Keep a VR seat's facing while its vehicle stands on its nose

A VR seat follows its vehicle's turns through the yaw of the vehicle's
nose. A nose pointing almost straight up or down keeps only a sliver of
horizontal direction to take that yaw from. mcc_2's launch arm stands the
drop pod 0.07 degrees short of the vertical. There, a hundredth of a degree
of sideways wobble would swing the yaw 8 degrees, and crossing the vertical
would flip it half a turn, turning the whole world in one frame. A nose
within six degrees of vertical now carries nothing, and the seat keeps its
facing until the nose has a heading again.

Every probed ride carries exactly as before. tram1b's tram pitches 9
degrees at most, storage1's crash pod 35, and mcc_2's pod keeps its heading
until it stands in the tube. The twist about the world's up of each frame's
rotation would also have avoided the flip. It was tried and dropped: it
parted from the nose's heading by up to 8.8 degrees over storage1's crash,
where the pod rolls to -48 degrees while pitched.

Probed both drop pod rides in VR. storage1's pod falls for 30 s, then
crashes through the tower; its largest turn in one tic is 2 degrees.
mcc_2's arm swings its pod from nose down to nose up in the launch tube,
after the level's opening bed ride. Every ride stays in stereo with a level
view. The VR smoke, the source contract and the affected CI tests pass.

Author: themuffinator

docs/dev/plans/2026-10-02-openxr-vr.md

@@ -416,7 +416,21 @@ not open an XR frame and draw to the window as before.
     track at a constant offset, and the gun tracked it. Medlabs' table ride
     (the Stroggification) carries the view down its track the same way: the
     eye looks from the head along the strapped body, in stereo, through
-    every stop and turn of 40 s of the ride.
+    every stop and turn of 40 s of the ride. So do mcc_2's opening bed and
+    both drop pods. storage1's pod falls for 30 s and then crashes through
+    the tower, pitching between -24 and +35 degrees, its largest turn in a
+    tic 2 degrees; mcc_2's launch arm swings its pod from nose down through
+    level to nose up in the tube, and the level view lets the pod turn about
+    the head instead of laying the player on their back. The arm stands the
+    pod 0.07 degrees short of the vertical, where the nose keeps a horizontal
+    sliver a thousandth long: a hundredth of a degree of sideways wobble
+    would swing its yaw 8 degrees, and crossing the vertical would flip it
+    half a turn. So a nose within six degrees of vertical carries nothing,
+    and the seat keeps its facing until the nose has a heading again. The
+    twist of each frame's rotation about the world's up has no such point,
+    but it is not the nose's heading: it parted from it by up to 8.8 degrees
+    over storage1's crash, where the pod rolls to -48 degrees while
+    pitched, and the nose is what the seat keeps in front of the player.
   - *The engine turns the stick into the weapon hand's aim*, which a vehicle
     never sees, so a hand pointed aside made the tank strafe. The game turns
     the stick back to the head, which the vehicle faces.

docs/dev/release-completion.md

@@ -95,7 +95,8 @@
   vignette during stick movement and smooth turning, scope zoom that
   magnifies each eye, single player vehicles and turrets in stereo (the eye
   on the turret's axis, the turret following the head, the seat keeping its
-  own facing through the vehicle bind), a weapon wheel that no longer turns
+  own facing through the vehicle bind; the tram gun, the medical table, the
+  bed and both drop pod rides probed start to finish), a weapon wheel that no longer turns
   the world, and a Virtual
   Reality section in
   Settings > Game Options (twelve rows, twelve languages). Building that section

docs/dev/releases/v0.13.2.md

@@ -82,7 +82,8 @@
   without lowering your view twice. In single player the hovertank, the
   walker and the mounted guns surround you in stereo too
   (`vr_vehicleStereo`): you sit where the turret turns, it turns where you
-  look, and the trigger fires.
+  look, and the trigger fires. Scripted rides, from the tram's rear gun to
+  the drop pods, carry you in stereo as well, with the horizon kept level.
   In menus the weapon-hand controller points and clicks. Temporal anti-aliasing and dynamic resolution switch off while VR
   presents. `vr_status`, `vr_recenter` and `vr_restart` help with setup, and the
   desktop window mirrors the left eye by default.

docs/user/vr.md

@@ -200,7 +200,10 @@ trigger to fire. The laser dot shows where the shots will land, and each shot
 kicks in your weapon hand. The left stick drives the way the vehicle faces.
 A vehicle that turns under you, such as a walker turning on the spot, turns
 your view with it, and the comfort vignette darkens the edges of your view
-while a vehicle moves you.
+while a vehicle moves you. Scripted rides seat you the same way, from the
+tram's rear gun to the drop pods. Your view stays level whatever the ride
+does, so a pod swung nose-up into its launch tube turns around you instead
+of tipping you onto your back.
 
 | Setting | Default | What it does |
 | --- | --- | --- |

src/game/Player.cpp

@@ -13527,7 +13527,16 @@ void idPlayer::UpdateVRVehicleFrame( void ) {
 		vrVehicleYawValid = false;
 		return;
 	}
-	const float yaw = vehicle->GetAxis().ToAngles().yaw;
+	// a nose within six degrees of straight up or down has no heading to
+	// follow: its yaw rests on a sliver of the axis and flips half a turn as
+	// it crosses the vertical (mcc_2's launch arm stands the drop pod 0.07
+	// degrees short of it), so the seat keeps its facing until one returns
+	const idVec3 &nose = vehicle->GetAxis()[ 0 ];
+	if ( nose.ToVec2().LengthSqr() < 0.01f ) {
+		vrVehicleYawValid = false;
+		return;
+	}
+	const float yaw = nose.ToYaw();
 	if ( vrVehicleYawValid ) {
 		vrVehicleDeltaYaw = idMath::AngleNormalize180( vrVehicleDeltaYaw + idMath::AngleDelta( yaw, vrVehicleYaw ) );
 	}