Depth Buffer Plugin

A plugin for Godot 4.3+ to make viewport depth buffers usable by other viewports or shaders. This enables effects that require the depth buffer from a separate viewport, or can be used in some cases where a stencil buffer would normally be used (a silhouette effect for example).

How it works

The plugin works by adding custom type of SubViewport called DepthBufferViewport. Since the depth buffer of a SubViewport can't be accessed by another Viewport currently in Godot, the depth buffer of DepthBufferViewports are encoded to a color image which can be accessed by other Viewports through the ViewportTexture. This encoded texture can then be decoded by a shader or CompositorEffect to get the original depth buffer back. The encoding works by converting the 32bit float depth buffer to 4x 8bit unsigned integers by direct bit copying and assinging those uints to the color channels of the viewport output.

How to use it

The plugin adds an autoload node called DepthBufferManager which can be used to add DepthBufferViewports from code. To create one, call

var dbv = DepthBufferManager.create_depth_buffer_viewport(cull_mask, viewport_target)

which will create one with the specified camera cull mask and target viewport. For ease of use, DepthBufferViewports have a target_viewport property which will make the camera of the DepthBufferViewport track the camera of the target_viewport. The DepthBufferViewport will not be ready for use immediately since the engine needs to render a frame before the viewport texture is valid. After creating the viewport, you can use

if not dbv.is_viewport_ready():
    await dbv.viewport_ready

to wait until the ViewportTexture is ready, for example to pass it to a shader. To decode the encoded depth buffer, you can add #include "addons/depth_buffer/depth_decode.gdshaderinc" to your fullscreen shader and call decode_depth(), or use the included silhouette CompositorEffect as an example to make your own CompositorEffect.

The plugin comes with a premade silhouette effect that can be used with the new CompositorEffect system in Godot 4.3. To use it, add a Compositor to either the scene's Environment or the Camera3D and add a PostProcessSilhouette effect (the effect will handle the creation of the DepthBufferViewports). The silhouette_mask is the render layer which will show up over other objects in the scene as the silhouette, and the obstruction_layer is the render layer which the silhouette will not show in front of. The effect callback type can be changed to either Pre Transparent or Post Transparent depending on if you want the silhouettes to show up behind or in front of transparent objects/materials.

Any mesh that you want to be shown as a silhouette needs to have its VisualInstance3D.layer set to include the silhouette_mask, and any mesh that you don't want the silhouette to be shown over should have its layer set to include obstruction_mask. You may need to set the node to allow children to be edited (right click > editable children) if you have imported a mesh into the editor and can't change its render layer. In the example image below, the enemies are on render layers 1 & 2 and the player is on render layers 1 & 3, with silhouette_mask set to layer 2 and obstruction_mask set to layer 3:

Limitations

• This plugin currently only works with the Forward+ renderer. It should also work with the Mobile renderer but this bug prevents the viewport's color texture from being written to in a compute shader.
• This requires the new CompositorEffects added in Godot 4.3 since the alpha channel of a viewport's texture can't be overwritten in a regular Godot shader.
• While I've tried my best to make this plugin performant, it still requires rendering extra viewports which can be expensive. If only a few objects are being re-rendered (as is the case with the silhouette) the impact is not too much, but still more than a native stencil buffer approach. When/if stencil buffer access is allowed in Godot, this plugin would probably be unneccessary.
• Antialiasing: The silhouette effect does not work well with AMD FSR 2.2 or TAA and does not work at all with MSAA, but does work with FXAA. (Could maybe be fixed, I just don't know how TAA or MSAA work lol)