1 files changed, 42 insertions, 0 deletions
diff --git a/source/blender/editors/space_view3d/view3d_project.c b/source/blender/editors/space_view3d/view3d_project.c
index 0afa44b6f4f..58538fc3ecb 100644
--- a/source/blender/editors/space_view3d/view3d_project.c
+++ b/source/blender/editors/space_view3d/view3d_project.c
@@ -633,6 +633,48 @@ bool ED_view3d_win_to_3d_on_plane_int(const ARegion *region,
 }
 
 /**
+ * A wrapper for #ED_view3d_win_to_3d_on_plane that projects onto \a plane_fallback
+ * then maps this back to \a plane.
+ *
+ * This is intended to be used when \a plane is orthogonal to the views Z axis where
+ * projecting the \a mval doesn't work well (or fail completely when exactly aligned).
+ */
+bool ED_view3d_win_to_3d_on_plane_with_fallback(const ARegion *region,
+                                                const float plane[4],
+                                                const float mval[2],
+                                                const bool do_clip,
+                                                const float plane_fallback[4],
+                                                float r_out[3])
+{
+  float isect_co[3], isect_no[3];
+  if (!isect_plane_plane_v3(plane, plane_fallback, isect_co, isect_no)) {
+    return false;
+  }
+  normalize_v3(isect_no);
+
+  /* Construct matrix to transform `plane_fallback` onto `plane`. */
+  float mat4[4][4];
+  {
+    float mat3[3][3];
+    rotation_between_vecs_to_mat3(mat3, plane, plane_fallback);
+    copy_m4_m3(mat4, mat3);
+    transform_pivot_set_m4(mat4, isect_co);
+  }
+
+  float co[3];
+  if (!ED_view3d_win_to_3d_on_plane(region, plane_fallback, mval, do_clip, co)) {
+    return false;
+  }
+  mul_m4_v3(mat4, co);
+
+  /* While the point is already on the plane, there may be some small in-precision
+   * so ensure the point is exactly on the plane. */
+  closest_to_plane_v3(r_out, plane, co);
+
+  return true;
+}
+
+/**
  * Calculate a 3d difference vector from 2d window offset.
  * note that #ED_view3d_calc_zfac() must be called first to determine
  * the depth used to calculate the delta.