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197 lines
8.3 KiB
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.. _doc_mesh_lod:
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Mesh level of detail (LOD)
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==========================
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Level of detail (LOD) is one of the most important ways to optimize rendering
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performance in a 3D project, along with :ref:`doc_occlusion_culling`.
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On this page, you'll learn:
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- How mesh LOD can improve your 3D project's rendering performance.
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- How to set up mesh LOD in Godot.
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- How to measure mesh LOD's effectiveness in your project
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(and alternatives you can explore if it doesn't meet your expectations).
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.. seealso::
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You can see how mesh LOD works in action using the
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`Occlusion Culling and Mesh LOD demo project <https://github.com/godotengine/godot-demo-projects/tree/master/3d/occlusion_culling_mesh_lod>`__.
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Introduction
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------------
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Historically, level of detail in 3D games involved manually authoring meshes
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with lower geometry density, then configuring the distance thresholds at which
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these lower-detailed meshes should be drawn. This approach is still used today
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when increased control is needed.
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However, in projects that have a large amount of detailed 3D assets, setting up
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LOD manually can be a very time-consuming process. As a result, automatic mesh
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decimation and LOD configuration is becoming increasingly popular.
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Godot provides a way to automatically generate less detailed meshes for LOD
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usage on import, then use those LOD meshes when needed automatically. This is
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completely transparent to the user.
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The `meshoptimizer <https://meshoptimizer.org/>`__ library is used for LOD mesh
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generation behind the scenes.
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Mesh LOD works with any node that draws 3D meshes. This includes MeshInstance3D,
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MultiMeshInstance3D, GPUParticles3D and CPUParticles3D.
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Visual comparison
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-----------------
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Here is an example of LOD meshes generated on import. Lower detailed meshes
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will be used when the camera is far away from the object:
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.. figure:: img/mesh_lod_comparison_shaded.png
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:align: center
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:alt: From most detailed (left) to least detailed (right), shaded view
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From most detailed (left) to least detailed (right), shaded view
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Here's the same image with wireframe rendering to make the decimation easier to see:
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.. figure:: img/mesh_lod_comparison_wireframe.png
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:align: center
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:alt: From most detailed (left) to least detailed (right), wireframe view
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From most detailed (left) to least detailed (right), wireframe view
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.. seealso::
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If you need to manually configure level of detail with artist-created meshes,
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use :ref:`doc_visibility_ranges` instead of automatic mesh LOD.
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Generating mesh LOD
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-------------------
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By default, mesh LOD generation happens automatically for imported 3D scenes
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(glTF, .blend, Collada, FBX). Once LOD meshes are generated, they will
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automatically be used when rendering the scene. You don't need to configure
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anything manually.
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However, mesh LOD generation does **not** automatically happen for imported 3D
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meshes (OBJ). This is because OBJ files are not imported as full 3D scenes by
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default, but only as individual mesh resources to load into a MeshInstance3D
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node (or GPUParticles3D, CPUParticles3D, ...).
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To make an OBJ file have mesh LOD generated for it, select it in the FileSystem
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dock, go to the Import dock, change its **Import As** option to **Scene** then
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click **Reimport**:
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.. figure:: img/mesh_lod_obj_import.png
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:align: center
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:alt: Changing the import type on an OBJ file in the Import dock
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Changing the import type on an OBJ file in the Import dock
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This will require restarting the editor after clicking **Reimport**.
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.. note::
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The mesh LOD generation process is not perfect, and may occasionally
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introduce rendering issues (especially in skinned meshes). Mesh LOD
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generation can also take a while on complex meshes.
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If mesh LOD causes a specific mesh to look broken, you can disable LOD
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generation for it in the Import dock. This will also speed up resource
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importing. This can be done globally in the 3D scene's import options, or on
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a per-mesh basis using the Advanced Import Settings dialog.
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See :ref:`Importing 3D scenes <doc_importing_3d_scenes_using_the_import_dock>`
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for more information.
