BGE Fix T45207: Camera actuator shakes with low height

The camera-aiming code was using a near-zero-length cross product, which
caused oscillations. Thresholding on the cross product length seems to
fix this.

Reviewers: lucky3, Matpi, lordloki

Reviewed By: lordloki

Projects: #game_engine

Differential Revision: https://developer.blender.org/D1387

1 files changed, 6 insertions, 2 deletions

diff --git a/source/gameengine/Ketsji/KX_CameraActuator.cpp b/source/gameengine/Ketsji/KX_CameraActuator.cpp
index 4dc30ae1672..3d1802bb2a6 100644
--- a/source/gameengine/Ketsji/KX_CameraActuator.cpp
+++ b/source/gameengine/Ketsji/KX_CameraActuator.cpp
@@ -290,11 +290,15 @@ bool KX_CameraActuator::Update(double curtime, bool frame)
 	
 	/* only for it lies: cross test and perpendicular bites up */
 	if (inp < 0.0f) {
-		if (fp1[0] * fp2[1] - fp1[1] * fp2[0] > 0.0f) {
+		/* Don't do anything if the cross product is too small.
+		 * The camera up-axis becomes unstable and starts to oscillate.
+		 * The 0.01f threshold is arbitrary but seems to work well in practice. */
+		float cross = fp1[0] * fp2[1] - fp1[1] * fp2[0];
+		if (cross > 0.01f) {
 			from[0] -= fac * fp1[1];
 			from[1] += fac * fp1[0];
 		}
-		else {
+		else if (cross < -0.01f) {
 			from[0] += fac * fp1[1];
 			from[1] -= fac * fp1[0];
 		}