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Render Pipelines in Atom Renderer

A render pipeline describes a sequence of steps to render a 3D scene on a 2D screen. In Atom Renderer, the render pipeline is completely modular and driven by a series of passes. This lets you customize the render pipeline for features such as shadows, lighting, and post effects. You can also completely change the rendering technique, for example, to Deferred, Checkerboard, or Forward+. By default, Atom implements Forward+ rendering in its pre-built render pipelines.

Atom’s pre-built render pipelines are ready to use and encompass common use cases for Open 3D Engine (O3DE):

Main Rendering Pipeline: The default render pipeline for Windows, Linux, and macOS platforms.
Low-end Rendering Pipeline: The default render pipeline for iOS and Android platforms.

The render pipeline consists of a root pass that drives a collection of passes to handle different graphics techniques in the render pipeline. For more information, refer to the Pass System section.

Supported features in pre-built render pipelines

This table lists the features that are supported by the Main Rendering Pipeline and the Low-end Rendering Pipeline by default.

Feature	Description	Main Pipeline	Low-end Pipeline
Skinning	Compute-shader-based mesh skinning for bone weighted animations and morph targets.	Yes	Yes
Directional shadows	Cascaded shadowmap to simulate sun light.	Yes	Yes
Punctual shadows	Shadows for punctual light types: point and spot lights.	Yes	Yes
Area lights	Spherical, disk, capsule, quad, and polygonal lights.	Yes	Yes
Light culling	Tile-based or cluster-based light culling for efficient light iteration in the Forward+ pass.	Yes	Yes
RTX global illumination	Real-time global illumination that uses NVIDIA RTXGI (RTX Global Illumination).	Yes	No
Reflection probe	Generates and uses reflection probes for specular image-based lighting (IBL).	Yes	Yes
Physically based sky	Basic sky rendering that uses physically-based atmospheric modeling.	Yes	No
HDR skybox	Uses an HDR image as a skybox.	Yes	Yes
Screen space ambient occlusion (SSAO)	Applies SSAO to the scene to create contact shadows where objects meet.	Yes	No
Subsurface scattering	Calculates subsurface scattering for materials, such as skin, by using a screen space technique.	Yes	No
Deferred fog	Applies a height-based fog to the scene.	Yes	No
Screen space reflections (SSR)	Implements SSR by using renderered lighting buffer to approximate real-time reflections.	Yes	No
Depth of field	Applies a bokeh-shaped depth of field effect to the scene that’s based on the scene’s depth and camera focus paramters.	Yes	No
Bloom	Applies a bloom effect to the rendered image, making bright pixels bleed out into neighboring pixels.	Yes	No
Subpixel morphological anti-aliasing (SMAA)	Applies SMAA as an anti-aliasing technique.	Yes	No
Temporal anti-aliasing (TAA)	TAA uses motion vectors and temporal super-sampling to reduce aliasing in the final image.	Yes	No
Light adaptation	Adjusts brightness of the final image based on the scene’s average luminance, so the image appears neither too dark nor too bright.	Yes	Yes
Look modification and color grading	Allows artists to adjust the final look of the image by using look up tables (LUTs).	Yes	Yes
Display mapping	Adjusts the color values of the final image based on the color range that the screen expects.	Yes	Yes
User interface (UI)	Renders 2D UI components.	Yes	Yes
Auxilary geometry (AuxGeom)	Renders various geometry and text for in-editor tools and debugging, such as gizmos, grids, debug lines, and shapes.	Yes	Yes

File structure

A render pipeline is stored as an .azasset file, which is a JSON file that links to the root pass of the render pipeline and contains other configurations. In this file, the RootPassTemplate parameter contains the name of pipeline’s root pass.

The following example is the MainRenderPipeline.azasset file. The root pass template is called MainPipeline, which links to the MainPipeline.pass file.

{
    "Type": "JsonSerialization",
    "Version": 1,
    "ClassName": "RenderPipelineDescriptor",
    "ClassData": {
        "Name": "MainPipeline",
        "MainViewTag": "MainCamera",
        "RootPassTemplate": "MainPipeline",
        "RenderSettings": {
            "MultisampleState": {
                "samples": 4
            }
        }
    }
}

You can find the following render pipeline files in your O3DE engine source:

Main Rendering Pipeline: Gems\Atom\Feature\Common\Assets\Passes\MainRenderPipeline.azasset
Low-end Rendering Pipeline: Gems\Atom\Feature\Common\Assets\Passes\LowEndRenderPipeline.azasset

Modifying the render pipeline

Because modular passes drive the render pipeline, it’s more accessible to add or remove passes and update connections. You can modify the main render pipeline or create a custom render pipeline for your team. You might want to use a custom render pipeline for a number of reasons, such as to:

Develop new features that impact the render pipeline.
Implement and use alternative rendering techniques.
Improve rendering performance.

Modifying the render pipeline involves enabling or disabling a pass and updating the connections. You can do this directly by manually editing the MainPipeline.pass with a text editor.

To enable or disable a pass in the render pipeline:

In the render pipeline’s .pass file, find the pass that you want to enable or disable in the PassRequests list.
Set the Enabled property to true or false.

When you enable or disable a pass, the pass system automatically updates the connections in the pass hierarchy. This requires that you’ve set up fallback connections. For more information, refer to PassTemplate File Specification

For example, in the PassRequests section of the MainPipeline.pass, disable DeferredFogPass in the render pipeline by setting Enabled to false.

    "PassRequests": [

        ...
        
        {
            "Name": "DeferredFogPass",
            "TemplateName": "DeferredFogPassTemplate",
            "Enabled": false,
            "Connections": [
                ...
            ],
            "PassData": { ... }
        },
        
        ...
    ]