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Comparing mesh LOD visuals and performance
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------------------------------------------
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To disable mesh LOD in the editor for comparison purposes, use the
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**Disable Mesh LOD** advanced debug draw mode. This can be done using the menu
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in the top-left corner of the 3D viewport (labeled **Perspective** or
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**Orthogonal** depending on camera mode):
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.. figure:: img/mesh_lod_disable_lod.png
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:align: center
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:alt: Disabling mesh LOD in the 3D viewport's top-left menu
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Disabling mesh LOD in the 3D viewport's top-left menu
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Enable **View Frame Time** in the same menu to view FPS in the top-right corner.
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Also enable **View Information** in the same menu to view the number of primitives
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(vertices + indices) rendered in the bottom-right corner.
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If mesh LOD is working correctly in your scene and your camera is far away
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enough from the mesh, you should notice the number of drawn primitives
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decreasing and FPS increasing when mesh LOD is left enabled (unless you are
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CPU-bottlenecked).
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To see mesh LOD decimation in action, change the debug draw mode to
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**Display Wireframe** in the menu specified above, then adjust the
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**Rendering > Mesh LOD > LOD Change > Threshold Pixels** project setting.
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Configuring mesh LOD performance and quality
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--------------------------------------------
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You can adjust how aggressive mesh LOD transitions should be in the root viewport
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by changing the **Rendering > Mesh LOD > LOD Change > Threshold Pixels** project
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setting. To change this value at run-time, set ``mesh_lod_threshold`` on the
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root viewport as follows:
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::
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get_tree().root.mesh_lod_threshold = 4.0
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Each viewport has its own ``mesh_lod_threshold`` property, which can be set
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independently from other viewports.
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The default mesh LOD threshold of 1 pixel is tuned to look *perceptually*
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lossless; it provides a significant performance gain with an unnoticeable loss
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in quality. Higher values will make LOD transitions happen sooner when the
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camera moves away, resulting in higher performance but lower quality.
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If you need to perform per-object adjustments to mesh LOD, you can adjust how
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aggressive LOD transitions should be by adjusting the **LOD Bias** property on
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any node that inherits from GeometryInstance3D. Values *above* ``1.0`` will make
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LOD transitions happen later than usual (resulting in higher quality but lower
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performance). Values *below* ``1.0`` will make LOD transitions happen sooner than
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usual (resulting in lower quality but higher performance).
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Additionally, ReflectionProbe nodes have their own **Mesh LOD Threshold** property
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that can be adjusted to improve rendering performance when the reflection probe
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updates. This is especially important for ReflectionProbes that use the **Always**
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update mode.
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.. note::
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When rendering the scene, mesh LOD selection uses a screen-space metric.
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This means it automatically takes camera field of view and viewport
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resolution into account. Higher camera FOV and lower viewport resolutions
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will make LOD selection more aggressive; the engine will display heavily
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decimated models earlier when the camera moves away.
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As a result, unlike :ref:`doc_visibility_ranges`, you don't need to do
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anything specific in your project to take camera FOV and viewport resolution
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into account.
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Using mesh LOD with MultiMesh and particles
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-------------------------------------------
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For LOD selection, the point of the node's :abbr:`AABB (Axis-Aligned Bounding Box)`
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that is the closest to the camera is used as a basis. This applies to any kind
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of mesh LOD (including for individual MeshInstance3D)s, but this has some implications
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for nodes that display multiple meshes at once, such as MultiMeshInstance3D,
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GPUParticles3D and GPUParticles3D. Most importantly, this means that all
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instances will be drawn with the same LOD level at a given time.
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If you are noticing incorrect LOD selection with GPUParticles3D, make sure
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the node's visibility AABB is configured by selecting the GPUParticles3D
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node and using **GPUParticles3D > Generate AABB** at the top of the 3D
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viewport.
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If you have instances in a MultiMesh that are far away from each other, they
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should be placed in a separate MultiMeshInstance3D node. Doing so will also
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improve rendering performance, as frustum and occlusion culling will be able to
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cull individual nodes (while they can't cull individual instances in a
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MultiMesh).
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