Hajime Hoshi (星一) (@hajimehoshi)

English version

Who am I?

• Hajime Hoshi (星一) @hajimehoshi
• Software Engineer
• Work: Non-game programming in C++
• Hobby: Game programming in Go
• Learned game programming myself
• Wanted and tried developing a game, but I started to enjoy developing tools to develop games, and I’ve not finished developing games yet.

What I’m talking today

• About Ebiten (海老天) - a 2D game library
• What Ebiten is
• Actual example
• Implementation (Mainly how tough�the implemention was)

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• Feel free to ask questions anytime

Go Gopher by Renée Franch is licensed under CC BY 3.0

What Ebiten is

Ebiten - What Ebiten is

• https://hajimehoshi.github.io/ebiten/
• An extremely simple 2D game lib
• About 50 functions for rendering and input
• Multi-platform
• Apache License 2.0

Ebiten - Features

Ebiten - Mainloop

• Specify a function called every frame to Run
• *ebiten.Image representing the screen is passed to the function
• In Ebiten, all images, offscreens, the screen are *ebiten.Image

func update(screen *ebiten.Image) error {

// Update

// Draw to the screen

return nil

}

​

func main() {

if err := ebiten.Run(update, 320, 240, 2,

“title”); err != nil {

log.Fatal(err)

}

}

Ebiten - Graphics

• Affine matrix
• Translation, scaling, and rotating are represented as one matrix
• API without confusion

*ebiten.Image (render target)

*ebiten.Image (render source)

The photograph by Chris Nokleberg is licensed under CC BY 3.0

Ebiten - Demo

• A kind of block-falling game
• Works both on desktop and�browser with the same Go code

Actual example

Actual example - Clock of Atonement

• 償いの時計 - Clock of Atonement
• http://blockbros.net/tsugunai/
• Android / iOS
• Worked with @daigo
• Introduced by Fami-tsu (ファミ通)
• https://app.famitsu.com/20170514_1042354/
• 15000 DL (Aug, 2017)
• Got a little money

Actual example - Clock of Atonement

• Developed a game editor as a web app
• Like RPG Maker
• Converted RPG Engine in Go to JavaScript with GopherJS
• Tested the game on browsers

Ebiten (Go)

RPG Engine (Go)

Game Editor

(Web App)

Test Player�(Web App)

RPG Engine

(JS)

Game Data

(JSON)

Actual example - Clock of Atonement

• Compiled the RPG engine with gomobile
• Embedded the game data JSON into the mobile app

Ebiten (Go)

RPG Engine (Go)

Game Editor

(Web App)

Game Player

(Mobile App)

RPG Engine�(Native bin.)

Game Data

(JSON)

Implementation (Summary)

Implementation - Graphics

• OpenGL
• Ebiten is a 2D game lib but utilizes GPU

Implementation - Sound

• https://github.com/hajimehoshi/oto (音)
• A low-level library to play sound on multiple platforms
• Used by other game libs (e.g. https://github.com/faiface/beep)

Implementation - Audio decoder

• Ported MP3 decoder after the patent problems were solved
• MP3 works on any browsers and Ebiten uses the native MP3 decorders
• Ogg doesn’t work on Safari natively

Implementation (Desktops)

Implementation (Desktops) - GLFW

• OpenGL / GLFW
• As GLFW works on multiplatforms, Ebiten automatically works on multiplatforms
• Probably works on *BSD other than FreeBDS
• github.com/go-gl/glfw: a go-gettable binding
• Cgo is inevitable

http://www.glfw.org/

Implementation (Desktops) - Cgo

• Cgo requires gcc/clang
• Inevitable Cgo as long as GLFW is used
• Windows - MinGW
• Hard to install from the official site
• Recommend TDM-GCC
• http://tdm-gcc.tdragon.net/
• macOS - clang
• Type clang, and follow the instruction
• Linux / FreeBSD - gcc/clang

Implementation (Browsers)

Implementation (Browsers)

• GopherJS
• Software to compile Go to JavaScript
• Can call any JavaScript funcs from Go
• Even goroutines work
• Performance is not so good
• Ebiten uses web technology like WebGL and�WebAudio

GopherJS Logo?

https://twitter.com/gopherjs

Implementation (Browsers) - GopherJS

package main��import (� "github.com/gopherjs/gopherjs/js"�)��func main() {� doc := js.Global.Get("document")� doc.Call("addEventListener", � "click",� func() {� println("clicked")� }� )�}

document.addEventListener(� 'click', function() {� console.log('clicked');� }�)

The above code represents a semantically-equivalent code, and actual compilation results will not be so clean.

