From 7730f8ba8b66978a3340213420c31c9e6d148d58 Mon Sep 17 00:00:00 2001
From: Hermet Park <hermet@lottiefiles.com>
Date: Thu, 24 Jul 2025 18:16:31 +0900
Subject: [PATCH] renderer: properly support stroke bounding box after scaling.

The stroke bounding box in ThorVG was previously only approximated,
as the stroke transformation was mistakenly omitted. This has now
been enhanced.

The stroke join/corner is still not taken into account in the
bounding box computation...
---
 examples/BoundingBox.cpp |  3 ++-
 src/renderer/tvgShape.h  | 13 +++++++++----
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/examples/BoundingBox.cpp b/examples/BoundingBox.cpp
index 98aa94d3..3d708dbd 100644
--- a/examples/BoundingBox.cpp
+++ b/examples/BoundingBox.cpp
@@ -256,10 +256,11 @@ struct UserExample : tvgexam::Example
             shape->moveTo(0, 0);
             shape->lineTo(150, 100);
             shape->lineTo(0, 100);
+            shape->close();
             shape->fill(255, 0, 255);
             shape->strokeWidth(30);
             shape->strokeFill(0, 255, 255);
-            shape->close();
+            shape->strokeJoin(tvg::StrokeJoin::Round);
 
             tvg::Matrix m = {1.8794f, -0.6840f, 0.0f, 0.6840f,  1.8794f, 0.0f, 0.0f, 0.0f, 1.0f};
             shape->transform(m);
diff --git a/src/renderer/tvgShape.h b/src/renderer/tvgShape.h
index 58ee4d01..2fff92cb 100644
--- a/src/renderer/tvgShape.h
+++ b/src/renderer/tvgShape.h
@@ -125,10 +125,15 @@ struct ShapeImpl : Shape
 
         //Stroke feathering
         if (stroking && rs.stroke) {
-            x -= rs.stroke->width * 0.5f;
-            y -= rs.stroke->width * 0.5f;
-            w += rs.stroke->width;
-            h += rs.stroke->width;
+            //Use geometric mean for feathering.
+            //Join, Cap wouldn't be considered. Generate stroke outline and compute bbox for accurate size?
+            auto sx = sqrt(m.e11 * m.e11 + m.e21 * m.e21);
+            auto sy = sqrt(m.e12 * m.e12 + m.e22 * m.e22);
+            auto feather = rs.stroke->width * sqrt(sx * sy);
+            x -= feather * 0.5f;
+            y -= feather * 0.5f;
+            w += feather;
+            h += feather;
         }
 
         pt4[0] = {x, y};