Among the various things I worked on this week, I spent a good portion of time implementing a basic fluid system in the game engine. The fluid system lets me create volumes of moving fluid that alter the player’s physics and can flow down off ledges and interact with objects, so that levels like the aqueducts look and feel more convincing.
The two major pieces of the fluid system are physics and rendering. The physics system is arguably more important, but ended up mostly being a matter of fine-tuning. Here’s how it works:
Previously, every frame I did a sweep of the area below onscreen entities to locate the nearest surface to stand on, and used that to perform some motion and collision detection calculations so that the player can properly move up/down slopes, stand on moving objects, and drop when standing over a gap.
To implement the water system, I extended the standing sweep code to also locate the nearest volume of fluid that intersects the entity, and store it. What this allows me to do is detect whether or not the player character is currently inside a fluid, regardless of whether or not he’s standing on a surface at the time (so it even works while jumping/falling).
Once I had this functioning, I added the ability to attach ‘Material’ information to any piece of geometry in the game, including fluids. These two pieces of functionality combined allow me to control the physical attributes of a surface, and then further alter those attributes when the player is standing inside water or other fluids. This not only allows me to implement slippery surfaces like ice, but cause the player to move more slowly and have different control characteristics when in the water.
In the future, this should also allow me to alter the characteristics of surfaces in response to gameplay events – enemies that spray sticky goo on the ground, spider webs that slow you down, or a potion that freezes water, turning it into slippery ice you can stand on.
After a bit of fine-tuning, I came up with some physics parameters for the basic material types that I’m happy with, that make the differences between various materials obvious without breaking gameplay mechanics. The jumping mechanics were a particularly tough detail – even minor changes to the acceleration and speed characteristics of the player render some of my jumping puzzle prototypes completely impossible to solve, so I had to carefully test against those puzzle prototypes each time I adjusted the physics values for a given material. In the final game, I’m going to have to address this by only using a specific set of materials in each level, so that it’s not necessary to retest the entire game after physics changes.
Of course, water isn’t particularly interesting if you can’t see it, so the other thing I did was build a fairly simple bit of rendering that allows fluids to flow across surfaces and down off ledges.
