pi

README

pi

The retro game development engine for Go, inspired by Pico-8 and powered by Ebitengine.

FAQ

Is this a new fantasy console?

No, it's not. It's rather a game development library with dev-tools which make it simple (and fun!) to write retro games in Go.

What is a retro game?

It's a game that resembles old 8-bit/16-bit games. This usually means:

• (extremely) Low resolution (like 128x128 pixels)
• Limited number of colors (like 16)
• Very small number of assets (like 256 sprites, map having up to 8K tiles)
• Small number lines of code (thousands rather than millions)
• Sound effects and music made using predefined synth instruments and effects

What similarities does Pi have with Pico-8?

• Most API function names are similar and behave the same way.
• Screen resolution is small, and the number of colors is limited. Although in Pi you can change the resolution and palette.
• You have one small sprite sheet.

Why would I use it?

Because it's the easiest way to write a game in Go. IMHO ;)

Is Pi ready to use?

Pi is under development. Only limited functionality is provided. API is not stable. See ROADMAP for details.

How to get started?

1. Install dependencies
   • Go 1.18+
   • If not on Windows, please install additional dependencies for Linux or macOS.
2. Try examples from examples directory.
3. Create a new game using provided Github template.
4. Read the documentation.

Documentation

Overview

Package pi provides API to develop retro games.

Please note that the entire pi package is not concurrency-safe. This means that it is unsafe to run functions and access package variables from go-routines started by your code.

Index

• Constants
• Variables
• func Atan2(dx, dy float64) float64
• func Boot() error
• func Btn(button Button) bool
• func BtnBits() int
• func BtnPlayer(button Button, player int) bool
• func Btnp(button Button) bool
• func BtnpBits() int
• func BtnpPlayer(button Button, player int) bool
• func Camera(x, y int) (prevX, prevY int)
• func CameraReset() (prevX, prevY int)
• func Circ(centerX, centerY, radius int, color byte)
• func CircFill(centerX, centerY, radius int, color byte)
• func Clip(x, y, w, h int) (prevX, prevY, prevW, prevH int)
• func ClipReset() (prevX, prevY, prevW, prevH int)
• func Cls()
• func ClsCol(col byte)
• func Cos(angle float64) float64
• func Line(x0, y0, x1, y1 int, color byte)
• func MouseBtn(b MouseButton) bool
• func MouseBtnp(b MouseButton) bool
• func MousePos() (x, y int)
• func MustBoot()
• func MustRun(runBackend func() error)
• func Pal(color byte, replacementColor byte)
• func PalDisplay(color byte, replacementColor byte)
• func PalReset()
• func Palt(color byte, transparent bool)
• func PaltReset()
• func Pget(x, y int) byte
• func Print(text string, x, y int, color byte) (rightMostX int)
• func PrintCustom(text string, x, y int, color byte) (rightMostX int)
• func Pset(x, y int, color byte)
• func Rect(x0, y0, x1, y1 int, color byte)
• func RectFill(x0, y0, x1, y1 int, color byte)
• func Reset()
• func Run(runBackend func() error) error
• func Sget(x, y int) byte
• func Sin(angle float64) float64
• func Spr(n, x, y int)
• func SprSize(n, x, y int, w, h float64)
• func SprSizeFlip(n, x, y int, w, h float64, flipX, flipY bool)
• func Sset(x, y int, color byte)
• func Stop()
• func Time() float64
• type Button
• type Font
  • func (f Font) Print(text string, x, y int, color byte) int
• type MouseButton

Constants

const (
	Left  = vm.ControllerLeft
	Right = vm.ControllerRight
	Up    = vm.ControllerUp
	Down  = vm.ControllerDown
	O     = vm.ControllerO // O is a first fire button
	X     = vm.ControllerX // X is a second fire button
)

Keyboard mappings:

player 0: [DPAD] - cursors  [O] - Z C N   [X] - X V M
player 1: [DPAD] - SFED     [O] - LSHIFT  [X] - TAB W Q A

First connected gamepad controller is player 0, second player 1 and so on. On XBox controller O is A and Y, X is B and X.

const (
	SpriteWidth, SpriteHeight = 8, 8
)

Variables

var (
	// Update is a user provided function executed each frame (30 times per second).
	//
	// The purpose of this function is to handle user input, perform calculations, update
	// game state etc. Typically, this function does not draw on screen.
	Update func()

	// Draw is a user provided function executed at most each frame (up to 30 times per second).
	// π may skip calling this function if previous frame took too long.
	//
	// The purpose of this function is to draw on screen.
	Draw func()

	// SpriteSheetWidth will be used if sprite-sheet.png was not found.
	SpriteSheetWidth = defaultSpriteSheetWidth
	// SpriteSheetHeight will be used if sprite-sheet.png was not found.
	SpriteSheetHeight = defaultSpriteSheetHeight
	// Palette has all colors available in the game. Up to 256.
	// Palette is taken from loaded sprite sheet (which must be
	// a PNG file with indexed color mode). If sprite-sheet.png was not
	// found, then default 16 color palette is used.
	//
	// Can be freely read and updated. Changes will be visible immediately.
	Palette = defaultPalette

	// ScreenWidth specifies the width of the screen (in pixels).
	ScreenWidth = defaultScreenWidth
	// ScreenHeight specifies the height of the screen (in pixels).
	ScreenHeight = defaultScreenHeight

	Resources fs.ReadFileFS // Resources contains files like sprite-sheet.png, custom-font.png
)

User parameters. Will be used during Boot (and Run).

