Detail Mode Overview
Unreal Engine 4 currently supports switching between 3 detail modes: Low, Medium, and High. In cases involving effects, detail modes are set for Medium, and High detail. This is used primarily for the case of split screen where draw calls can reach performance lowering levels. The primary objective being to reduce draw calls in Medium detail mode by culling or reducing emission rates.
Changing the value in the Medium Detail Spawn Rate Scale field on an emitter in Cascade, will reduce its emission rate when the game is running in Medium Detail Mode. These settings can be used to reduce overdraw and draw calls depending upon requirements. In the case of split screen, draw call reduction is key.
How do I determine Draw Calls?
The detail mode can be set in Cascade using the View Menu -> Detail Mode setting, or for the entire Unreal Editor by changing the Quick Settings -> Engine Scalability Settings -> Effects quality level. Change to Medium to see Medium Detail Spawn Rate Scale and you should see red text displaying the detail mode currently active. It is critical to work in Medium Detail Mode only when adjusting effects for this mode. Changes will not be visible in game by default, unless your project defaults to Medium Detail Mode.
Medium Detail Spawn Rate Scale is a multiplier, sticking to a 0-1 range will set the appropriate value. Essentially setting to .5 will halve particle emission counts, a value of 1 will use the current emission rate. The values are clamped and will not go above 1.
It is good practice to consider split screen when in the process of placing ambient effects. If the effect is non-essential to split screen and has a high number of draw calls, it might be a good place to start reducing emission counts especially if the effect consists only of mesh emitters.
It is important to remember draw calls for particles are determined by material, and are a fixed cost regardless of emission count. For example, a 1 pass material on an emitter with an emission rate of 30 is only 1 draw call.
Commands
There are several commands which are useful in determining draw call costs.
To switch to Split Screen Mode in Play in Editor or the game, use the console command debugCreatePlayer 1.
If you only have one controller, use the command ssswapControllers to jump between players.
View draw calls for a given area using the command DumpParticleFrameRenderingStats. This command dumps a spreadsheet and a screenshot of where the action took place. This screenshot is an estimation since it is taken after the data is dumped and is more of a visual reference.
Viewing the stats in the spreadsheet is straight forward. It should look something like this:
In this case, the fix is pretty straight forward. We have one effect that must call a bunch of meshes and they loop spawn most likely. This can be tricky however, because if one effect is the primary culprit, and you wish to keep the effect's current visuals there is only one option, convert the meshes to sprites.
If however the emitter is not necessary for split screen, set the mediumDetailSpawn rate to 0.00.
Spreadsheet stat columns are named with appropriate descriptions:
$ RenderTime ms : How many milliseconds it takes to render the effect. $ NumComponents : How many instances of the particle system are in view and active. $ NumPasses : How many passes are in the material being called by the emitter. $ NumEmitters : How many emitters are in the particle system. $ NumDraws : The accumulated number of draw calls for all instances of the particle system.