The Shader class

In Unity, when you work with shaders that are part of the graphics pipeline, you usually work with instances of the Shader class. An instance of the Shader class is called a Shader object.

A Shader object is a Unity-specific way of working with shader programs; it is a wrapper for shader programs and other information. It lets you define multiple shader programs in the same file, and tell Unity how to use them.

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Shader object fundamentals

A Shader object contains shader programs, instructions for changing settings on the GPU (collectively called the render state), and information that tells Unity how to use them.

You use Shader objects with materials to determine the appearance of your scene.

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You can create Shader objects in two ways. Each has its own type of asset:

• You can write code to create a shader asset, which is a text file with the .shader extension.
• You can use Shader Graph to create a Shader Graph asset.

Whichever way you create your Shader object, Unity represents the results in the same way internally.

Inside a Shader object

A Shader object has a nested structure. It organizes information into structures called SubShaders and Passes. It organises shader programs into shader variants.

Shader object

A Shader object contains:

• Information about itself, such its name
• An optional fallback Shader object, which Unity uses if it can’t use this one
• One or more SubShaders

You can also define additional information such as shared shader code, or whether to use a custom editor. For information on defining a Shader object, see ShaderLab: defining a Shader object.

SubShaders

SubShaders let you separate your Shader object into parts that are compatible with different hardware, render pipelines, and runtime settings.

A SubShader contains:

• Information about which hardware, render pipelines, and runtime settings this SubShader is compatible with
• SubShader tags, which are key-value pairs that provide information about the SubShader
• One or more Passes

You can also define additional information, such as render state that is common to all of its Passes. For information on everything you can define in a SubShader, see ShaderLab: defining a SubShader.

Passes

A Pass contains:

• Pass tags, which are key-value pairs that provide information about the Pass
• Instructions for updating the render state before running its shader programs
• Shader programs, organised into one or more shader variants

You can also define additional information such as a name. For information on everything you can define in a Pass, see ShaderLab: defining a Pass.

Shader variants

The shader programs that a Pass contains are organised into shader variants. Shader variants share common code, but have different functionality when a given keyword is enabled or disabled.

The number of shader variants in a Pass depends on how many keywords you define in your shader code, and the target platform. Each Pass contains at least one variant.

For more information, see Shader keywords and variants.

Order of operations during rendering

This is a high-level description of how Unity uses Shader objects to render geometry, in all render pipelines.

Before Unity uses a Shader object:

1. Unity creates a list of SubShaders for the Shader object. It adds all of the SubShaders defined in the Shader object, and then adds all of the SubShaders in any fallback Shader objects, in order.

When Unity first renders geometry using the Shader object, or when the shader LOD value or active render pipeline changes:

1. Unity iterates over the list of all SubShaders and examines them to determine if they are: compatible with the device hardware; at or below the current shader LOD value; and compatible with the active render pipeline.
2. If the list contains one or more SubShaders that meet these requirements, it selects the first one. This is the active SubShader.
3. If the list does not contain any SubShaders that meet all of the requirements:
   1. If the list contains one or more SubShaders that meet the hardware requirements (but do not meet the LOD or render pipeline requirements), Unity selects the first one. This is the active SubShader.
   2. If the list does not contain any SubShaders that meet the hardware requirements, it shows an error material.

Unity can identify geometry that uses the same shader variant and organize it into batches for more efficient rendering. Once per frame, for every batch of geometry:

1. Unity determines which of the Passes in the active SubShader it should render, and at which point in the frame. This behavior varies by render pipeline.
2. For every Pass that it renders:
   1. If the current render state does not match the render state defined in the Pass, Unity sets the render state as defined in the Pass.
   2. The GPU renders the geometry using the relevant shader variant.