274 lines
11 KiB
Markdown
274 lines
11 KiB
Markdown
---
|
|
title: "Gamebridge: Fitting Square Pegs into Round Holes since 2020"
|
|
date: 2020-05-09
|
|
series: howto
|
|
tags:
|
|
- witchcraft
|
|
- sm64
|
|
- twitch
|
|
---
|
|
|
|
Recently I did a stream called [Twitch Plays Super Mario 64][tpsm64]. During
|
|
that stream I both demonstrated and hacked on a tool I'm calling
|
|
[gamebridge][gamebridge]. Gamebridge is a tool that lets you allow games to
|
|
interoperate with programs they really shouldn't be able to interoperate with.
|
|
|
|
[tpsm64]: https://www.twitch.tv/videos/615780185
|
|
[gamebridge]: https://github.com/Xe/gamebridge
|
|
|
|
Gamebridge works by aggressively hooking into a game's input logic (through a
|
|
custom controller driver) and uses a pair of [Unix fifos][ufifo] to communicate
|
|
between it and the game it is controlling. Overall the flow of data between the
|
|
two programs looks like this:
|
|
|
|
[ufifo]: http://man7.org/linux/man-pages/man7/fifo.7.html
|
|
|
|
![A diagram explaining how control/state/data flows between components of the
|
|
gamebridge stack](/static/blog/gamebridge.png)
|
|
|
|
You can view the [source code of this diagram in GraphViz dot format
|
|
here](/static/blog/gamebridge.dot).
|
|
|
|
The main magic that keeps this glued together is the use of _blocking_ I/O.
|
|
This means that the bridge input thread will be blocked _at the kernel level_
|
|
for the vblank signal to be written, and the game will also be blocked at the
|
|
kernel level for the bridge input thread to write the desired input. This
|
|
effectively uses the Linux kernel to pass around a scheduling quantum like you
|
|
would in the L4 microkernel. This design consideration also means that
|
|
gamebridge has to perform _as fast as possible as much as possible_, because it
|
|
realistically only has a few hundred microseconds at best to respond with the
|
|
input data to avoid humans noticing any stutter. As such, gamebridge is written
|
|
in Rust.
|
|
|
|
## Implementation
|
|
|
|
When implementing gamebridge, I had a few goals in mind:
|
|
|
|
- Use blocking I/O to have the kernel help with this
|
|
- Use threads to their fullest potential
|
|
- Unix fifos are great, let's use them
|
|
- Understand linear interpolation better
|
|
- Create a surreal demo on Twitch
|
|
- Only have one binary to start, the game itself
|
|
|
|
As a first step of implementing this, I went through the source code of the
|
|
Mario 64 PC port (but in theory this could also work for other emulators or even
|
|
Nintendo 64 emulators with enough work) and began to look for anything that
|
|
might be useful to understand how parts of the game work. I stumbled across
|
|
`src/pc/controller` and then found two gems that really stood out. I found the
|
|
interface for adding new input methods to the game and an example input method
|
|
that read from tool-assisted speedrun recordings. The controller input interface
|
|
itself is a thing of beauty, I've included a copy of it below:
|
|
|
|
```c
|
|
// controller_api.h
|
|
#ifndef CONTROLLER_API
|
|
#define CONTROLLER_API
|
|
|
|
#include <ultra64.h>
|
|
|
|
struct ControllerAPI {
|
|
void (*init)(void);
|
|
void (*read)(OSContPad *pad);
|
|
};
|
|
|
|
#endif
|
|
```
|
|
|
|
All you need to implement your own input method is an init function and a read
|
|
function. The init function is used to set things up and the read function is
|
|
called every frame to get inputs. The tool-assisted speedrunning input method
|
|
seemed to conform to the [Mupen64 demo file spec as described on
|
|
tasvideos.org][mupendemo], and I ended up using this to help test and verify
|
|
ideas.
