Optimizing draw calls

To draw geometry on the screen, Unity issues draw calls to the graphics API. A draw call tells the graphics API what to draw and how to draw it. Each draw call contains all the information the graphics API needs to draw on the screen, such as information about textures, shadersA program that runs on the GPU. More info
See in Glossary, and buffers. Draw calls can be resource intensive, but often the preparation for a draw call is more resource intensive than the draw call itself.

To prepare for a draw call, the CPU sets up resources and changes internal settings on the GPU. These settings are collectively called the render state. Changes to the render state, such as switching to a different material, are the most resource-intensive operations the graphics API performs.

Because render-state changes are resource intensive, it is important to optimize them. The main way to optimize render-state changes is to reduce the number of them. There are two ways to do this:

• Reduce the total number of draw calls. When you decrease the number of draw calls, you also decrease the number of render-state changes between them.
• Organize draw calls in a way that reduces the number of changes to the render state. If the graphics API can use the same render state to perform multiple draw calls, it can group draw calls together and not need to perform as many render-state changes.

Optimizing draw calls and render-state changes has a number of benefits for your application. Mainly, it improves frame times, but it also:

• Reduces the amount of electricity your application requires. For battery-powered devices, this reduces the rate at which batteries run out. It also reduces the amount of heat a device produces when running your application.
• Improves maintainability of future development on your application. When you optimize draw calls and render-state changes earlier and maintain them at an optimized level, you can add more GameObjectsThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
  See in Glossary to your sceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
  See in Glossary without producing large performance overheads.

There are several methods you can use in Unity to optimize and reduce draw calls and render-state changes. Some methods are more suited for certain scenes than others. The following methods are available in Unity:

• GPU instancing: Render multiple copies of the same meshThe main graphics primitive of Unity. Meshes make up a large part of your 3D worlds. Unity supports triangulated or Quadrangulated polygon meshes. Nurbs, Nurms, Subdiv surfaces must be converted to polygons. More info
  See in Glossary at the same time. GPU instancing is useful for drawing geometry that appears multiple times in a scene, for example, trees or bushes.
• Draw call batching: Combine meshes to reduce draw calls. Unity provides the following types of built-in draw call batching:
  • Static batchingA technique Unity uses to draw GameObjects on the screen that combines static (non-moving) GameObjects into big Meshes, and renders them in a faster way. More info
    See in Glossary: Combine meshes of static GameObjects in advance. Unity sends the combined data to the GPU, but renders each mesh in the combination individually. Unity can still cull the meshes individually but each draw call is less resource-intensive since the state of the data never changes.
  • Dynamic batchingAn automatic Unity process which attempts to render multiple meshes as if they were a single mesh for optimized graphics performance. The technique transforms all of the GameObject vertices on the CPU and groups many similar vertices together. More info
    See in Glossary: Transforms mesh vertices on the CPU, groups vertices that share the same configuration, and renders them in one draw call. Vertices share the same configuration if they store the same number and type of attributes. For example, position and normal.
• Manually combining meshes: Manually combine multiple meshes into a single mesh, using the Mesh.CombineMeshes function. Unity renders the combined mesh in a single draw call instead of one draw call per mesh.
• SRP Batcher: If your Project uses a Scriptable Render PipelineA series of operations that take the contents of a Scene, and displays them on a screen. Unity lets you choose from pre-built render pipelines, or write your own. More info
  See in Glossary (SRP), use the SRP Batcher to reduce the CPU time Unity requires to prepare and dispatch draw calls for materials that use the same shader variant.

Optimization priority

You can use multiple draw call optimization methods in the same scene but be aware that Unity prioritizes draw call optimization methods in a particular order. If you mark a GameObject to use more than one draw call optimization method, Unity uses the highest priority method. The only exception to this is the SRP Batcher. When you use the SRP Batcher, Unity also supports static batching for GameObjects that are SRP Batcher compatible. Unity prioritizes draw call optimizations in the following order:

1. SRP Batcher and static batching
2. GPU instancing
3. Dynamic batching

If you mark a GameObject for static batching and Unity successfully batches it, Unity disables GPU instancing for that GameObject, even if the renderer uses an instancing shader. When this happens, the InspectorA Unity window that displays information about the currently selected GameObject, asset or project settings, allowing you to inspect and edit the values. More info
See in Glossary window displays a warning message that suggests that you disable static batching. Similarly, if Unity can use GPU instancing for a mesh, Unity disables dynamic batching for that mesh.