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tookie

90´s Platformer Collisions

5 posts in this topic

Hello!

I want to create a 2D platformer game. My intention is resemble the classic 1990´s games, like Prince of Persia and Another World.

The game should handle several floor "levels", falls, floors with inclinations, and maybe deformable floors too.

The character should be able to walk, jump, go up a floor level, go down.

 

I want to do all the programming in C++, I don´t want to use an engine like Box2D.

My plan is to use data structures storing the floor geometry as line segments. Something like this:

struct Segment
{
  vec2 pointA, pointB;
};

// All line segments in a floor level
typedef std::vector<Segment> t_floorLevel;

// All geometry of the game map
typedef std::vector<t_floorLevel> t_terrainGeom;

So the question is...How can I handle the collisions?

Should SAT be overkill for this?

Should I represent the characters as circles, and check if they hit a line segment?

 

Thanks, Paul.

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I would start by using an acceleration structure. In the case of the floor, you can have the map divided into vertical slices (like a typical regular grid, but 1D instead of 2D). Apart from that, it all depends on what accuracy you want. A simple bounding circle-segment test could do, as you mention. Or an axis-aligned square - segment test. Or you can go and implement a full-fledged polygon-line test...

 

In case you go the polygon-segment test approach, you may test it easily by intersecting the segment with the polygon's segments... (maybe checking if the entire segment is inside the polygon in case you think that's possible).

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I suggest starting from the rules, and optimizing geometric acceleration structures later.

You can implement a generic query (e.g. what platform segments, at what time, and from which side intersect this object if it moves between these two positions) in a simple, inefficient and obviously correct way, use it as the foundation of a complete collision detection system that does what you want (e.g. one-sided barriers, sliding on slippery surfaces, etc.), and when you have refined your requirements try optimized, more complex and less general algorithms.

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Regarding the specific questions:


Should SAT be overkill for this?

It might or might not be a good choice of clever algorithm for "phase 2".


Should I represent the characters as circles, and check if they hit a line segment?

A convex polygon or (if the character isn't convex) a simple polygon is going to be much more suitable for a character than an ellipse; rounded corners can cause the character to stand in a wrong position, something quite unacceptable in a platformer. Ellipses are popular collision shapes for bullets, which are actually round and tolerate some approximation because they explode on contact.

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I suggest starting from the rules, and optimizing geometric acceleration structures later.

Just to clarify, when I said "I would start by" I meant "the algorithm for collision detection should start by". When it comes to the development timeline, I totally agree with LorenzoGatti's suggestion of having something that works first an optimizing later...

Edited by Javier Meseguer de Paz
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