BICUBIC);
+$im = imagerotate($im, 45, 0xff0000);
+
+test_image_equals_file(__DIR__ . "/gd223.png", $im);
+?>
+--EXPECT--
+The images are equal.