Cleanup: use determinant_m3(m) < 0 to implement is_negative_m3/m4

Use a more direct method of checking if a matrix is negative instead of
using cross & dot product.

Also replace some determinant_m3() < 0 checks with is_negative_m3.

1 files changed, 11 insertions, 0 deletions

diff --git a/source/blender/blenlib/BLI_math_matrix.h b/source/blender/blenlib/BLI_math_matrix.h
index 15264dbe8b7..87a01e0c264 100644
--- a/source/blender/blenlib/BLI_math_matrix.h
+++ b/source/blender/blenlib/BLI_math_matrix.h
@@ -528,7 +528,18 @@ void interp_m3_m3m3(float R[3][3], const float A[3][3], const float B[3][3], flo
  */
 void interp_m4_m4m4(float R[4][4], const float A[4][4], const float B[4][4], float t);
 
+/**
+ * Return true when the matrices determinant is less than zero.
+ *
+ * \note This is often used to check if a matrix flips content in 3D space,
+ * where transforming geometry (for example) would flip the direction of polygon normals
+ * from pointing outside a closed volume, to pointing inside (or the reverse).
+ *
+ * When the matrix is constructed from location, rotation & scale
+ * as matrix will be negative when it has an odd number of negative scales.
+ */
 bool is_negative_m3(const float mat[3][3]);
+/** A version of #is_negative_m3 that takes a 4x4 matrix. */
 bool is_negative_m4(const float mat[4][4]);
 
 bool is_zero_m3(const float mat[3][3]);