Clamp Collision Response

I've a player with a rectangular collision box which collides with a scene object again with a simple rectangular collision box. Both objects are set to "CLAMP" response type and most of the time everything works fine.

With the debug banner enabled, contacts shows as 0. When the player is moved into contact with the scene object and the movement keys are kept held down, contacts shows 2 as expected, upon releasing the keys contacts returns to 0 as expected. The player can then be moved left/right/up without problem, and trying to move down will once more cause the clamp response and contacts becomes 2 until release.

However, occasionally after releasing the keys contacts remains at 2 and from that time on, the player can only move parallel to the scene objects surface, any attempt to move off it results in the Y velocity been reset to 0 by the collision response code.

I've had a quick look in the engine in t2dPhysics.cc and the culprit appears to be "resolveClampCollision"

// Solve Source?
    if ( solveSrc )
    {
        // Fetch Linear Velocity / Collision Normal.
        const t2dVector linearVelocity = pCollisionStatus->mpSrcPhysics->getGrossLinearVelocity();
        t2dVector collisionNormal = pCollisionStatus->mCollisionNormal;
        // Normalise Collision Normal.
        collisionNormal.normalise();

        // Calculate Dot Velocity Normal.
        const F32 dotVelocityNormal = collisionNormal * linearVelocity;

        // Any velocity to clamp?
        if ( mNotZero(dotVelocityNormal) )
        {
           // Set new Linear Velocity.
           // NOTE:-   vO = vI - ((N*vI)N) 
           pCollisionStatus->mpSrcPhysics->setGrossLinearVelocity( linearVelocity-(collisionNormal*dotVelocityNormal) );
        }
    }
.. solveDst snipped ..

When the bug occurs, the collisionNormal is (-0, -1.0) and linearVelocity is (0, -3.25), dot velocity ends up as (0, 3.25) and thus in set new linear velocity we end up setting the Y velocity to -3.25 - (-1*3.25) = 0.

If the above is instead modified to test for velocity been in the same direction as the collision normal, then when the bug occurs and we normally end up stuck to the sceneobject, we can now move away from it.

// Solve Source?
    if ( solveSrc )
    {
        // Fetch Linear Velocity / Collision Normal.
        const t2dVector linearVelocity = pCollisionStatus->mpSrcPhysics->getGrossLinearVelocity();
        t2dVector collisionNormal = pCollisionStatus->mCollisionNormal;
        // Normalise Collision Normal.
        collisionNormal.normalise();

        // Any velocity to clamp?

        const F32 dotVelocityNormal = collisionNormal.dot(linearVelocity);
        if ( dotVelocityNormal < 0 )
        {
            // Set new Linear Velocity.
            // NOTE:-   vO = vI - ((N*vI)N) 
            pCollisionStatus->mpSrcPhysics->setGrossLinearVelocity( linearVelocity-(collisionNormal*dotVelocityNormal) );
        }
    }
... same fix in the solveDst too ...

This is the first time I've really looked at the TGB source and I've only had a 5 minute glance over things, but even so this "fix" does not feel right. Although it solves the problem, it doesn't really address the cause.

The player shouldn't be getting stuck in the first place. I.E after releasing the movement keys contacts is remaining at 2 when normally it should return to 0 until we try to move into the sceneobject again. Why this is occurring though, I'm not sure.

Anyone have any ideas?

EDIT: Updated "fix" in case anyone reads this without reading all the thread.