[6.40] [asimage] Fix filled shapes drawn with a semi-transparent colour

graf2d/asimage/CMakeLists.txt

@@ -68,3 +68,5 @@ if (x11)
 endif()
 
 ROOT_INSTALL_HEADERS()
+
+ROOT_ADD_TEST_SUBDIRECTORY(test)

graf2d/asimage/src/libAfterImage/draw.c

@@ -340,7 +340,7 @@ apply_tool_point_colored(ASDrawContext *ctx, int curr_x, int curr_y, CARD32 rati
 		dst += curr_y * cw + curr_x;
 		if (get_flags(ctx->flags, ASDrawCTX_UsingScratch))
 		{
-			CARD32 value = (ARGB32_ALPHA8(ctx->tool->matrix[0])*ratio)/255 ;
+			CARD32 value = ratio ;	/* coverage only; alpha applied on merge */
 			if( *dst < value ) 
 				*dst = value ;
 		}		
@@ -391,7 +391,7 @@ fill_hline_notile_colored(ASDrawContext *ctx, int x_from, int y, int x_to, CARD3
 		{
 			while( x1 <= x2 ) 
 			{
-				CARD32 value = (ARGB32_ALPHA8(ctx->tool->matrix[0])*ratio)/255 ;
+				CARD32 value = ratio ;	/* coverage only; alpha applied on merge */
 				if( dst[x1] < value ) 
 					dst[x1] = value ;
 				++x1 ;

graf2d/asimage/test/CMakeLists.txt

@@ -0,0 +1,7 @@
+# Copyright (C) 1995-2026, Rene Brun and Fons Rademakers.
+# All rights reserved.
+#
+# For the licensing terms see $ROOTSYS/LICENSE.
+# For the list of contributors see $ROOTSYS/README/CREDITS.
+
+ROOT_ADD_GTEST(TASImageDraw tasimage_draw.cxx LIBRARIES ASImage)

graf2d/asimage/test/tasimage_draw.cxx

@@ -0,0 +1,68 @@
+#include "gtest/gtest.h"
+
+#include "TASImage.h"
+
+namespace {
+
+constexpr UInt_t kSize = 64;
+
+constexpr UInt_t kPixels = kSize * kSize;
+
+// Index of the four canvas corners.
+constexpr UInt_t kCorners[4] = {0, kSize - 1, (kSize - 1) * kSize, kPixels - 1};
+
+// Draw a filled circle of `colour` well inside a kSize x kSize image and check
+// that the fill stayed inside it. The corner values are compared against what
+// they were before drawing rather than against a constant, so the test does not
+// depend on how a fresh TASImage is initialised.
+void CheckFilledCircleStaysInside(const char *colour)
+{
+   TASImage img(kSize, kSize);
+
+   UInt_t *argb = img.GetArgbArray();
+   ASSERT_NE(argb, nullptr);
+
+   UInt_t before[4];
+   for (int i = 0; i < 4; ++i)
+      before[i] = argb[kCorners[i]];
+   const UInt_t centre = (kSize / 2) * kSize + kSize / 2;
+   const UInt_t centreBefore = argb[centre];
+
+   img.DrawCircle(kSize / 2, kSize / 2, kSize / 4, colour, -1);
+
+   argb = img.GetArgbArray();
+   ASSERT_NE(argb, nullptr);
+
+   for (int i = 0; i < 4; ++i)
+      EXPECT_EQ(argb[kCorners[i]], before[i]) << "the fill escaped the circle and reached corner " << i;
+
+   EXPECT_NE(argb[centre], centreBefore) << "the circle was not filled at all";
+}
+
+} // namespace
+
+// https://github.com/root-project/root/issues/23014
+//
+// libAfterImage scaled the coverage it wrote into the scratch canvas by the
+// brush alpha, which made the flood fill that closes a filled shape depend on
+// that alpha. Two symptoms followed, and this geometry shows both: at an alpha
+// of 0x8C or below the fill never terminated, and between 0x8D and 0xFE it
+// returned but leaked through the anti-aliased outline and covered the whole
+// image. Only a fully opaque brush behaved correctly.
+
+TEST(TASImage, FilledCircleOpaque)
+{
+   CheckFilledCircleStaysInside("#FFFF0000");
+}
+
+// Used to leak out of the circle and fill the whole image.
+TEST(TASImage, FilledCircleHighAlpha)
+{
+   CheckFilledCircleStaysInside("#C0FF0000");
+}
+
+// Used to hang: the colour from the issue report.
+TEST(TASImage, FilledCircleSemiTransparent)
+{
+   CheckFilledCircleStaysInside("#7FFF0000");
+}

test/stressGraphics.ref

@@ -49,7 +49,7 @@
           waves   3303861       100    712368      3000     62688     19000    123056      7000   3396789      1000
            tf12      3954       100     14714       300     17378      7800      9629      2000      3794       200
         tspline      9073       100     22210       300     27691      7800     20712      2000      9157       200
-       tscatter     15595       200     22620       400     28599      7800     42618      2000     15460       200
+       tscatter     15595       200     22620       400     28599      7800     44900      2000     15460       200
     tefficiency     12926       200     24364       300     24021      6800     12637      2000     12584       200
      profile_2d     29664       200     19414       300     23024      6500     12821      2000     31317       200
  profile_2dpoly     26463       400     23542       300     38792      8500     30458      4000     26759       400

test/stressGraphics_zlibng.ref

@@ -49,7 +49,7 @@
           waves   3303861       100    709707      3000     62761     19000    122867      7000   3396789      1000
            tf12      3954       100     14714       300     17378      7800      9629      2000      3794       200
         tspline      9073       100     22210       300     27691      7800     20712      2000      9157       200
-       tscatter     15595       200     22420       400     28599      7800     42618      2000     15460       200
+       tscatter     15595       200     22420       400     28599      7800     44760      2000     15460       200
     tefficiency     12926       200     24364       300     24021      6800     12637      2000     12584       200
      profile_2d     29664       200     19414       300     23024      6500     12821      2000     31317       200
  profile_2dpoly     26463       400     23542       300     38792      8500     30458      4000     26759       400