In videogames, the world you see and the world you collide with are quite different. What you see are well-defined, accurate, and highly-detailed objects. What lies behind those objects is the world of collision, built around player comfort and performance, and as a result, is not entirely what you would expect.
• Visual Geometry vs. Collision Geometry: Almost all assets in an environment (e.g., trees, rocks, and buildings) are made of high-resolution geometry. In the collision world, a simplified shape exists underneath their high-detail counterparts. They mimic the high-res geometry's shape as closely as possible to define how bullets, grenades, or characters interact with this object when encountering it.
Below is an example of a scene from our game and what the corresponding collision world looks like. You can clearly see the simplified representation of the meshes in the collision world versus what you actually see. Simplified shapes are used for collision detection as it can be a very costly process for computers to calculate. The fewer triangles (or shapes) that need to be detected, the better the game's performance will be.
• Collision Shape types: There are three types of collision shapes commonly used in games. The simplest and least expensive forms are called "Primitives." These are basic mathematical shapes like spheres/boxes/capsules that are cheap performance-wise. You can also connect multiple "Primitives" and create more elaborate compound collision shapes to represent your 3D models.
Second on the list are "Convex Meshes." These shapes are also mathematically generated but use triangles to "shrink-wrap" and fit the object's shape more accurately with less manual setup.
Finally, there are "3D Meshes." These shapes can be 1:1 duplicates of the visual models they are based on or a less detailed version. Although this is the most precise representation of the visual asset, it is highly costly for the game's performance, making it an unpopular choice in most cases.
Presented from left to right is an example of a "Primitive," "Convex Mesh," and "3D Mesh" asset.
• Character Collision: Similar to environmental objects, characters also use collision shapes to allow them to interact with the environment around them. In LEMNIS GATE, we use capsules (See "Primitives" above) to prevent characters from passing through walls while allowing them to walk into and over objects. Capsule shapes are widely used to represent characters as they have the ability to slide smoothly across surfaces and not get stuck on ledges and outcroppings.
The image below shows a character and its respective collision capsule. This simple "Primitive" shape determines how this character will physically react to other objects in the environment.
• Tricks and Illusions: For the benefit of gameplay, collision geometry often differs from its visual counterpart. Staircases are a good example; developers often use ramps or navigation-friendly shapes to represent a set of stairs to promote smooth movement and avoid getting stuck when walking on top of objects during gameplay.
On a different note, hidden geometry can also be used to keep players outside of unwanted areas. Hidden collision geometry is often used to contain and direct players to benefit their experience. We're sure many of you have experienced first-hand, the classic "Invisible wall" in at least one of your favourite games!
• Collision Layers: In addition to the tricks mentioned above, the collision world is usually split into many different "layers." These layers let us control which types of objects are allowed to collide with others. For example, hidden geometry like the previously discussed invisible wall (in our case, a dome over the entire map) will prevent characters from moving through it but won't prevent bullets or grenades from passing through. It would look rather weird if projectiles hit all invisible walls in a game, right?
Although players can't see it during gameplay, the invisible dome shown above is a piece of hidden geometry that keeps players within the intended areas of a map.
The dome creates a level boundary that restricts characters from passing through and leaving the combat area.
Back to the visual world, players won't see the dome but will undoubtedly get blocked by it if they try to cross it. As the example shows, projectiles can go through it, but characters don't stand a chance!
And there you have it! Mostly hidden from the players' eyes, the underlying world of collision is an often overlooked but fundamental part of a game as it affects immersion, gameplay, and game performance.
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