There are two kinds of collisions that can happen in a game of Contra: object vs. object collisions, and object vs. level collisions. Each is handled by completely different code and detected using completely different data, so they are basically two separate systems.
Object vs. Object Collision
In the first post of this series, I mentioned that the game keeps track of three different kinds of objects: players, bullets from players, and enemies. Collision detection is one of the areas where this separation comes into play. Instead of checking for collisions between any and all of these objects, the game only checks collisions between the groups that will react to each other. Players never interact with other players or their bullets, and enemies never interact with other enemies, so the only collisions that matter are those between players and enemies, and those between player bullets and enemies.
Given that enemies are always involved in the object/object collisions that matter, collision detection between objects is done as part of updating all of the enemies. Each frame, the game loops over each active enemy and runs its particular update logic. After an enemy updates, the collision system kicks in and checks that enemy for collisions with players, and then collisions with player bullets. Each enemy has a pair of flags that can be used to indicate to the collision system whether either or both of these kinds of collisions are enabled for that enemy. This is how some enemies, like the bodies of turrets, are able to be shot by the player (by allowing collisions with player bullets) but not able kill you if you walk into them (by not allowing collisions with players.) Beyond these flags, no special logic is used to narrow down the list of potential collisions for an enemy. Every enemy is simply checked against every player and every bullet.
Checking for a collision between a given enemy and a given player is based on a point vs. rectangle test. The point that represents the player’s current position is checked against the rectangle that represents the current hit box that the enemy is using. If the point is inside the rectangle at the moment the check is performed then a collision has occurred. At first glance this seems a little strange. You need the player to be able to be shot in the head as well as the feet, but only the point that represents the player’s current position (which is usually fairly close the the center of the player sprite) is checked for collision with the enemy hit box. The key to making the system work is that the enemy hit boxes are not just tight bounds around the enemy sprites themselves, but rather representations of the area where collisions would occur if we were doing a more traditional rectangle vs. rectangle test. Put another way, the hit boxes that enemies use represent the space where the player position would have to be to make the player sprite overlap the enemy sprite.
You can see this process in action in the image above. In each of the three sections, the player’s position is indicated by a single green pixel in the center of a green circle. The pink rectangle represents the hit box that the white enemy bullet is using to check for collisions against the player’s position. Notice how the enemy bullet uses a different hit box depending on what the player is doing. This is how the game effectively changes where the player is vulnerable without actually having a hit box around the player to work with. Each enemy has a version of their hit box to use against a player in the water, a jumping player, a player on the ground, a standing/running player, and a player bullet. The particular type of player bullet is not taken into account when picking a hit box to test against so there is no difference between the different guns you can get from a bullet collision perspective.
Object vs. Level Collision
Collisions between objects and the level itself are not handled by a system in the way that collisions among objects are. Instead, level collisions are tested for directly wherever they are needed. If a player needs to know if he has run off the end of a platform then the code that updates a running player will query for a collision at the player’s feet every frame until it finds that there is nothing there. Most enemies don’t need to know about collisions with the level at all, but those that do will similarly check for them at specific times as needed. The only kind of query that is supported is checking a point against the level collision map that we’ve talked about before. The collision query returns the collision code for tile under the point that was checked (empty, solid, water or platform) and then the caller will do whatever it needs to in response to the different results.
Collisions in the pseudo-3D base levels work almost exactly the same as in the 2D levels but with certain constraints to make the effect hold up in fake 3D. Enemies are not allowed to collide with the player if their screen space Y position is too small (i.e. too far toward the top of the screen which means too far back “into” the screen.) This works because players are known to always be at the bottom of the screen during these levels. Player bullets are timed and only allowed to collide with most enemies once they have been on screen long enough to have reached the “back” of the room. For enemies that can be shot at any Z position within the room (like the rolling grenades that you have to blow up before they roll down and kill you) the timer on your bullets is not used. Instead, a flag is checked to see if your bullet was fired while you were lying on the ground, and if so, normal 2D collision detection is allowed to continue as usual. This works only because you are detecting a collision between two objects that are both known to be on the ground where Y position maps directly to Z position (i.e. if two things look like they are colliding in 2D, then you know they are colliding in 3D as well, but only along the ground.) With these constraints in place, the normal, flat 2D collision detection system can be used to detect collisions between objects even in a pseudo-3D environment.
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