Development Showcase: Renderer (Vulkan vs OpenGL)
Afternoon wherever you are in the world! I'm Caleb, developer of Harwell. If you're not familiar with it, it's a capitalist RTS where you build up factories to make stacks of profit to buy out the other players, and it's pretty neat if I do say so myself.
I've decided to move the game from Godot's Compatibility renderer (which uses OpenGL) to Forward+ (which uses Vulkan). It's been coming for a little while now but I finally decided to make the decision, and I thought some people might be interested in the details!
Compatibility vs Forward+
If you don't know, Godot has multiple renderers which you can choose from: Forward+, Mobile, and Compatibility. Forward+ being the modern Vulkan renderer, Mobile being slightly older but compatible with more low-power devices, and Compatibility being the OpenGL renderer which has great compatibility and performance.
Godot recommends Compatibility for any games based on mobile, or any 2D games as OpenGL has great 2D performance, while they usually recommend Vulkan for more modern features (such as certain post-processing) or for 3D games on modern devices.
Why start with Compatibility?
Harwell is fully 2D and therefore has very little overhead. Running on PC, Harwell takes up about 160MB, runs almost all effects on the CPU, and uses very little GPU processing and VRAM.
Choosing compatibility meant I could guarantee performance on all devices, and even upload a HTML version if I wanted to. Pretty neat huh?
Why the Switch?
Throughout the development process I was encountering restrictions in various areas - primarily post-processing and GPU particles. On top of this it turns out ARM support in OpenGL is extremely poor.
Performance
It's not my primary focus, but I did various testing in OpenGL and in Vulkan in order to work out what performs best. I was testing this on a device with a fairly decent GPU (6700XT), but even then I was getting around 180FPS in OpenGL compared to 200-300 FPS in Vulkan. I'm aware AMD has historically performed better in Vulkan so these results may not carry over, but I also know that Godot have put more development time in their Forward+ (Vulkan) renderer than they have the Compatbility renderer, so there was always going to be some improvement there.
Particles
I also noticed a peculiarity whilst using OpenGL - GPU particles were considerably heavier on performance than CPU particles were. The game uses particle effects in many places - It's used to create the fog, the Mars dust, the icons that 'jump' out of the buildings, the smoke from buildings, etc. I tried switching just some of these over to GPU particles instead of CPU particles which should normally be more performant, but noticed that I lost FPS. I went from around the 180FPS mark to around 120FPS. Considerable. Now, that may just be my implementation, and it was a rushed one because it was just for testing, but this didn't seem to make much sense to me.
ARM Support
I have a Snapdragon laptop, and ARM devices are becoming more common. I want the game to perform well on all devices, but even though OpenGL boasts compatbility, I noticed extremely poor performance on my ARM-based laptop.
Performance was never going to be great, it's only using an iGPU as it's only for very light gaming, but even then when I tested Vulkan on my device the difference was measurable. Using OpenGL, I was getting around 15FPS. After switching to Vulkan, this went up to 40FPS.
After looking into it, ARM support is extremely poor in OpenGL due to the architecture, especially using Godot's Compatibility renderer which may not be using the latest versions of OpenGL with all the latest and greatest features.
Benefits of Forward+
Switching to the Forward+ renderer didn't just bring performance improvements - it brought features.
Many of Godot's post-processing effects are restricted to the Forward+ renderer, including SSR, SSAO, SDFGI, etc. To be honest, most of these are pretty useless in a 2D game and won't be used anyway, but the fact that they are there if needed is very useful.
One feature that is useful in 2D is HDR, and the compatibility renderer doesn't fully support it. Support for HDR may not be fully, properly implemented in the Forward+ renderer either, but it will be the first to get any updates from the Godot team.
That's another benefit - updates. The Forward+ renderer is the focus of the Godot team and with it being an open source engine development time is in short supply. I want Harwell to get the most out of the engine in all areas, and I think the Forward+ renderer will help.
Development
So, how much work did I need to do to implement this? To be honest, really not that much! I was worried some of the code may break due to renderer-specific features, but in actuality I saw almost no issues at all.
The only work I had to do was adjust the WorldEnvironment node, which manages the post-processing for the game. The switch caused the post-processing settings to look completely different - contrast was severely boosted, saturation was overcooked, and things just needed dialing back. I had to go through and re-adjust the post-processing effects and check the particle systems to make sure they looked right with the new settings. I couldn't get it to match, but I think the game looks improved since I redid it anyway, which is good.
I've been busy working on development for Harwell and haven't had much time to write these development diaries - but I shall try to keep up with all of the changes!
(Part of it is the fact I write these on my lunch at work as I have a full-time job, and I've been off work rewriting large parts of Harwell for the past two weeks!).
There's some really cool things I'd love to talk about (implementing AI companies for example) so keep your eyes peeled for future ones.
Have a lovely week,
Caleb