Allow using a custom Viewport as a basis for Movie Maker mode

[Bethleem]

Describe the project you are working on

Video capture of cinematics.

Describe the problem or limitation you are having in your project

The max screen resolution is lower than the optimal target resolution.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

In 3.5 branch I made an animation frame capturer that, in order to avoide the screen resolution limitations, rendered the contet into a customizable viewport anf then capture this viewport data and save it as png files.
I tried it in godot 4 and just had to make some adjustments for getting the viewport data and save it to disk but it still works. So, my suggestion is, can you made movie-writer to render into a customizable viewport than can be sized bigger than the screen resolution? I mean, I can do it manually frame by frame so it would be cool to have it implemented in the movie-writer.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Movie-writer will create a size customizable viewport and the render the scene in it, so screen resolution limitations will be avoided.
For the frame capture it will use get_texture() in the SubViewport, then convert into Image with get_image() and saved to disk with save_png(), or passed to the video muxer.

If this enhancement will not be used often, can it be worked around with a few lines of script?

Yes, it works easily with png format, but will be cool to have it implemented to work with video in movie-writer.

Is there a reason why this should be core and not an add-on in the asset library?

Well, if movie-writer is core, then this should be core because is an enhacement.

[Calinou]

Related to #5790.

Note that you can already resize the window to be larger than the screen, but this requires third-party tools that may not be readily available on Windows or macOS.

Please double-check first if using the viewport stretch mode and setting the viewport_width and viewport_height accordingly in the Project Settings does the trick 🙂

If that doesn't work, then a project setting could be added to allow you to specify a custom node path to use as the root viewport: https://github.com/godotengine/godot/blob/d93bec61c3e8f5e047f12ae3ce6a68fbc7a37273/main/main.cpp#L3239-L3244

[Calinou]

It turns out this is already feasible; it's just not documented. I've opened a pull request to document this: godotengine/godot-docs#6398

Closing, as the feature is already implemented so to speak.

Testing project: test_movie_maker_viewport.zip