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 some additional tools (like a console) 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)
• Simple rules (opposite to Paradox grand strategy games)
• 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 and a map.

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+
• Pi is powered by Ebitengine which has its own dependencies. See instructions how to install them.

1. Create a new game using provided Github template.

See also examples directory and documentation.

Roadmap

• Present game on the screen
  • ability to change the resolution and palette
  • sprite-sheet loader
  • more options: full screen, specifying tps and scale
  • Game loop
• Implement Graphics API
  • drawing sprites and pixels with camera and clipping support
  • add the ability to directly access pixels on the screen and sprite-sheet
  • palette transparency
  • palette swapping
  • printing text on the screen
    • system font
    • custom font and additional features: escape characters, offsets
  • drawing shapes
    • lines, rectangles, circles, ovals
    • fill patterns
  • math API
    • Cos, Sin, Atan2
    • Min, Max, Mid
  • stretching sprites
• Game controller support: gamepad and keyboard
• Mouse support (dev mode)
• Full keyboard support (dev mode)
• Map API
• Menu screen
• Development console
  • stopping, resuming the game
    • add a programmatic way to stop the game
    • resume the game using console command
  • running public functions
  • sprite-sheet editor
  • map editor
  • sound editor
  • music editor
• Documentation
  • Go docs
• Examples
  • simple programs for beginners
  • interactive programs describing how functions work
    • Sin,Cos,Atan2 visualization

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 BootOrPanic()
• 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 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 Color(col byte) (prevCol byte)
• func ColorReset() (prevCol byte)
• func Cos(angle float64) float64
• func Cursor(x, y int) (prevX, prevY int)
• func CursorReset() (prevX, prevY int)
• 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 int)
• func Pset(x, y int)
• func Reset()
• func Run() error
• func RunOrPanic()
• func Sget(x, y int) byte
• func Sin(angle float64) float64
• func Snap() (string, error)
• 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

Constants

const (
	SpriteWidth, SpriteHeight = 8, 8
)

Variables

var (
	Update = func() {} // Update is a user provided function executed each frame.
	Draw   = func() {} // Draw is a user provided function executed each frame (if he can).

	Resources fs.ReadFileFS // Resources contains files like sprite-sheet.png

	// 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 [256]image.RGB = defaultPalette

	// 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

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

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

var (
	// ScreenData contains pixel colors for the screen visible by the player.
	// Each pixel is one byte. It is initialized during pi.Boot.
	//
	// Pixels on the screen are organized from left to right,
	// top to bottom. Slice element number 0 has pixel located
	// in the top-left corner. Slice element number 1 has pixel color
	// on the right and so on.
	//
	// Can be freely read and updated. Useful when you want to use your own
	// functions for pixel manipulation.
	// Pi will panic if you try to change the length of the slice.
	ScreenData []byte
)

Screen-specific data

var (
	// SpriteSheetData contains pixel colors for the entire sprite sheet.
	// Each pixel is one byte. It is initialized during pi.Boot.
	//
	// Pixels in the sprite-sheet are organized from left to right,
	// top to bottom. Slice element number 0 has pixel located
	// in the top-left corner. Slice element number 1 has a pixel color
	// on the right and so on.
	//
	// Can be freely read and updated.
	// Useful when you want to use your own functions for pixel manipulation.
	// Pi will panic if you try to change the length of the slice.
	SpriteSheetData []byte
)

Sprite-sheet data

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 color, camera position and clipping region.

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

func BootOrPanic

func BootOrPanic()

BootOrPanic 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 Btn

func Btn(button Button) bool

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

func BtnBits

func BtnBits() int

BtnBits returns the state of all 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 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 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 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 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 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 Color

func Color(col byte) (prevCol byte)

Color sets the color used by Pset.

Color returns previously used color.

func ColorReset

func ColorReset() (prevCol byte)

ColorReset resets the color to default value which is 6.

ColorReset returns previously used color.

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 Cursor

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

Cursor set cursor position (in pixels) used by Print.

Cursor returns previously set cursor position.

func CursorReset

func CursorReset() (prevX, prevY int)

CursorReset resets cursor position used by Print to 0,0.

CursorReset returns previously set cursor position.

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 and SprSizeFlip.

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 int)

Print prints text on the screen. It takes into consideration cursor position, color, clipping region and camera position.

After printing all characters Print goes to the next line. When there is no space left on screen the clipping region is scrolled to make room.

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 Pset

func Pset(x, y int)

Pset sets a pixel color on the screen to Color.

func Reset

func Reset()

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

func Run

func Run() error

Run boots the game, opens the window and run the game loop. It must be called from the main thread.

It returns error when something terrible happened during initialization.

func RunOrPanic

func RunOrPanic()

RunOrPanic 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 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 Snap

func Snap() (string, error)

Snap takes a screenshot and saves it to temp dir.

Snap returns a filename. If something went wrong error is returned.

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 is finished, but before Draw. For now the entire app will be closed, but later it will show a dev console instead, where developer will be able to resume the game.

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.

const (
	Left  Button = 0
	Right Button = 1
	Up    Button = 2
	Down  Button = 3
	O     Button = 4 // O is a first fire button
	X     Button = 5 // 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.

type Font

type Font struct {
	// Data contains all 256 characters sorted by their ascii-like number.
	// Each character is 8 subsequent bytes, starting from the top.
	// Left-most pixel in a line is bit 0. Right-most pixel in a line is bit 7.
	Data [8 * 256]byte
	// Width in pixels for all characters below 128
	Width int
	// WidthSpecial is a with of all special characters (code>=128)
	WidthSpecial int
	Height       int
}

Font contains all information about loaded font.