`Camera2D` disrupts low processor mode

[Kiisu-Master]

Tested versions

Godot 4.2, 4.3

System information

Windows 10 - AMD Radeon R7 Graphics, OpenGL3 angle and Vulkan

Issue description

The Camera2D disrupts low processor mode in GUI projects like GodSVG or Pixelorama, which use (or used to use) it for dynamic viewports. It causes high GPU usage on idle.

Note: this was not reproducible by other people on Linux or other hardware for some reason. It could also be that my hardware is so underpowered that it's just more noticeable on it.

Steps to reproduce

Add a Camera2D to a project that uses low processor mode and observe the GPU usage staying high on idle.

Minimal reproduction project (MRP)

Temporary solution

The Camera2D can be replaced with a custom implementation using the Viewport canvas_transform property.

[MewPurPur]

Can you reproduce it in a new project where you simply change that setting and add a camera?

It also might be something to do with the specific way our projects do cameras, like with them being a single blue 2D node among countless green ones.

[Kiisu-Master]

Can you reproduce it in a new project where you simply change that setting and add a camera?

Yes I can.
In an empty project the GPU usage stays quite low though, around 10%. That's why I didn't post an MRP.

I was also able to reproduce in Pixelorama, It would stay at 20% - 40% GPU usage, around the same as in GodSVG.

[OverloadedOrama]

Note: this was not reproducible by other people on Linux or other hardware for some reason. It could also be that my hardware is so underpowered that it's just more noticeable on it.

I was mistaken, I can indeed reproduce the issue on Linux (Godot v4.2.1.stable - openSUSE Tumbleweed 20240314 - X11 - GLES3 (Compatibility) - NVIDIA GeForce GTX 1060 6GB (nvidia) - AMD Ryzen 5 1600 Six-Core Processor (12 Threads))

On a MRP with low processor mode enabled, when a Camera2D is disabled, idle GPU usage stays at 0%, while when it is enabled, it jumps from 3-5% on average. An MRP can be found here:
Camera2DGPU.zip

On Pixelorama, when Camera2Ds are disabled, idle GPU usage is 0%, while with Camera2Ds enabled, it jumps from 5-20%.

Taking a look at Camera2D's source code, it seems that the _update_scroll function which is responsible for updating `Viewport.canvas_transform is being called on internal process/physics_process. This probably explains why there is GPU usage even when the camera's properties do not change.

What's weird is that Godot 3's Camera2D seems to be working the same way (at least according to my limited knowledge of Godot's source code), but GPU usage on idle is 0% on the MRP. It could be that It could be that Godot 4.x's Viewport.canvas_transform updating is more taxing on the GPU than Godot 3.x. Or it could be that Godot 3.x properly respects low processor mode.

EDIT: Taking a better look at Godot 3's Camera2D code, it seems that process/physics_process is only enabled when smoothing is enabled, while in Godot 4, either process or physics process is always set to true. Indeed, when I test the MRP in Godot 3 with smoothing enabled, GPU usage is at 2%, while when it's disabled it's at 0%. I wonder if Godot 4's Camera2D could be optimized the same way?

EDIT 2: I took a look at Camera2D's history and came across #46717 which implements the optimization I mentioned in my previous paragraph. It was targeted for 3.2 but it seems like it never got implemented in 4.x. I haven't done any testing, but if my hypothesis is correct, merging a PR with similar changes for 4.x could fix this issue, at least when camera smoothing is disabled.