|
|
|
|
[mupendemo]: http://tasvideos.org/EmulatorResources/Mupen/M64.html
|
|
|
|
The thing that struck me was how _simple_ the format was. Every frame of input
|
|
uses its own four-byte sequence. The constants in the demo file spec also helped
|
|
greatly as I figured out ways to bridge into the game from Rust. I ended up
|
|
creating two [bitflag][bitflag] structs to help with the button data, which
|
|
ended up almost being a 1:1 copy of the Mupen64 demo file spec:
|
|
|
|
[bitflag]: https://docs.rs/bitflags/1.2.1/bitflags/
|
|
|
|
```rust
|
|
bitflags! {
|
|
// 0x0100 Digital Pad Right
|
|
// 0x0200 Digital Pad Left
|
|
// 0x0400 Digital Pad Down
|
|
// 0x0800 Digital Pad Up
|
|
// 0x1000 Start
|
|
// 0x2000 Z
|
|
// 0x4000 B
|
|
// 0x8000 A
|
|
pub(crate) struct HiButtons: u8 {
|
|
const NONE = 0x00;
|
|
const DPAD_RIGHT = 0x01;
|
|
const DPAD_LEFT = 0x02;
|
|
const DPAD_DOWN = 0x04;
|
|
const DPAD_UP = 0x08;
|
|
const START = 0x10;
|
|
const Z_BUTTON = 0x20;
|
|
const B_BUTTON = 0x40;
|
|
const A_BUTTON = 0x80;
|
|
}
|
|
}
|
|
```
|
|
|
|
### Input
|
|
|
|
This is where things get interesting. One of the more interesting side effects
|
|
of getting inputs over chat for a game like Mario 64 is that you need to [hold
|
|
buttons or even the analog stick][apress] in order to do things like jumping
|
|
into paintings or on ledges. When you get inputs over chat, you only have them
|
|
for one frame. Therefore you need some kind of analog input (or an emulation of
|
|
that) that decays over time. One approach you can use for this is [linear
|
|
interpolation][lerp] (or lerp).
|
|
|
|
[apress]: https://youtu.be/kpk2tdsPh0A?list=PLmBeAOWc3Gf7IHDihv-QSzS8Y_361b_YO&t=13
|
|
[lerp]: https://www.gamedev.net/tutorials/programming/general-and-gameplay-programming/a-brief-introduction-to-lerp-r4954/
|
|
|
|
I implemented support for both button and analog stick lerping using a struct I
|
|
call a [Lerper][lerper] (the file it is in is named `au.rs` because [.au.][au] is
|
|
the lojban emotion-particle for "to desire", the name was inspired from it
|
|
seeming to fake what the desired inputs were).
|
|
|
|
[lerper]: https://github.com/Xe/gamebridge/blob/b2e7ba21aa14b556e34d7a99dd02e22f9a1365aa/src/au.rs
|
|
[au]: http://jbovlaste.lojban.org/dict/au
|
|
|
|
At its core, a Lerper stores a few basic things:
|
|
|
|
- the current scalar of where the analog input is resting
|
|
- the frame number when the analog input was set to the max (or
|
|
above)
|
|
- the maximum number of frames that the lerp should run for
|
|
- the goal (or where the end of the linear interpolation is, for most cases in
|
|
this codebase the goal is 0, or neutral)
|
|
- the maximum possible output to return on `apply()`
|
|
- the minimum possible output to return on `apply()`
|
|
|
|
Every frame, the lerpers for every single input to the game will get applied
|
|
down closer to zero. Mario 64 uses two signed bytes to represent the controller
|
|
input. The maximum/minimum clamps make sure that the lerped result stays in that
|
|
range.
