From 0342fb5d20cd13b495a7511ccfdfc3044d83cb04 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sybren=20A=2E=20St=C3=BCvel?= <sybren@blender.org>
Date: Tue, 3 Aug 2021 13:43:24 +0200
Subject: Fix T90387: division by zero when trying to invert scale

Fix division by zero when `BKE_bone_parent_transform_invert()` inverts a
scale vector with zero components.

Zero values in the to-be-inverted vector are now simply skipped, i.e.
remain zero after inversion. This at least ensures that
`invert_v3_safe(invert_v3_safe(vector))` results in the same vector.

This commit does NOT fix the conceptual problem that an inversion of a
potentially non-invertible vector is relied upon. It just avoids the
division by zero.
---
 source/blender/blenkernel/intern/armature.c          |  2 +-
 source/blender/blenlib/BLI_math_vector.h             |  1 +
 source/blender/blenlib/intern/math_vector_inline.c   | 13 +++++++++++++
 source/blender/blenlib/tests/BLI_math_vector_test.cc | 18 ++++++++++++++++++
 4 files changed, 33 insertions(+), 1 deletion(-)

(limited to 'source')

diff --git a/source/blender/blenkernel/intern/armature.c b/source/blender/blenkernel/intern/armature.c
index 434d696f9bd..1f02b084534 100644
--- a/source/blender/blenkernel/intern/armature.c
+++ b/source/blender/blenkernel/intern/armature.c
@@ -1882,7 +1882,7 @@ void BKE_bone_parent_transform_invert(struct BoneParentTransform *bpt)
 {
   invert_m4(bpt->rotscale_mat);
   invert_m4(bpt->loc_mat);
-  invert_v3(bpt->post_scale);
+  invert_v3_safe(bpt->post_scale);
 }
 
 void BKE_bone_parent_transform_combine(const struct BoneParentTransform *in1,
diff --git a/source/blender/blenlib/BLI_math_vector.h b/source/blender/blenlib/BLI_math_vector.h
index 2f4cf1721af..860ba14a3ed 100644
--- a/source/blender/blenlib/BLI_math_vector.h
+++ b/source/blender/blenlib/BLI_math_vector.h
@@ -185,6 +185,7 @@ MINLINE void negate_v3_db(double r[3]);
 
 MINLINE void invert_v2(float r[2]);
 MINLINE void invert_v3(float r[3]);
+MINLINE void invert_v3_safe(float r[3]); /* Invert the vector, but leaves zero values as zero. */
 
 MINLINE void abs_v2(float r[2]);
 MINLINE void abs_v2_v2(float r[2], const float a[2]);
diff --git a/source/blender/blenlib/intern/math_vector_inline.c b/source/blender/blenlib/intern/math_vector_inline.c
index 55f7a152b83..dddefd60b1b 100644
--- a/source/blender/blenlib/intern/math_vector_inline.c
+++ b/source/blender/blenlib/intern/math_vector_inline.c
@@ -847,6 +847,19 @@ MINLINE void invert_v3(float r[3])
   r[2] = 1.0f / r[2];
 }
 
+MINLINE void invert_v3_safe(float r[3])
+{
+  if (r[0] != 0.0f) {
+    r[0] = 1.0f / r[0];
+  }
+  if (r[1] != 0.0f) {
+    r[1] = 1.0f / r[1];
+  }
+  if (r[2] != 0.0f) {
+    r[2] = 1.0f / r[2];
+  }
+}
+
 MINLINE void abs_v2(float r[2])
 {
   r[0] = fabsf(r[0]);
diff --git a/source/blender/blenlib/tests/BLI_math_vector_test.cc b/source/blender/blenlib/tests/BLI_math_vector_test.cc
index 7e75a521d4c..955f02b8065 100644
--- a/source/blender/blenlib/tests/BLI_math_vector_test.cc
+++ b/source/blender/blenlib/tests/BLI_math_vector_test.cc
@@ -45,3 +45,21 @@ TEST(math_vector, ClampVecWithVecs)
   EXPECT_FLOAT_EQ(1.0f, c[0]);
   EXPECT_FLOAT_EQ(3.0f, c[1]);
 }
+
+TEST(math_vector, test_invert_v3_safe)
+{
+  float v3_with_zeroes[3] = {0.0f, 2.0f, 3.0f};
+  invert_v3_safe(v3_with_zeroes);
+  EXPECT_FLOAT_EQ(0.0f, v3_with_zeroes[0]);
+  EXPECT_FLOAT_EQ(0.5f, v3_with_zeroes[1]);
+  EXPECT_FLOAT_EQ(0.33333333333f, v3_with_zeroes[2]);
+
+  float v3_without_zeroes[3] = {1.0f, 2.0f, 3.0f};
+  float inverted_unsafe[3] = {1.0f, 2.0f, 3.0f};
+  invert_v3_safe(v3_without_zeroes);
+  invert_v3(inverted_unsafe);
+
+  EXPECT_FLOAT_EQ(inverted_unsafe[0], v3_without_zeroes[0]);
+  EXPECT_FLOAT_EQ(inverted_unsafe[1], v3_without_zeroes[1]);
+  EXPECT_FLOAT_EQ(inverted_unsafe[2], v3_without_zeroes[2]);
+}
-- 
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