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Implementation (Browsers) - Performance

• Take profile
• Easiest to do compared to other platforms
• Avoid copying structs
• Avoid locks, channels and goroutines
• Might cause setTimeout
• Avoid defer
• defer is 25x slower. Actually this is slow with the original Go (gopherjs/gopherjs#648)
• Avoid reflection (like encoding/json)
• We are trying to change JSON to MessagePack in our game engine for Clock of Atonement

Implementation (Browsers) - GopherJS status

• GopherJS is still unstable but developed very actively
• Bug when using complex128 as a map keys (gopherjs/gopherjs#187)
• Bug in ‘shift’ operation with a uint variable (gopherjs/gopherjs#418)
• str[idx] doesn’t cause ‘out of index’ error (gopherjs/gopherjs#543)
• (a-b)/c becomes a - b/c (gopherjs/gopherjs#649)
• Found June this year
• I fixed this and became a contributor!

Implementation (Mobiles)

Implementation (Mobiles) - gomobile

• gomobile (golang.org/x/mobile)
• Tools to compile Go progrsm for Android/iOS
• gomobile build
• Create an application (Android: *.apk, iOS: *.ipa)
• gomobile bind
• Create a library (Android: *.aar, iOS: *.framework)

Implementation (Mobiles) - gomobile

• Only gomobile bind is available with Ebiten
• Need to write Java/Objective-C to some extent
• gomobile build is inflexible
• Creates an app with only one OpenGL surface
• Lacks features for actual apps in the stores
• Signature, advertisements, collaboration with other apps, etc.

Implementation (Mobiles) - Difficulty

• Detecting the culprit is hard
• Ebiten is wrong?
• gomobile is wrong?
• Go itself is wrong?
• GPU is wrong?
• Context losts
• Performance tuning

Implementation (Mobiles) - Bugs (1/4)

• Usage of OpenGL
• Unforced errors with initializing consts (c07f16d9)
• OpenGL doesn’t tell what is wrong :-(
• The screen is black until I noticed the problem
• Solve with guts
• glTexSubImage2D doesn’t work (0c611d87)
• glFlush solved that
• iOS’s target framebuffer is non-0 (5eb77d2f)
• The screen is black until I noticed the problem

Implementation (Mobiles) - Bugs (2/4)

• Various GPU behaviors
• Can’t compare two mat4s with == (24bb5b5c)
• Found iOS first time. Why?
• Divide by 0 (f537378f)
• Crash on Kindle Fire (hajimehoshi/ebiten#390)

Implementation (Mobiles) - Bugs (3/4)

• Bugs in gomobile
• Wrong arguments in a GL function (golang/go#14403)
• int is treated as 32bit on ObjC side via gomobile bind (golang/go#16182)
• Buts in Go
• 100 < 1 is true on ARM 32bit (golang/go#19403) (hajimehoshi/ebiten#329)
• Casting int -> float32 16 times in a row caused that

Implementation (Mobiles) - Bugs (4/4)

• Bugs in Ebiten due to gomobile bind (0ee6b440)
• Ebiten’s game is embedded as a dynamic lib with gomobile bind
• init function was called unexpectedly → crash!
• Global variables that should be set from Java/JNI were uninitialized unexpectedly
• No clue other than that “init was called” from the stack trace
• Lesson: init functions in a dynamic lib in Go can be called before any function in the lib is called in the first time
• You can’t expect that init is called as you expect

Implementation (Mobiles) - Context Lost

• OpenGL ES causes context lost
• Actually WebGL causes the same thing, but it was so rare that I couldn’t notice that
• *ebiten.Image pixel data was only on GPU
• All data on GPU is cleared
• Crash when switching an app
• Crash when turning the power off

Implementation (Mobiles) - Context Lost

• Adopted B
• Record draw commands as much as possible, and restore with them
• Now Ebiten users don’t have to care about context lost

Implementation (Mobiles) - Performance Tuning

• Android
• The tools are not useful for native binaries
• Go with println
• Unstable measurement results
• Depends on e.g. if the device is connected with power supply or not
• Take relative time instead of absolute time
• Take three time points ABC, then calculate (the average of) (B-A) / (C-A)
• See the detail in an article in Qiita (Japanese)
• iOS: Easy since the tools were useful
• Reduce draw calls anyway (same as general OpenGL progamming)

Conclusion

How is game developing in Go?

• Go doesn’t change the situation extremely
• Depends on your preference
• Does GC matter? → Not sure
• GC is not problematics for us so far
• Because Go’s GC works very well or�we’ve not developed so huge games that GC matters
• Small number of games in Go
• A little while ago: There are more game libs in Go than games in Go…
• Now the number of games has been increasing e.g. thanks to Gopher Game Jam
• However, I’d like to see more games in Go

Conclusion

• Ebiten - A simple 2D game library
• Actual example: Clock of Atonement
• Desktops
• Use GLFW, that requires Cgo
• Browsers
• Use GopherJS
• Mobiles
• Use gomobile bind. The toughest implementation
• I wish there would be more games in Go

Thank You!