Render to texture functionality on demand

[gtsiam]

I am trying to implement a ping pong texture (a texture that is sent to the gpu, goes through a shader, and again, and again, and so on).

I know it is possible to do by setting up multiple Viewports and ViewportTextures, but it is a bit wasteful. if you want to do a few of those per frame.

So maybe it would be a good idea to have viewports that can render on demand. Compute shaders would be nice, but I assume it is a bit too much to implement.

I am willing to work on this myself, but I would like a few pointers, since I am not too sure where the rendering happens in the engine (or to know if there is an obvious alternative method I missed).

EDIT: Actually, I just remembered this post, so I can probably do without the pointers...

[Zylann]

I kind of miss this, everytime I have to use shaders for texture generation. It is very inconvenient to setup a bunch of viewports and have them working in the correct order to execute a series of steps, sometimes with multiple iterations, without knowing for sure when the final frame is ready (each _process, I assume?), and even worse if you need to have that output fed into further passes.

My current use case is terrain generation: I want to support erosion in my terrain generator, but this requires to run a slope-dependent blur shader a lot of times on the same image, and the more I tweak my generator, the more I think I'll need a node-based system which however feels very cumbersome to setup with the current viewports, because unlike the recently added visual shader editor which only has to generate one shader to one output, I would need to plug outputs of different shaders to the inputs of others...
So for now I try to get along with a single viewport or two, waiting dozens of frames for everything to complete in order, which causes pseudo-laggy edition behavior, and quickly becomes unusable.

About rendering on demand, I found there is VisualServer.draw(), which was used for onion-skinning so the editor is able to draw the entire scene again with some alpha (the entire editor is also redrawn btw). So, in order to have render on demand, it would mean to have this feature per viewport, instead of being global.

This could benefit to any baking tool as well which needs to run iterations on textures.

[Calinou]

So maybe it would be a good idea to have viewports that can render on demand. Compute shaders would be nice, but I assume it is a bit too much to implement.

Godot 4.0 makes use of compute shaders and will likely expose a compute shader API of some sort.

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