How/where to hook into godot's renderer to dump framebuffers to CPU?

Question

I'd like to use Godot to render visualizations to be overlayed on a video stream in real-time, and I need a way to get the frames out of godot with the alpha channel intact.

However, I'm not sure about the where to to go to pull image data off the GPU. I don't mind writing a module if need be, but I don't know how I can hook in to the render pipeline to call glReadPixels or something.

Any suggestions? Are there better ways to go about this that I'm missing?

Thanks!

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Ideally I'd like to be able to do the rendering off-screen as well.

Answer 1

I know you can download the last drawn frame from a Viewport to main memory by using Viewport.get_texture().get_data(). This will return an Image object containing the pixel data.
Note that this is a very expensive operation, it can lag a bit if the resolution is too big.

Each Godot game runs under a main viewport representing the screen, which you can get with get_viewport().
You can render offscreen by putting your scene under a custom viewport, and making it render always (by default I think it won't render if not being made visible by a viewpot container node).

As for hooking up in the renderer itself, I don't know. Maybe there is another way involving modification of rasterizer_gles3.cpp?
If the destination of the frame is to be rendered using the same graphics card, I imagine there should be a better way than having to download the screen from it 60 times per second.