I submitted a game to Bevy Jam 4 and it was pretty cool

2023 Dec 16 8 minute read

I submitted an entry to the 4th #bevy jam this December, with the theme of " That's a LOT of Entities!". As a big fan of the Marble Blast Ultra game on the 360 arcade, I though I might try a stab at something similar.

Bevy Blast Ultra

With subtle acknowledgement of its inspiration, Bevy Blast Ultra brings several minutes of non-stop MVP marble action to any browser Windows, Mac, or Linux machine.

How's Bevy 0.12?

There's a couple Epic Rust Moments and issues I ran into while creating this game. Bevy has grown tremendously since I last wrote about it in version 0.6, and this is an effort to document what I found of note.

Bevy editor, pls

While Bevy is a capable engine, its lack of an editor means procedural generation is much more achievable in comparison to handcrafted levels. A game like MBU, however, requires intentionally designed levels (or a wizardly knowledge of procedural generation).

Marble Blast Ultra screenshot

A few 3rd party editors for Bevy exist at the moment, but are understandably limited in comparison to established engines like Unreal.

Fortunately, Bevy is able to import GLTFs, and a workflow for exporting to Bevy is already available. Using a single .blend file, I could edit all levels as different scenes, and create reusable prefabs. Most importantly, I could create any kind of level geometry that Blender could export.

Early development footage

Assets are still kind of tricky, but not as much

Assets used to be among the more tedious game logic to implement. With the advent of helpful crates like bevy_asset_loader, this isn't much of an issue.

Before I decided on going the Blender route, I considered writing a bunch of .ron scene files by hand. I would have liked to specify a bunch of prefab .ron scenes, and use them within level .ron scenes. Apparently, that still isn't a possibility just yet, but I hear an overhaul of the way scenes are handled in Bevy is under way. I imagine this might rely on some of the new asset v2 preprocessing features released in 0.12.

Annoyingly, it's still impossible to load folders with WASM (which isn't a Bevy issue, but still a bummer when you have to hard-code filenames in your project for web support).

Z will always be up to me

ok seriously though. "Up" is (0.0,0.0,1.0), and I'm Tired of People trying to Tell Me Otherwise! It is not (0.0,1.0,0.0). I will die on this very tall Z-aligned hill.

This brings me to The worst moment of the Bevy Game Jam 2023: figuring out skyboxes.

I spent 6 very misplaced hours right before the due date attempting to figure out how to convince my skybox image that Z is up.

This was, of course, completely unnecessary compared to fixing core issues with the camera logic, but I persisted and brute-forced my way to figuring it out.

The wretched skybox journey

Bevy provides a few files in the example assets for showing a skybox image, but it has some minor issues - mostly the fact that it is a little bit rotated.

Game screenshot showing a skybox that is rotated 90 degrees counter-clockwise

But that shouldn't be an issue, right? With the right editing, I could certainly rotate it back in the right direction. But the files Bevy provides are ktx2 images. Most tools to convert to and from this format are CLI based (or need compilation) which I wanted to avoid. NVIDIA does provide a GUI application for viewing and exporting many of these filetypes, but it claimed my precious ktx2 files were corrupt.

Settings view of NVIDIA Texture Tools Exporter

This application has many, many settings, and I know what approximately 0 of them actually do.

Thankfully, the Bevy repo also provide a .png skybox example, I just needed to map portions on the image in the correct sequence and rotation to match, which I figured out slowly and arduously by changing the image until everything properly tiled together.

A cubemap from Bevy's examples

I'm sure this is a solved problem, and there's a document somewhere specifying where to put which bit of image, but I instead spent hours brute forcing this. But hey, it eventually worked!

I skipped over a lot with this story. It also involved forking a cubemap generator javascript project so I could up the photo resolution, and lots of aggravated scribbling in Microsoft paint

I do not know how to compile a C project

One alternative route was to use Patrick Walton's automatic tool, but this requires a beginner level of C knowledge and some patience. After scouring the internet for the right header files to download, and reading source code to figure out where to place them, I gave up. But it looks like a great application for this exact purpose, if you're able to get it running.

The world's most difficult to remember acronym

Since I last used Bevy, its added another physics engine to the mainstream - bevy_xpbd. The xpbd part stands for Extended Position Based Dynamics. Without a bookmark, prepare to enter every permutation of xpbd's letters in your search engine.

XPBD to my understanding is a newer (but less battle-tested) way to calculate physics. The YouTube channel Two Minute Papers does a pretty good job summarizing the differences between XPBD and more traditional methods. One interesting feature of XPBD is that it can simulate situations where more traditional methods would fail, such as marbles on a track, which I incorporated into my game.

Marble on rails in game

I've used Rapier almost exclusively in the past, and while it's matured nicely, it's still a library intended for general use. That means it needs the glue of bevy_rapier to translate into our game world, and the Rapier and Bevy world don't always play together without issues. bevy_xpbd on the other hand, is designed specifically for use in Bevy, which (in my opinion) makes its resulting API far friendlier and intuitive to use. I didn't encounter any bugs or strange behavior and will use it as my physics engine of choice moving forwards.

I only wish it had some built in forces (outside of gravity) like wind or magnetism - but those are easy enough to implement yourself.

Graphics are too good

As I alluded to earlier in this post, Bevy Blast Ultra is not playable in-browser, because it runs like rotten garbage, clocking in a spectacular ~0.5FPS despite my desperate fumbling with wasm-opt. After playing a few other submissions, I suspect the issue is the shadow graphics, which were recently improved and look great, but might cost too much for a browser game. But hey, it looks really nice!

Adjusting game settings

I used a big .ron file to store many game settings. Whenever there was some scalar that affected gameplay, I tried to include it here, so I could tweak values without needing to recompile. (Also, since there's no settings UI, this is at least an option for changing sensitivity).

    game: (
        player: (
            // More torque = ball goes faster
            torque_strength: 4.8,
            // Impulse applied to the Z AXIS
            jump_strength: 6.0,
            friction: 0.9,
            // XPDBDPX calculates the mass of our collider, but /
            // by adjusting the density we can fine-tune things
            density: 1.0,
            // Because theres multiple marbles, and we can't 
            // just have one "main" player, the focal point is 
            // actually the average of player positions. This 
            // applies a force to marbles to keep them towards 
            // the focus point. To keep players near the center 
            // from vibrating, a "deadzone" is applied
            magnetism_deadzone: 0.5,
            // How fast the pull towards the average center
            magnetism_strength: 1.0,
            // How far the magnetic influence reaches
            influence_radius: 5.0,
        camera: (
            controller_sensitivity: (3.0, 1.5),
            mouse_sensitivity: (5.0, 5.0),
            fov: 90,
            // lol this doesn't actually do anything anymore
            fov_multiplier_range: RangeInclusive( start: 1.2, end: 0.9 ),
            // Not sure why it has to be between these two 
            // numbers, but it does work!
            angle_range: RangeInclusive( start: 200, end: 270 ),
            // How far away to position the camera from the 
            // player center
            distance: 7.0,
    // etc...

I am a big fan of Ranges in Rust. Am I the only one?

The best Bevy

Bevy is (objectively) a good game engine because Rust is (objectively) a good programming language. The best parts of Bevy are when it leverages the best parts of Rust, most notably in the incredibly satisfying generic Query API. I find the more trying tasks within Bevy consist of working with dynamic data (like asset loading). Still, I've found Bevy to be among the more enjoyable projects to hack on in Rust, contrast to my web escapades in Tauri which left me feeling bitter and empty.