The general advice when making games is to use existing tried and tested libraries as much as possible. The idea is that you can then focus on the actual game rather than its implementation. However, I've always found it much more fun to create things from scratch, so I do :)

I recently made Amazeballs, a game where you shoot paintballs in an invisible maze in order to navigate yourself to the exit. For this game I needed a 2D collision detection system.

I have previously used a system of having axis-aligned bounding boxes (“hitboxes”) associated with every object which can collide or be collided with. When you update an object’s position, you loop through all other objects and see if the bounding boxes intersect. If they do, there is a collision.

For instance, let’s imagine that we had three objects in the world, A, B and C, and wanted to update the position of C. We would calculate the new position of C and see if it intersects with A or B. See below:

In this case the movement applied to C would mean that it interacts with B; therefore, there is a collision.

However this method has one major drawback.

Each time the physics engine updates, it has to calculate the new position of each object and check if it intersects with any of the other objects. The amount by which each object is moved is typically set by the frame rate and given as a “delta time”. Delta time is the number of milliseconds each frame last, often abbreviated to “dt”.

For example, if an object were traveling at 5 meters per second and the screen was updated every 16 milliseconds (60 frames per second) then the object would move 0.08 meters each frame (5 meters per second × 0.016 seconds).

However, if the user had a less powerful machine and could only run the game at 30 frames per second, then the screen would be updated every 33 milliseconds. In this case the same object traveling at 5 meters per second would now travel 0.16 meters each frame (5 meters per second × 0.033 seconds).

This means that if two users, playing on different machines, were to run at a wall, it would be possible for the 60 frames per second user to be blocked, and the 30 frames per second user to pass right through. This is because the new position doesn’t intersect with the wall, and so no collision was detected. This is called clipping.

There are common preventions for clipping. For instance you can dictate that the user’s machine must be able to run the game at a minimum frame rate, and then ensure all walls are thick enough that you cannot clip through them. However, I never really liked that the behaviour of the game could be affected by the user’s machine…

Therefore I used the Amazeballs project to try out an alternative method for implementing collision detection. This time based on line segment intersections.

The player is represented as a single point and the walls are represented as line segments i.e. no longer as boxes.

Now when a player moves, a new movement line segment is created with one end being the player’s starting position and the other end being the potential new position. Then all the walls are looped through to see if this movement line segment intersects with them. If it does, then there has been a collision.

It is now no longer possible to clip through walls!

In the case of low frame rates (large delta time), the movement line segment gets longer, but still intersects with the wall. This means that the collision is always detected. So the game’s physics engine runs exactly the same, regardless of the user’s machine.

Although it wasn’t needed in Amazeballs, this line intersection method can also be extended to objects which have an area. One way of doing this is to bound each object with a series of line segments; just like a hitbox but it doesn't have to be a rectangle. Now when you update the position of an object, you create movement line segments for each of the points of the hitbox. You then see if any of them intersect with the line segments of the other objects hitboxes. And that’s it!

Thanks for taking the time to read this post and I hope at least some of you found it useful / interesting.

If you want to see it in action head over to Amazeballs. It’s a free game which runs in the browser and only takes a few minutes to play :)

Thanks!

Turnaround games

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