Fix #43828: broken transparency of imagearc for truecolor in blendingmode

No pixel of a filled arc must ever be drawn multiple times. Otherwise we get artifacts regarding transparency. That happens with the current implementation of gdImageFilledArc() unless gdChord or gdNoFill are set. When gdPie is set, however, the filled arc is drawn in wedges, which are polygons of three points, and so some overlap is natural. To resolve the issue, we stick with the current algorithm of calculating the wedges, but instead of drawing each polygon separately, we put the relevant points in a large array, and draw a single polygon. That also is supposed to improve the performance considerably. Note that this modification will change the results when gdImageSetStyle() or gdImageSetBrush() are used, but we believe that this modification is also an improvement in this regard, even though it still might not make much sense to use these functions with gdImageFilledArc(). The respective fix for libgd is <libgd/libgd@e7e20d6>.

Author: cmb69

ext/gd/libgd/gd.c

@@ -1625,9 +1625,7 @@ long lsqrt (long n)
 /* s and e are integers modulo 360 (degrees), with 0 degrees
  being the rightmost extreme and degrees changing clockwise.
  cx and cy are the center in pixels; w and h are the horizontal
- and vertical diameter in pixels. Nice interface, but slow.
- See gd_arc_f_buggy.c for a better version that doesn't
- seem to be bug-free yet. */
+ and vertical diameter in pixels. */
 
 void gdImageArc (gdImagePtr im, int cx, int cy, int w, int h, int s, int e, int color)
 {
@@ -1636,8 +1634,8 @@ void gdImageArc (gdImagePtr im, int cx, int cy, int w, int h, int s, int e, int
 
 void gdImageFilledArc (gdImagePtr im, int cx, int cy, int w, int h, int s, int e, int color, int style)
 {
-gdPoint pts[3];
-int i;
+gdPoint pts[363];
+int i, pti;
 int lx = 0, ly = 0;
 int fx = 0, fy = 0;
 
@@ -1665,7 +1663,7 @@ void gdImageFilledArc (gdImagePtr im, int cx, int cy, int w, int h, int s, int e
 }
 }
 
-for (i = s; i <= e; i++) {
+for (i = s, pti = 1; i <= e; i++, pti++) {
 int x, y;
 x = ((long) gdCosT[i % 360] * (long) w / (2 * 1024)) + cx;
 y = ((long) gdSinT[i % 360] * (long) h / (2 * 1024)) + cy;
@@ -1674,19 +1672,28 @@ void gdImageFilledArc (gdImagePtr im, int cx, int cy, int w, int h, int s, int e
 if (style & gdNoFill) {
 gdImageLine(im, lx, ly, x, y, color);
 } else {
-/* This is expensive! */
-pts[0].x = lx;
-pts[0].y = ly;
-pts[1].x = x;
-pts[1].y = y;
-pts[2].x = cx;
-pts[2].y = cy;
-gdImageFilledPolygon(im, pts, 3, color);
-}
+if (y == ly) {
+pti--; /* don't add this point */
+if (((i > 270 || i < 90) && x > lx) || ((i > 90 && i < 270) && x < lx)) {
+/* replace the old x coord, if increasing on the
+ right side or decreasing on the left side */
+pts[pti].x = x;
+}
+} else {
+pts[pti].x = x;
+pts[pti].y = y;
+}
+	}
 }
 } else {
 fx = x;
 fy = y;
+if (!(style & (gdChord | gdNoFill))) {
+pts[0].x = cx;
+pts[0].y = cy;
+pts[pti].x = x;
+pts[pti].y = y;
+}
 }
 lx = x;
 ly = y;
@@ -1713,6 +1720,10 @@ void gdImageFilledArc (gdImagePtr im, int cx, int cy, int w, int h, int s, int e
 gdImageLine(im, cx, cy, lx, ly, color);
 gdImageLine(im, cx, cy, fx, fy, color);
 }
+} else {
+pts[pti].x = cx;
+pts[pti].y = cy;
+gdImageFilledPolygon(im, pts, pti+1, color);
 }
 }
 }

ext/gd/tests/bug43828.phpt

@@ -0,0 +1,23 @@
+--TEST--
+Bug #43828 (broken transparency of imagearc for truecolor in blendingmode)
+--SKIPIF--
+<?php
+if (!extension_loaded('gd')) die('skip ext/gd not available');
+?>
+--FILE--
+<?php
+
+$im = imagecreatetruecolor(100,100);
+
+$transparent = imagecolorallocatealpha($im, 255, 255, 255, 80);
+imagefilledrectangle($im, 0,0, 99,99, $transparent);
+$color = imagecolorallocatealpha($im, 0, 255, 0, 100);
+imagefilledarc($im, 49, 49, 99,99, 0 , 360, $color, IMG_ARC_PIE);
+
+ob_start();
+imagepng($im);
+echo md5(ob_get_clean());
+imagedestroy($im);
+?>
+--EXPECT--
+3d82e4525f19790ae1055366e2a36917

ext/gd/tests/imagecolorallocatealpha_basic.phpt

@@ -26,5 +26,5 @@ var_dump(md5(base64_encode($imgsrc)));
 var_dump($corA);
 ?>
 --EXPECT--
-string(32) "2a6424e4cb4e1b7391dfff74bf136bde"
+string(32) "f95489d97f4f1a5c4dc265388922d1ec"
 int(842163455)