Node3D needs an immediate draw mode

[blackears]

At the moment, if you want to draw 3D graphics to screen, your only optin is to create a mesh, add it to a MeshInstance3D and then add it to the scene tree. Sometimes it makes more sense for the node to draw its own objects rather than having to defer to a MeshInstance3D. In my case, I'd like to draw tool helpers in an editor viewport, but I can also see this being useful if one wants to create a painting application and needs to draw brush strokes immediately and not at some deferred time.

I'd propose adding a _draw() call to Node3D that acts similar to the _draw() call on a CanvasItem. This method would be called very time the viewport is rendered. The Node3D would also provide a method that can be called from within the _draw() call which accepts a mesh object and a shader. This would then draw the given mesh with the indicated shader when the viewport completes its next render pass. This would only appear in the next pass and you'd need to supply it again in the next _draw() call if you wanted to have it draw in the next refresh.

[Calinou]

• Related to Add singletons for 2D and 3D debug drawing #5196.

I believe there is a technical reason why Node3D (or rather, VisualInstance3D/GeometryInstance3D) doesn't have a _draw() function like CanvasItem. It might be due to optimization reasons, such as instancing support.

[blackears]

Perhaps. I don't know much about how this has been implemented in the engine. But it would be nice to have a more flexible rendering pipeline. After working with OpenGL, using Godot feels fairly limiting and you're always needing to create elaborate work arounds if you need to do anything other than draw meshes with Godot's lighting system.

[theraot]

This reminds me of ImmediateGeometry Godot 3, which was replaced with ImmediateMesh in Godot 4. In fact, for debug I've been 3D adding a MeshInstance3D with a ImmediateMesh with some code to add geometry to it, and a custom material override created in runtime... Only if get_tree().debug_collisions_hint is true.

I'm guessing that having it only draw/update when it should visible would be quite a challenge.

[blackears]

That's not quite the same thing. Godot 3's ImmediateGeometry is a node that lets you build your geometry by issuing OpenGL 1 like commands - but it's still just another type of mesh node. I'm talking more about accessing the rendering pipeline directly. You've already built your mesh and you want say "draw this mesh with this shader on the next pass and only the next pass'.

[mieldepoche]

I wish 3d debug drawing was integrated in the engine, but for now there's a pretty good addon for it: https://github.com/DmitriySalnikov/godot_debug_draw_3d