Optimization of Draw Calls

I want to talk about what I think is a pretty cool trick for optimizing the number of draw calls in Netstorm 2 an RTS.  Dealing with draw calls in an RTS is more challenging than, say, an FPS because the player can create a large number of objects over the course of the game.  In an FPS, you know how many objects there will be (except maybe for missiles).  In an RTS, the point of the game is to dynamically create units for your army, and each one of these might be a draw call, which means it can get pretty expensive, pretty quickly.

The Bridges of RisingStorm County

There are a large number of bridge units built in a game of Netstorm

Netstorm has an additional challenge: one of the principle “units” a player will place is a bridge.  This is free (although limited to how many you can place every few seconds) and is used to maneuver.  It is in the player’s best interest to place bridges as fast as possible.  In an average game there are hundreds of bridge pieces being built.

Bridges of RisingStorm

The bridges have Tetris-like shapes

Each bridge piece is a Tetris-like shape that is made up of smaller atomic pieces, e.g., a straight, a corner and a tee.  Each of these under normal circumstances will be a separate draw call.  However, thanks to the Mesh Combiner script provided with Unity, these are very easily combined into the Tetris-like shapes.  However, there are still hundreds of Tetris-like shapes to deal with.

Small diversion:  A few years ago, a friend of mine and I worked on a game engine ourselves.  For better or worse, I pushed us into using threads aggressively.  One cool benefit was that since systems acted in independent threads, we could avoid the situation where the may game loop has to run as fast as possible to service any possible need.  Thus the CPU would only be used as much as it was needed, which for many situations was hardly at all.

In order to deal with these many independent systems we set up a publish/subscribe mechanism to decouple dependencies between the systems.  We also had several systems which would only run based on certain events happening, rather than running in a loop.  We created a central dispatch and scheduler which we called the Hub to manage the execution of these systems and dispatch messages to them.

Recently, I saw a way of bringing back some aspects of the Hub into Unity.  Specifically the dispatcher and the scheduler.  The dispatcher uses Unity’s SendMessage interface, but instead of needing to know which game object you want to send to, you just send the message to the dispatcher.  Any gameobject that wants to see that message simply needs to register for it.  One of the ways I use this inside Netstorm is to turn shadows on and off when the FPS are too low or safely high enough.  When the FPS drops below 15, a message is sent to the dispatcher saying “LowFPS” and the shadows disable.  Once the FPS goes back above 60 another message is sent and shadows are turned back on.  Because the FPS checker doesn’t know who is getting the message other effects could also register for the low and high FPS to decide when to turn off.  I could also use this for Level of Detail changes.

But, going back to the bridges, there’s an even cooler trick.  Running the mesh combiner is expensive and Unity’s dynamic batching is limited to 300 polygon object.  It would be nice to have something in between that we can choose to run as needed.  This is where the scheduler comes in.

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