# Introduction to custom lighting in URP

> An overview of approaches to implementing custom lighting.

Implementing a custom lighting function or model gives you more control over the visual style and performance of your application.

For example, a simpler, cartoon style rendering can be significantly faster than realistic physically-based rendering, and can have a unique appealing look.

A good example of a custom lighting implementation is the [cockpit scene in the URP 3D Sample project](https://unity.com/demos/urp-3d-sample#the-cockpit).

Unity provides several ways of implementing custom lighting using HLSL, shader graph custom function nodes, or through URP source code modification.

## HLSL shaders and shader graph shaders

Custom HLSL shaders provide you with a wide range of features and flexibility to implement custom lighting functions. You have full control over render passes, input and output variables, attributes, and internal shader functions.

Pre-built Shader Graph nodes let you use common shader functions, but provide few options for modifying their behavior. Shader Graph also provides [custom function nodes](https://docs.unity3d.com/Packages/com.unity.shadergraph@latest/index.html?subfolder=/manual/Custom-Function-Node.html) that let you inject custom HLSL code inside a node.

You can find examples of custom function nodes in the **URP 3D Sample** project, and in the [Custom Lighting sample in the Shader Graph package](https://docs.unity3d.com/Packages/com.unity.shadergraph@latest/index.html?subfolder=/manual/Shader-Graph-Sample-Custom-Lighting.html).

### Limitations of custom function nodes

Custom function nodes let you insert your code into the vertex shader function or the fragment shader function. These functions correspond to the **Vertex** and **Fragment** shader stages in Shader Graph, or to the functions `vert(Attributes IN)` and `frag(Varyings IN)` in an HLSL shader.

A custom function node does not provide the following functionality:

* Change or add attributes in the input and output structures (often defined as `struct Attributes` and `struct Varyings` in shader code). For example, this means that you can't add an extra custom option to the **Fragment** or **Vertex** shader stages.

* Define extra shader passes.

* Define extra render state commands such as `ColorMask`.

If your shader requires any of the features above, consider writing a ShaderLab shader.

## Methods for using different types of light

Techniques for implementing custom lighting vary depending on which types of lights your custom effects require.

To use the main light, use the `GetMainLight` shader function. For more information, refer to [Use lighting in a custom URP shader](/engine/6000.7/manual/materials-and-shaders/shaders/writing-custom-shaders-urp/use-built-in-shader-methods/lighting.md).

The Forward and Forward+ rendering paths handle extra directional lights and additional lights differently. The Forward+ rendering path doesn't have a limit on real-time lights per GameObject, so the `GetAdditionalLightsCount` method always returns 0. For information on how to write a rendering loop that uses additional lights in Forward and Forward+ rendering paths, refer to [Render additional lights in a shader in URP](/engine/6000.7/manual/lighting-overview/lighting/custom-lighting/use-built-in-shader-methods-additional-lights-fplus.md).

## Additional resources

* [Lighting in URP](/engine/6000.7/manual/lighting-overview/lighting.md)

* [HLSL in Unity](/engine/6000.7/manual/materials-and-shaders/shaders/writing-custom/shader-writing/sl/writing-shader-writing-shader-programs-hlsl.md)

* [Custom Lighting sample in the Shader Graph package](https://docs.unity3d.com/Packages/com.unity.shadergraph@latest/index.html?subfolder=/manual/Shader-Graph-Sample-Custom-Lighting.html)