|
|
|
|
### Twitch Integration
|
|
|
|
This is one of the first times I have ever used asynchronous Rust in conjunction
|
|
with synchronous rust. I was shocked at how easy it was to just spin up another
|
|
thread and have that thread take care of the Tokio runtime, leaving the main
|
|
thread to focus on input. This is the block of code that handles [running the
|
|
asynchronous twitch bot in parallel to the main thread][twitchrs]:
|
|
|
|
[twitchrs]: https://github.com/Xe/gamebridge/blob/b2e7ba21aa14b556e34d7a99dd02e22f9a1365aa/src/twitch.rs#L12
|
|
|
|
```rust
|
|
pub(crate) fn run(st: MTState) {
|
|
use tokio::runtime::Runtime;
|
|
Runtime::new()
|
|
.expect("Failed to create Tokio runtime")
|
|
.block_on(handle(st));
|
|
}
|
|
```
|
|
|
|
Then the rest of the Twitch integration is boilerplate until we get to the
|
|
command parser. At its core, it just splits each chat line up into words and
|
|
looks for keywords:
|
|
|
|
```rust
|
|
let chatline = msg.data.to_string();
|
|
let chatline = chatline.to_ascii_lowercase();
|
|
let mut data = st.write().unwrap();
|
|
const BUTTON_ADD_AMT: i64 = 64;
|
|
|
|
for cmd in chatline.to_string().split(" ").collect::<Vec<&str>>().iter() {
|
|
match *cmd {
|
|
"a" => data.a_button.add(BUTTON_ADD_AMT),
|
|
"b" => data.b_button.add(BUTTON_ADD_AMT),
|
|
"z" => data.z_button.add(BUTTON_ADD_AMT),
|
|
"r" => data.r_button.add(BUTTON_ADD_AMT),
|
|
"cup" => data.c_up.add(BUTTON_ADD_AMT),
|
|
"cdown" => data.c_down.add(BUTTON_ADD_AMT),
|
|
"cleft" => data.c_left.add(BUTTON_ADD_AMT),
|
|
"cright" => data.c_right.add(BUTTON_ADD_AMT),
|
|
"start" => data.start.add(BUTTON_ADD_AMT),
|
|
"up" => data.sticky.add(127),
|
|
"down" => data.sticky.add(-128),
|
|
"left" => data.stickx.add(-128),
|
|
"right" => data.stickx.add(127),
|
|
"stop" => {data.stickx.update(0); data.sticky.update(0);},
|
|
_ => {},
|
|
}
|
|
}
|
|
```
|
|
|
|
This implements the following commands:
|
|
|
|
| Command | Meaning |
|
|
|----------|----------------------------------|
|
|
| `a` | Press the A button |
|
|
| `b` | Press the B button |
|
|
| `z` | Press the Z button |
|
|
| `r` | Press the R button |
|
|
| `cup` | Press the C-up button |
|
|
| `cdown` | Press the C-down button |
|
|
| `cleft` | Press the C-left button |
|
|
| `cright` | Press the C-right button |
|
|
| `start` | Press the start button |
|
|
| `up` | Press up on the analog stick |
|
|
| `down` | Press down on the analog stick |
|
|
| `left` | Press left on the analog stick |
|
|
| `stop` | Reset the analog stick to center |
|
|
|
|
Currently analog stick inputs will stick for about 270 frames and button inputs
|
|
will stick for about 20 frames before drifting back to neutral. The start button
|
|
is special, inputs to the start button will stick for 5 frames at most.
|
|
|
|
### Debugging
|
|
|
|
Debugging two programs running together is surprisingly hard. I had to resort to
|
|
the tried-and-true method of using `gdb` for the main game code and excessive
|
|
amounts of printf debugging in Rust. The [pretty\_env\_logger][pel] crate (which
|
|
internally uses the [env_logger][el] crate, and its environment variable
|
|
configures pretty\_env\_logger) helped a lot. One of the biggest problems I
|
|
encountered in developing it was fixed by this patch, which I will paste inline:
|
|
|
|
[pel]: https://docs.rs/pretty_env_logger/0.4.0/pretty_env_logger/
|
|
[el]: https://docs.rs/env_logger/0.7.1/env_logger/
|
|
|
|
```diff
|
|
diff --git a/gamebridge/src/main.rs b/gamebridge/src/main.rs
|
|
index 426cd3e..6bc3f59 100644
|
|
@@ -93,7 +93,7 @@ fn main() -> Result<()> {
|
|
},
|
|
};
|
|
|
|
- sticky = match stickx {
|
|
+ sticky = match sticky {
|
|
0 => sticky,
|
|
127 => {
|
|
ymax_frame = data.frame;
|
|
```
|
|
|
|
Somehow I had been trying to adjust the y axis position of the stick by
|
|
comparing the x axis position of the stick. Finding and fixing this bug is what
|
|
made me write the Lerper type.
|
|
|
|
---
|
|
|
|
Altogether, this has been a very fun project. I've learned a lot about 3d game
|
|
design, historical source code analysis and inter-process communication. I also
|
|
learned a lot about asynchronous Rust and how it can work together with
|
|
synchronous Rust. I also got to make a fairly surreal demo for Twitch. I hope
|
|
this can be useful to others, even if it just serves as an example of how to
|
|
integrate things into strange other things from unixy first principles.
|
|
|
|
You can find out slightly more about [gamebridge][gamebridge] on its GitHub
|
|
page. Its repo also includes patches for the Mario 64 PC port source code,
|
|
including one that disables the ability for Mario to lose lives. This could
|
|
prove useful for Twitch plays attempts, the 5 life cap by default became rather
|
|
limiting in testing.
|
|
|
|
Be well.
|