[Bf-blender-cvs] [8d163d59765] blender-v2.82-release: Docs: describe turn-table gimble lock mitigation

[Campbell Barton]

Commit: 8d163d597652aec1f2ca926ba978447dd8476619
Author: Campbell Barton
Date:   Wed Jan 22 02:51:01 2020 +1100
Branches: blender-v2.82-release
https://developer.blender.org/rB8d163d597652aec1f2ca926ba978447dd8476619

Docs: describe turn-table gimble lock mitigation

===================================================================

M	source/blender/editors/space_view3d/view3d_edit.c

===================================================================

diff --git a/source/blender/editors/space_view3d/view3d_edit.c b/source/blender/editors/space_view3d/view3d_edit.c
index 3dcde401216..a4714249da2 100644
--- a/source/blender/editors/space_view3d/view3d_edit.c
+++ b/source/blender/editors/space_view3d/view3d_edit.c
@@ -758,7 +758,32 @@ static void viewrotate_apply(ViewOpsData *vod, const int event_xy[2])
     quat_to_mat3(m, vod->curr.viewquat);
     invert_m3_m3(m_inv, m);
 
-    /* avoid gimble lock */
+    /* Avoid Gimble Lock
+     *
+     * Even though turn-table mode is in use, this can occur when the user exits the camera view
+     * or when aligning the view to a rotated object.
+     *
+     * We have gimble lock when the user's view is rotated +/- 90 degrees along the view axis.
+     * In this case the vertical rotation is the same as the sideways turntable motion.
+     * Making it impossible to get out of the gimble locked state without resetting the view.
+     *
+     * The logic below lets the user exit out of this state without any abrupt 'fix'
+     * which would be disorienting.
+     *
+     * This works by blending two horizons:
+     * - Rotated-horizon: `cross_v3_v3v3(xaxis, zvec_global, m_inv[2])`
+     *   When only this is used, this turntable rotation works - but it's side-ways
+     *   (as if the entire turn-table has been placed on it's side)
+     *   While there is no gimble lock, it's also awkward to use.
+     * - Un-rotated-horizon: `m_inv[0]`
+     *   When only this is used, the turntable rotation can have gimbal lock.
+     *
+     * The solution used here is to blend between these two values,
+     * so the severity of the gimbal lock is used to blend the rotated horizon.
+     * Blending isn't essential, it just makes the transition smoother.
+     *
+     * This allows sideways turn-table rotation on a Z axis that isn't world-space Z,
+     * While up-down turntable rotation eventually corrects gimble lock. */
 #if 1
     if (len_squared_v3v3(zvec_global, m_inv[2]) > 0.001f) {
       float fac;