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URP Rendering Flow

· Rendering & TA ·
#Unity#URP#Rendering#Pipeline

A condensed summary of Unity’s official documentation on the Universal Render Pipeline flow—useful when reading URP source code or writing ScriptableRendererFeature.

Original doc: https://docs.unity3d.com/Packages/com.unity.render-pipelines.universal@8.1/manual/rendering-in-universalrp.html

URP renders a frame in five phases:

  1. Setup Culling Parameters
  2. Culling
  3. Build Rendering Data
  4. Setup Rendering
  5. Execute Renderer

1. Setup Culling Parameters

Goal: configure ScriptableCullingParameters.

  • Which camera, FOV, near/far planes
  • Layer masks for rendering / shadows
  • Shadow distance, cascades, which lights cast shadows
  • Quality or per-camera overrides

URP pulls data from Camera, UniversalAdditionalCameraData, QualitySettings, etc. Override hooks let you customize the parameters (e.g., render only certain layers).

2. Culling

With parameters prepared, URP culls the scene:

  • Iterates renderers, lights, shadow casters
  • Tests against frustum, layers, shadow settings
  • Outputs a CullingResults

CullingResults contains visible renderers, lights, and the data required for shadow maps. Good culling values reduce unnecessary work; e.g., shrinking shadow distance prevents faraway casters from consuming resources.

3. Build Rendering Data

URP combines:

  • CullingResults
  • URP Asset settings (rendering path, MSAA, shadow quality, additional light limits…)
  • Camera settings (viewport, render scale, post-processing toggles…)
  • Platform capabilities

The outcome is RenderingData, broken down into:

  • CameraData
  • LightData
  • ShadowData
  • Other feature toggles

Think of it as the “contract” for the rest of the pipeline: what we render, how we render it, and with which quality.

4. Setup Rendering

Using RenderingData, URP builds the render pass queue:

  • Creates built-in ScriptableRenderPass instances (depth prepass, main light, transparent rendering, post-processing, UI…)
  • Calls EnqueuePass for custom passes (from ScriptableRendererFeature)
  • Determines order based on dependencies

This stage is where most customization happens—you choose which passes run for each camera.

5. Execute Renderer

Finally URP executes the queued passes:

  1. Initialize render targets
  2. For each pass:
    • Configure targets / clears
    • Set viewport/scissor
    • Issue DrawRenderers, Blit, custom commands
  3. Output the final image to the camera target

Opening Frame Debugger shows this stage unrolled command by command.

Summary

URP frame loop:

  1. Prepare culling parameters
  2. Cull renderers/lights/shadows
  3. Build rendering data (camera/light/shadow info)
  4. Queue render passes
  5. Execute passes and present

Knowing these checkpoints helps when:

  • Reading URP source
  • Writing custom renderer features
  • Tweaking URP Asset or camera settings

Original article (Chinese) on CSDN “uniGame”, CC BY-SA 4.0.
https://blog.csdn.net/alla_Candy/article/details/121559633