From cea221e2d56ee530c360a84af404857862344ae7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lorc=C3=A1n=20Mc=20Donagh?= <lorcan@lmdsp.com>
Date: Thu, 7 Mar 2024 16:13:33 +0100
Subject: [PATCH] sw_engine: fix radial gradient when focal point is outside
 circle on ARM/Apple Silicon
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sw_engine: radial gradient
[issues 2014: radial gradient](#2014)

Radial gradient results in a corrupted image when the focal point is outside the circle on Apple Silicon.
This happens because some compilers use FMA to optimize the a = dr² - dx² - dy² calculation,
which cause loss of precision.

We rely on temporary variables to prevent FMA.
We could also use compiler specific float contraction control pragmas to avoid this if this doesn't work in the future
---
 AUTHORS                                              |  1 +
 .../radial_gradient_focal_outside_circle.svg         | 12 ++++++++++++
 src/renderer/sw_engine/tvgSwFill.cpp                 |  8 +++++++-
 3 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 src/examples/resources/radial_gradient_focal_outside_circle.svg

diff --git a/AUTHORS b/AUTHORS
index be3f4093..da584810 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -26,3 +26,4 @@ SergeyLebedkin <sergii@lottiefiles.com>
 Jinny You <jinny@lottiefiles.com>
 Nattu Adnan <nattu@reallynattu.com>
 Gabor Kiss-Vamosi <kisvegabor@gmail.com>
+Lorcán Mc Donagh <lorcan@lmdsp.com>
diff --git a/src/examples/resources/radial_gradient_focal_outside_circle.svg b/src/examples/resources/radial_gradient_focal_outside_circle.svg
new file mode 100644
index 00000000..8d0bd78f
--- /dev/null
+++ b/src/examples/resources/radial_gradient_focal_outside_circle.svg
@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg width="800" height="590" version="1.1" id="https://github.com/thorvg/thorvg/issues/2014" xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg">
+   <defs>
+      <radialGradient cx="429" cy="3" fx="429" fy="-600" gradientUnits="userSpaceOnUse" id="b" r="600">
+         <stop offset="0" stop-color="red" id="stop10" />
+         <stop offset="1" stop-color="white" id="stop11" />
+      </radialGradient>
+   </defs>
+
+   <path d="M 800,200 H 0 V 590 H 800 V 0 Z" fill="url(#b)" id="svg_4" />
+</svg>
\ No newline at end of file
diff --git a/src/renderer/sw_engine/tvgSwFill.cpp b/src/renderer/sw_engine/tvgSwFill.cpp
index 8956cd9f..60763068 100644
--- a/src/renderer/sw_engine/tvgSwFill.cpp
+++ b/src/renderer/sw_engine/tvgSwFill.cpp
@@ -201,7 +201,13 @@ bool _prepareRadial(SwFill* fill, const RadialGradient* radial, const Matrix* tr
         fill->radial.fy = cy + r * (fy - cy) / dist;
         fill->radial.dx = cx - fill->radial.fx;
         fill->radial.dy = cy - fill->radial.fy;
-        fill->radial.a = fill->radial.dr * fill->radial.dr - fill->radial.dx * fill->radial.dx - fill->radial.dy * fill->radial.dy;
+        // Prevent loss of precision on Apple Silicon when dr=dy and dx=0 due to FMA
+        // https://github.com/thorvg/thorvg/issues/2014
+        auto dr2 = fill->radial.dr * fill->radial.dr;
+        auto dx2 = fill->radial.dx * fill->radial.dx;
+        auto dy2 = fill->radial.dy * fill->radial.dy;
+
+        fill->radial.a = dr2 - dx2 - dy2;
     }
 
     if (fill->radial.a > 0) fill->radial.invA = 1.0f / fill->radial.a;