Discuss about `rvx, rvy` in `collection-map.js` there is a function with arguments named `rvx, rvy` , What do they mean? what's the full name ? https://github.com/phoboslab/Impact/blob/7768fd29c70ca924a78673d93081baab5a72fbe6/lib/impact/collision-map.js#L63 ------ the `_traceStep()` be called with vx & vy https://github.com/phoboslab/Impact/blob/7768fd29c70ca924a78673d93081baab5a72fbe6/lib/impact/collision-map.js#L44 Why do you pass `vx , vy` ? in my opinion , at line 44 , It should pass `sx ,sy` ------ the function `_traceStep` has arguments `rvx, rvy` , and pass them into `_checkTileDef()` https://github.com/phoboslab/Impact/blob/7768fd29c70ca924a78673d93081baab5a72fbe6/lib/impact/collision-map.js#L95 https://github.com/phoboslab/Impact/blob/7768fd29c70ca924a78673d93081baab5a72fbe6/lib/impact/collision-map.js#L104 https://github.com/phoboslab/Impact/blob/7768fd29c70ca924a78673d93081baab5a72fbe6/lib/impact/collision-map.js#L142 https://github.com/phoboslab/Impact/blob/7768fd29c70ca924a78673d93081baab5a72fbe6/lib/impact/collision-map.js#L150 Why do you pass `rvx , rvy` ? in my opinion , at line 44 , It should pass `vx ,vy`. --------- I post a PR , Just for comparing the differences in the code https://github.com/phoboslab/Impact/pull/100/files If change the code to my version, will there be any problems?