In situations where the motion path of the cue ball is similar to the object ball (a hit angle near 180 degrees), everything works fine and the object ball sinks in the pocket. But it seems that the more angular the shot path is, the greater the margin of error is for the impulse I'm generating. I've tried many things to fix this: adjust the ball's collision margin, scaling the world larger, turning off friction and restitution and lots of others but nothing seems to changes this behavior.
Here are the relevant bits of my code:
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//assume p = pocket center, b = object ball center, c = cue ball center
//first find the position of the ghost ball, ie the target point of collision for the cue ball
btVector3 ghostPos = b+(b-p).normalize()*(2.0f*BALL_RADIUS);
//then use the normal between the ghostball and cue ball as the impulse, scaled by the shots total distance
btVector3 cueImpulse = (ghostPos-c).normalize()*((p.distance(b)+ghostPos.distance(c))*HIT_RATIO);
//finally apply the impulse to the cueball's center of mass (using general form of applyImpulse to later use rel_pos for english
cueBallBody->applyImpulse(cueImpulse,btVector3());
