Shadow Volumes

1. Introduction

Shadow volumes are a method of rendering dynamic shadows. "Dynamic" means that everything (light source, shadow casters, shadow receivers) can move and change each frame and the shadows will reflect that.

To activate them in Castle Game Engine, just set the light’s Shadows property to true. You can do this in the editor, and observe the effects immediately.

Shadow volumes in editor Shadow volumes example

Important limitations:

  • The models that cast shadows must be 2-manifold. This means that every edge has exactly 2 (not more, not less) neighbor faces, so the whole shape is a closed volume.

  • Right now only one light source can cast shadows using shadow volumes. Do not set Shadows to true on multiple lights.

2. Examples

Check out these CGE examples:

3. Features

Featues of shadows by shadow volumes (in particular, how does this technique compare to shadow maps):

  • Shadow volumes produce hard shadows. That’s both an advantage (they are as sharp as your geometry, no problems with texture resolution like in shadow maps) and disadvantage (when simulating large area lights, hard shadows may look unrealistic).

  • Shadow volumes are easier to activate. To see the shadows, just choose one light in the scene (probably the brightest one) and set Shadows to true.

    That’s it. Compared to shadow maps, you don’t need to tweak anything to deal with shadow maps resolution, bias etc.

  • Shadow volumes require the shadow casters to be 2-manifold. So you need to be more careful when modeling. Shadow volumes usually don’t work if you try to activate them on a random 3D scene. More about this below.

  • It’s difficult to compare the speed of "shadow volumes" vs "shadow maps". Both techniques are expensive in different ways. On one hand, shadow volumes require extra rendering passes (additional rendering of "shadow quads" to the stencil buffer, and then additional render to color buffer, for each shadow-casting light). On the other hand, shadow maps require updating the shadow textures (one for each light source).

  • Our current shadow volumes implementation allows for only one light casting shadow volumes. (This may be improved some day. Give us a shout at the forum if needed and support the engine development to make this happen sooner.)

    Note that shadow volumes will require more and more rendering passes when multiple light sources may cast shadows.

    In contrast, shadow maps support as many lights as you want already, and each light only requires to keep yet another shadow texture up-to-date.

  • Note that it’s perfectly fine to use both shadow volumes and shadow maps in a single scene.

  • Applying a texture with transparency (alpha testing) on the object has no effect on the shadows cast.

    Leaf texture with transparent parts

    E.g. if you have a leaf designed as a simple quad, with a leaf texture that has transparent parts — the shadows cast by shadow volumes will look like cast by quads, they will not reflect the leaf shape. Similarly if you make a fence as a plane with a texture.

    In general, shadow volumes cannot account for the object visual appearance like a texture. They only account for the object topology (polygons, edges, vertices).

    In contrast, shadow maps interact with objects using texture with alpha-testing nicely (fully transparent places will not cast shadows).

    Note that shadow casters may use partial transparency (by alpha testing or alpha blending). They can use material with non-zero transparency, they can use textures with alpha channel. These shapes render OK, they just cast shadows just as if they were completely opaque.

4. Shadow casters 3D geometry must be 2-manifold (closed volume)

By default, for shadow volumes, all shapes that cast shadows must be 2-manifold. This means that every edge has exactly 2 (not more, not less) neighbor faces, so the whole shape is a closed volume. Also, faces must be oriented consistently (e.g. CCW outside). This requirement is often quite naturally satisfied for natural objects.

Note that satisfying the above requirements (2-manifold, consistent ordering) means that you can also use backface culling which improves rendering performance. You typically turn on backface culling in your 3D authoring software (e.g. check it in Blender material settings) which will export it to the glTF or X3D file. Our engine will automatically follow this information.

You can inspect whether your shapes are detected as a 2-manifold by Castle Model Viewer: see menu item "Help → Manifold Edges Information". To check which edges are actually detected as "border edges" use "View → Fill mode → Silhouette and Border Edges", manifold silhouette edges are displayed yellow and border edges (you want to get rid of them!) are blue.

You can also check manifold edges in Blender: you can easily detect why the mesh is not manifold by "Select → Select All By Trait → Non Manifold" command (press F3 to find this command in edit mode). Also, remember that faces must be ordered consistently CCW — in some cases "Recalculate normals outside" (this actually changes vertex order in Blender) may be needed to reorder them properly.

Select Non Manifold in Blender

Note that each shape must be 2-manifold (by default, when TCastleRenderOptions.WholeSceneManifold is false). It’s not enough (it’s also not necessary) for the whole scene (TCastleScene) to be 2-manifold.

The shape is a smallest unbreakable unit that we pass to GPU when rendering. It corresponds exactly to glTF primitive and X3D TShapeNode. When working with Blender, each Blender object corresponds to as many shapes as you use materials in that Blender object (so, usually just one).

Putting the requirement to be 2-manifold on shape, not on scene, has advantages and disadvantages:

  • Advantage: We prepare and render shadow volumes per-shape, so we work efficiently with dynamic models. Transforming a shape (move, rotate…​), or changing the active shapes has zero cost, no data needs to be recalculated.

