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Talk:Developing a 2D game in Java ME - Part 3

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Featured article, August 24th 2008 (week 35)

Anum Amin - Why need to track old Position?

Can someone tell me why we need to keep track of old position of the Entity? OldX and OldY have been used in Collision detection. Can anyone explain me these lines:

// check if collision resulted from x movement

if ((this.x < (other.x + other.width)) &&

         (this.x + width > other.x) &&
         (this.oldY < (other.y + other.height)) && 
         (this.oldY + height > other.y)
       if (oldX < x) {
         collision |= COLLISION_RIGHT;
       } else {
         collision |= COLLISION_LEFT;
Sorry if it is lame Question... I am going to develop game for first time... Thanks.

Anum Amin 11:53, 21 August 2012 (EEST)

Hamishwillee - Probably best to ask on forums

This article is 4 years old and the author might not be watching it any more, so best to ask on the forums. Without looking deeply, I suspect that if you have two moving objects the collision will occur at some point between the current and last position. If you just calculate the current position, you might miss a collision that already occured between frames.


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