PhysicsCharacter.onGround() is unreliable

[stephengold]

I saw strange character behaviors while writing tutorials for LBJ. Most of the time PhysicsCharacter.onGround() returns the correct value, but occasionally it returns true when the character is unsupported.

This allows jumping to unlimited height in tests such as HelloWalk, TestPhysicsCharacter, and TestQ3. It might be related the character's vertical velocity being nearly zero.

[stephengold]

Interesting that the issue doesn't appear in HelloCharacter.

[stephengold]

It appears in HelloGhost, but not if I configure the character's gravity to 59.9 psu/sec2 instead of 60. And note that HelloCharacter uses the default gravity, which is 29.4 psu/sec2.

[stephengold]

Sure enough:

bool btKinematicCharacterController::onGround() const
{
    return (fabs(m_verticalVelocity) < SIMD_EPSILON) && (fabs(m_verticalOffset) < SIMD_EPSILON);
}

[stephengold]

btKinematicCharacterController is chock full of thoughtless hacks and copypasta. It could benefit greatly from a complete rewrite. But not today! The less I modify it, the happier I'll be. Ideally, I'd simply add an isOnGround() method to the jmeKcc subclass and invoke that in place of btKinematicCharacterController::onGround().

btKinematicCharacterController contains a few protected fields that seem relevant:

• m_wasOnGround is unfortunately set using the existing onGround() method, so no help there.
• full_drop has the opposite sense of onGround(). Unfortunately it is updated only if m_verticalVelocity <= 0.0, so it's not useful during the rising half of a jump.
• m_wasJumping has the opposite sense of onGround().

Of those 3 fields, m_wasJumping seems closest to providing the desired functionality. I've run a few tests, and it looks good.

[stephengold]

Okay, the fix is in Libbulletjme v14.5.0. In hindsight, I wish I'd made it return onGround() && !m_wasJumping;---less risk of a false positive that way.

[stephengold]

As written, TestPhysicsCharacter, TestQ3, and TestWalkingChar do not invoke onGround()---they permit jumping even when the character is airborne. In order to test this fix, those apps need minor modifications

[stephengold]

There's an issue with the current solution because it returns true after the character steps off the supporting surface. See the Forum discussion: https://hub.jmonkeyengine.org/t/linear-velocity-inconsistency-on-collisionlistener/47446/4