Functions

func Atan2

func Atan2(dx, dy float64) float64

Atan2 converts DX, DY into an angle from 0..1

Similar to Cos and Sin, angle is taken to run anticlockwise in screenspace. For example:

atan(0,-1)  // returns 0.25

func Boot

func Boot() error

Boot initializes the engine based on user parameters such as ScreenWidth and ScreenHeight. It loads the resources like sprite-sheet.png.

If sprite-sheet.png was not found in pi.Resources, then empty sprite-sheet is used with the size of pi.SpriteSheetWidth * pi.SpriteSheetHeight.

Boot also resets all draw state information like camera position and clipping region.

Boot can be run multiple times. This is useful for writing unit tests.

func Btn

func Btn(button Button) bool

Btn returns true if a controller button is being pressed at this moment by player 0.

func BtnBits

func BtnBits() int

BtnBits returns the state of all controller buttons for players 0 and 1 as bitset.

The first byte contains the button states for player 0 (bits 0 through 5, bits 6 and 7 are unused). The second byte contains the button states for player 1 (bits 8 through 13).

Bit 0 is Left, 1 is Right, bit 5 is the X button.

A bit of 1 means the button is pressed.

func BtnPlayer

func BtnPlayer(button Button, player int) bool

BtnPlayer returns true if a controller button is being pressed at this moment by specific player. The player can be 0..7.

func Btnp

func Btnp(button Button) bool

Btnp returns true when the controller button has just been pressed. It also returns true after the next 15 frames, and then every 4 frames. This simulates keyboard-like repeating.

func BtnpBits

func BtnpBits() int

BtnpBits returns the state of all controller buttons for players 0 and 1 as bitset.

The first byte contains the button states for player 0 (bits 0 through 5, bits 6 and 7 are unused). The second byte contains the button states for player 1 (bits 8 through 13).

Bit 0 is Left, 1 is Right, bit 5 is the X button.

A bit of 1 means the button has just been pressed.

func BtnpPlayer

func BtnpPlayer(button Button, player int) bool

BtnpPlayer returns true when the controller button has just been pressed. It also returns true after the next 15 frames, and then every 4 frames. This simulates keyboard-like repeating. The player can be 0..7.

func Camera

func Camera(x, y int) (prevX, prevY int)

Camera sets the camera offset used for all subsequent draw operations.

func CameraReset

func CameraReset() (prevX, prevY int)

CameraReset resets the camera offset to origin (0,0).

func Circ

func Circ(centerX, centerY, radius int, color byte)

Circ draws a circle

Circ takes into account camera position, clipping region and draw palette.

func CircFill

func CircFill(centerX, centerY, radius int, color byte)

CircFill draws a filled circle

CircFill takes into account camera position, clipping region and draw palette.

func Clip

func Clip(x, y, w, h int) (prevX, prevY, prevW, prevH int)

Clip sets the clipping region in the form of rectangle. All screen drawing operations will not affect any pixels outside the region.

Clip returns previous clipping region.

func ClipReset

func ClipReset() (prevX, prevY, prevW, prevH int)

ClipReset resets the clipping region, which means that entire screen will be clipped.

func Cls

func Cls()

Cls cleans the entire screen with color 0. It does not take into account any draw state parameters such as clipping region or camera.

func ClsCol

func ClsCol(col byte)

ClsCol cleans the entire screen with specified color. It does not take into account any draw state parameters such as clipping region or camera.

func Cos

func Cos(angle float64) float64

Cos returns the cosine of angle which is in the range of 0.0-1.0 measured clockwise.

If you want to use conventional radian-based function use math.Cos.

func Line

func Line(x0, y0, x1, y1 int, color byte)

Line draws a line between points x0,y0 and x1,y1 (inclusive).

Line takes into account camera position, clipping region and draw palette.

func MouseBtn

func MouseBtn(b MouseButton) bool

MouseBtn returns true if the mouse button is being pressed at this moment.

func MouseBtnp

func MouseBtnp(b MouseButton) bool

MouseBtnp returns true when the mouse button has just been pressed. It also returns true after the next 15 frames, and then every 4 frames. This simulates keyboard-like repeating.

func MousePos

func MousePos() (x, y int)

MousePos returns the position of mouse in screen coordinates.

func MustBoot

func MustBoot()

MustBoot does the same as Boot, but panics instead of returning an error.

Useful for writing unit tests and quick and dirty prototypes. Do not use on production ;)

func MustRun

func MustRun(runBackend func() error)

MustRun does the same as Run, but panics instead of returning an error.

