SubViewport + ViewportTexture doesn't work when created programmatically

[bluenote10]

Godot version

v4.0.stable.official [92bee43]

System information

Ubuntu 22.04, GeForce GTX 980

Issue description

Creating a SubViewport + ViewportTexture programmatically doesn't seem to work. The content of the viewport is simply not visible, and there are debugger errors:

The message Viewport Texture must be set to use it on first glance suggests that this is another manifestation of #66247, but in this case it is not a false error: The issue in #66247 seems to go away when setting the update mode to "always". That's not the case with the issue I'm facing. Also, creating exactly the same scene tree statically in the editor works, which indicates that the issue has rather something to do with creating the SubViewport + ViewportTexture programmatically.

Steps to reproduce

Simply use a single Node as the scene root with this script attached:

extends Node


func _ready():
    # Create the viewport, add something to it, and add it to the scene tree:
    # Note that setting UPDATE_ALWAYS doesn't help.
    var viewport = SubViewport.new()
    viewport.render_target_update_mode = SubViewport.UPDATE_ALWAYS
    add_something_to_viewport(viewport)
    add_child(viewport)
    
    # Now that the viewport has a path, we can create a texture and connect it:
    var viewport_texture = ViewportTexture.new()
    viewport_texture.viewport_path = viewport.get_path()

    # Finally create a sprite that uses the viewport texture:
    var sprite = Sprite2D.new()
    sprite.centered = false
    sprite.texture = viewport_texture # <== this line causes debugger errors
    add_child(sprite)


func add_something_to_viewport(viewport: SubViewport):
    var label = Label.new()
    label.text = "Hello world"
    viewport.add_child(label)

The example builds this scene tree dynamically:

For comparison I have created another minimal project, where I'm creating exactly the same scene tree statically in the editor:

In this case the SubViewport + ViewportTexture works, i.e., the contents of viewport are visible.

Minimal reproduction project

Full minimal reproduction:

• ProgrammaticViewport.zip
• Code on GitHub

[Rindbee]

In this case, it is recommended to obtain ViewportTexture directly from Viewport.

# var viewport_texture = ViewportTexture.new()
# viewport_texture.viewport_path = viewport.get_path()
var viewport_texture = viewport.get_texture()

The way of directly specifying viewport_path is just for convenience in the editor.

[smix8]

When you create ViewportTextures in script you creating an unlinked / empty texture as rendertarget.
Setting the path on the ViewportTexture does nothing expect setting the path variable on the ViewportTexture.

You need to use the texture_proxy_create() and texture_proxy_update() functions of the RenderingServer to link rendertargets together. If you use Viewport.get_texture() this is done for you in the background. The returned ViewportTexture is just a wrapper for a texture that was proxy linked in the background. You can link any texture with the same format as your Viewport texture together using the RenderingServer API, it does not have to be a ViewportTexture at all.

[bluenote10]

@Rindbee @smix8 Thanks for the clarification! Indeed, using Viewport.get_texture() works just fine. I took the opportunity to simply document this externally on StackOverflow, which is maybe already sufficient in terms of documenting the behavior?