  • Advantage: We can avoid rendering per-shape. We can reject shadow volume rendering for shape if we know shape’s shadow will never be visible from the current camera view.

  • Advantage: Not the whole scene needs to be 2-manifold. If a shape is 2-manifold, it casts shadow. If your scene has both 2-manifold and non-2-manifold shapes, it will work OK, just only a subset of shapes (the 2-manifold ones) will cast shadows.

  • Disadvantage: The whole shape must be 2-manifold. You cannot create 2-manifold scenes by summing multiple non-2-manifold shapes.

4.1. Optional: Treat whole scene as 2-manifold

If you set MyScene.RenderOptions.WholeSceneManifold to true then the whole TCastleScene must be 2-manifold. In turn, we don’t require in this case that each shape is 2-manifold separately.

This is useful when your model is composed from multiple shapes that together are 2-manifold. For example, if your mesh in Blender is 2-manifold but it uses multiple materials. In such case, exporting it to glTF or X3D splits each mesh into multiple shapes. Each shape is not 2-manifold but whole scene is.

Use TCastleRenderOptions.WholeSceneManifold to cast shadows from such models. See the Alpaca model from examples/viewport_and_scenes/shadow_volumes_whole_scene_manifold for an example.

WholeSceneManifold example in editor

Note that when MyScene.RenderOptions.WholeSceneManifold is true, right now we assume that your whole scene is 2-manifold. We do not check it! You have to make sure it is true (e.g. check in Blender using "Select → Select All By Trait → Non Manifold" mentioned above). If the scene is not actually 2-manifold, rendering artifacts will appear.

5. Adjust what casts shadows

By default, every shape that is 2-manifold casts shadows, and every scene with TCastleRenderOptions.WholeSceneManifold = true casts shadows.

To stop some objects from casting shadows, set TCastleTransform.CastShadows to false.

6. Advanced: Control from X3D

Fountain level model, with shadow volumes The same fountain level model, with shadow volumes. After some interactive fun with moving/rotating stuff around :)
This section is relevant only for X3D authors that directly edit X3D nodes. Most Castle Game Engine users can ignore it. Just control the shadows by toggling Shadows property.

You can use X3D nodes to design your lights and shadow volumes on them. In the simplest case, just activate shadow volumes on a light source by setting fields shadowVolumes and shadowVolumesMain both to TRUE.

6.1. Example models

Demo 3D models that use dynamic shadow volumes are inside our demo_models, see subdirectory shadow_volumes/. Open them with Castle Model Viewer and play around.

6.2. X3D fields shadowVolumes and shadowVolumesMain

To all X3D light nodes, we add two fields:

*Light {
  ... all normal *Light fields ...
  SFBool  [in,out]  shadowVolumes      FALSE
  SFBool  [in,out]  shadowVolumesMain  FALSE # meaningful only when shadowVolumes = TRUE

The idea is that shadows are actually projected from only one light source (with shadow volumes, number of light sources is limited, since more light sources mean more rendering passes; for now, I decided to use only one light). The scene lights are divided into three groups:

  • First of all, there’s one and exactly one light that makes shadows. Which means that shadows are made where this light doesn’t reach. This should usually be the dominant, most intensive light on the scene.

    This is taken as the first light node with shadowVolumesMain and shadowVolumes = TRUE. Usually you will set shadowVolumesMain to TRUE on only one light node.

  • There are other lights that don’t determine where shadows are, but they are turned off where shadows are. This seems like a nonsense from "realistic" point of view — we turn off the lights, even though they may reach given scene point ? But, in practice, it’s often needed to put many lights in this group. Otherwise, the scene could be so light, that shadows do not look "dark enough".

    All lights with shadowVolumes = TRUE are in this group. (As you see, the main light has to have shadowVolumes = TRUE also, so the main light is always turned off where the shadow is).

  • Other lights that light everything. These just work like usual X3D lights, they shine everywhere (actually, according to X3D light scope rules). Usually only the dark lights should be in this group.

    These are lights with shadowVolumes = FALSE (default).

Usually you have to experiment a little to make the shadows look good. This involves determining which light should be the main light (shadowVolumesMain = shadowVolumes = TRUE), and which lights should be just turned off inside the shadow (only shadowVolumes = TRUE). This system tries to be flexible, to allow you to make shadows look good — which usually means "dark, but not absolutely unrealistically black".

In Castle Model Viewer you can experiment with this using Edit → Lights Editor.

If no "main" light is found (shadowVolumesMain = shadowVolumes = TRUE) then shadows are turned off on this model.

Trick: note that you can set the main light to have on = FALSE. This is the way to make "fake light" — this light will determine the shadows position (it will be treated as light source when calculating shadow placement), but will actually not make the scene lighter (be sure to set for some other lights shadowVolumes = TRUE then). This is a useful trick when there is no comfortable main light on the scene, so you want to add it, but you don’t want to make the scene actually brighter.

6.3. X3D field Appearance.shadowCaster

If you edit X3D nodes, you can control what casts shadows for each particular shape using the Appearance.shadowCaster field. See Appearance.shadowCaster documentation.

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