Useful for writing unit tests and quick and dirty prototypes. Do not use on production ;)

func Pal

func Pal(color byte, replacementColor byte)

Pal replaces color with another one for all subsequent drawings (it is changing the so-called draw palette).

Affected functions are Pset, Spr, SprSize, SprSizeFlip, Rect and RectFill.

func PalDisplay

func PalDisplay(color byte, replacementColor byte)

PalDisplay replaces color with another one for the whole screen at the end of a frame (it is changing the so-called display palette).

func PalReset

func PalReset()

PalReset resets all swapped colors for all palettes.

func Palt

func Palt(color byte, transparent bool)

Palt sets color transparency. If true then the color will not be drawn for next drawing operations.

Color transparency is used by Spr, SprSize and SprSizeFlip.

func PaltReset

func PaltReset()

PaltReset sets all transparent colors to false and makes color 0 transparent.

func Pget

func Pget(x, y int) byte

Pget gets a pixel color on the screen.

func Print

func Print(text string, x, y int, color byte) (rightMostX int)

Print prints text on the screen using system font. It takes into consideration clipping region and camera position.

Only unicode characters with code < 256 are supported. Unsupported chars are printed as question mark. The entire table of available chars can be found here: https://github.com/elgopher/pi/blob/master/internal/system-font.png

Print returns the right-most x position that occurred while printing.

func PrintCustom

func PrintCustom(text string, x, y int, color byte) (rightMostX int)

PrintCustom prints text in the same way as Print, but using custom font.

func Pset

func Pset(x, y int, color byte)

Pset sets a pixel color on the screen.

func Rect

func Rect(x0, y0, x1, y1 int, color byte)

Rect draws a rectangle between points x0,y0 and x1,y1 (inclusive).

Rect takes into account camera position, clipping region and draw palette.

func RectFill

func RectFill(x0, y0, x1, y1 int, color byte)

RectFill draws a filled rectangle between points x0,y0 and x1,y1 (inclusive).

RectFill takes into account camera position, clipping region and draw palette.

func Reset

func Reset()

Reset resets all user parameters to default values. Useful in unit tests.

func Run

func Run(runBackend func() error) error

Run boots the game and runs the game loop using specified backend. For ebitengine backend it must be called from the main thread, for example:

err := Run(ebitengine.Backend)

Run does not boot the game once the game has been booted. Thanks to this, the user can call Boot directly and draw to the screen before the game loop starts.

It returns error when something terrible happened during initialization.

func Sget

func Sget(x, y int) byte

Sget gets the pixel color on the sprite sheet.

func Sin

func Sin(angle float64) float64

Sin returns the sine of angle which is in the range of 0.0-1.0 measured clockwise.

Sin returns an inverted result to suit screen space (where Y means "DOWN", as opposed to mathematical diagrams where Y typically means "UP"):

sin(0.25) // returns -1

If you want to use conventional radian-based function without the y inversion, use math.Sin.

func Spr

func Spr(n, x, y int)

Spr draws a sprite with specified number on the screen. Sprites are counted from left to right, top to bottom. Sprite 0 is on top-left corner, sprite 1 is to the right and so on.

func SprSize

func SprSize(n, x, y int, w, h float64)

SprSize draws a range of sprites on the screen.

n is a sprite number in the top-left corner.

Non-integer w or h may be used to draw partial sprites.

func SprSizeFlip

func SprSizeFlip(n, x, y int, w, h float64, flipX, flipY bool)

SprSizeFlip draws a range of sprites on the screen.

If flipX is true then sprite is flipped horizontally. If flipY is true then sprite is flipped vertically.

func Sset

func Sset(x, y int, color byte)

Sset sets the pixel color on the sprite sheet. It does not update the global Color.

func Stop

func Stop()

Stop will stop the game loop after Update or Draw is finished. If you are using devtools, the game will be paused. Otherwise, the game will be closed.

func Time

func Time() float64

Time returns the amount of time since game was run, as a (fractional) number of seconds.

Calling Time() multiple times in the same frame will always return the same result.

Types

type Button

type Button int

Button is a virtual button on any game controller. The game controller can be a gamepad or a keyboard.

Button is used by Btn, Btnp, BtnPlayer, BtnpPlayer, BtnBits and BtnpBits.

type Font

type Font vm.Font

Font contains all information about loaded font and provides method to Print the text.

func (Font) Print

func (f Font) Print(text string, x, y int, color byte) int

Print prints text on the screen at given coordinates. It takes into account clipping region and camera position.

Only unicode characters with code < 256 are supported. Unsupported chars are printed as question mark. The entire table of available chars can be found here: https://github.com/elgopher/pi/blob/master/internal/system-font.png

Print returns the right-most x position that occurred while printing.

type MouseButton

type MouseButton int

const (
	MouseLeft   MouseButton = vm.MouseLeft
	MouseMiddle MouseButton = vm.MouseMiddle
	MouseRight  MouseButton = vm.MouseRight
)