Ebiten example - airship

// +build example

package main

import (
        "fmt"
        "image"
        "image/color"
        _ "image/jpeg"
        "log"
        "math"

        "github.com/hajimehoshi/ebiten"
        "github.com/hajimehoshi/ebiten/ebitenutil"
)

const (
        screenWidth  = 320
        screenHeight = 240
        maxAngle     = 256
        maxLean      = 16
)

var (
        skyColor  = color.RGBA{0x66, 0xcc, 0xff, 0xff}
        thePlayer = &player{
                x16:   16 * 100,
                y16:   16 * 200,
                angle: maxAngle * 3 / 4,
        }
        gophersImage           *ebiten.Image
        repeatedGophersImage   *ebiten.Image
        groundImage            *ebiten.Image
        perspectiveGroundImage *ebiten.Image
        fogImage               *ebiten.Image
)

func init() {
        var err error
        gophersImage, _, err = ebitenutil.NewImageFromFile("_resources/images/gophers.jpg", ebiten.FilterDefault)
        if err != nil {
                panic(err)
        }
        groundImage, _ = ebiten.NewImage(screenWidth*2, screenHeight*2/3+50, ebiten.FilterDefault)
        perspectiveGroundImage, _ = ebiten.NewImage(screenWidth*2, screenHeight, ebiten.FilterDefault)

        const repeat = 5
        w, h := gophersImage.Size()
        repeatedGophersImage, _ = ebiten.NewImage(w*repeat, h*repeat, ebiten.FilterDefault)
        for j := 0; j < repeat; j++ {
                for i := 0; i < repeat; i++ {
                        op := &ebiten.DrawImageOptions{}
                        op.GeoM.Translate(float64(w*i), float64(h*j))
                        repeatedGophersImage.DrawImage(gophersImage, op)
                }
        }

        const fogHeight = 8
        w, _ = perspectiveGroundImage.Size()
        fogRGBA := image.NewRGBA(image.Rect(0, 0, w, fogHeight))
        for j := 0; j < fogHeight; j++ {
                a := uint32(float64(fogHeight-1-j) * 0xff / (fogHeight - 1))
                clr := skyColor
                r, g, b, oa := uint32(clr.R), uint32(clr.G), uint32(clr.B), uint32(clr.A)
                clr.R = uint8(r * a / oa)
                clr.G = uint8(g * a / oa)
                clr.B = uint8(b * a / oa)
                clr.A = uint8(a)
                for i := 0; i < w; i++ {
                        fogRGBA.SetRGBA(i, j, clr)
                }
        }
        fogImage, _ = ebiten.NewImageFromImage(fogRGBA, ebiten.FilterDefault)
}

// player represents the current airship's position.
type player struct {
        // x16, y16 represents the position in XY plane in fixed float format.
        // The fractional part has 16 bits of precision.
        x16 int
        y16 int

        // angle represents the player's angle in XY plane.
        // angle takes an integer value in [0, maxAngle).
        angle int

        // lean represents the player's leaning.
        // lean takes an integer value in [-maxLean, maxLean].
        lean int
}

func round(x float64) float64 {
        return math.Floor(x + 0.5)
}

// MoveForward moves the player p forward.
func (p *player) MoveForward() {
        w, h := gophersImage.Size()
        mx := w * 16
        my := h * 16
        s, c := math.Sincos(float64(p.angle) * 2 * math.Pi / maxAngle)
        p.x16 += int(round(16*c) * 2)
        p.y16 += int(round(16*s) * 2)
        for mx <= p.x16 {
                p.x16 -= mx
        }
        for my <= p.y16 {
                p.y16 -= my
        }
        for p.x16 < 0 {
                p.x16 += mx
        }
        for p.y16 < 0 {
                p.y16 += my
        }
}

// RotateRight rotates the player p in the right direction.
func (p *player) RotateRight() {
        p.angle++
        if maxAngle <= p.angle {
                p.angle -= maxAngle
        }
        p.lean++
        if maxLean < p.lean {
                p.lean = maxLean
        }
}

// RotateLeft rotates the player p in the left direction.
func (p *player) RotateLeft() {
        p.angle--
        if p.angle < 0 {
                p.angle += maxAngle
        }
        p.lean--
        if p.lean < -maxLean {
                p.lean = -maxLean
        }
}

// Stabilize tries to move the player in the stable position (lean).
func (p *player) Stabilize() {
        if 0 < p.lean {
                p.lean--
        }
        if p.lean < 0 {
                p.lean++
        }
}

// Position returns the player p's position.
func (p *player) Position() (int, int) {
        return p.x16, p.y16
}

// Angle returns the player p's angle.
func (p *player) Angle() int {
        return p.angle
}

// updateGroundImage updates the ground image according to the current player's position.
func updateGroundImage(ground *ebiten.Image) {
        ground.Clear()

        x16, y16 := thePlayer.Position()
        a := thePlayer.Angle()
        gw, gh := ground.Size()
        w, h := gophersImage.Size()
        op := &ebiten.DrawImageOptions{}
        op.GeoM.Translate(float64(-x16)/16, float64(-y16)/16)
        op.GeoM.Translate(float64(-w*2), float64(-h*2))
        op.GeoM.Rotate(float64(-a)*2*math.Pi/maxAngle + math.Pi*3.0/2.0)
        op.GeoM.Translate(float64(gw)/2, float64(gh)-32)
        ground.DrawImage(repeatedGophersImage, op)
}

// drawGroundImage draws the ground image to the given screen image.
func drawGroundImage(screen *ebiten.Image, ground *ebiten.Image) {
        perspectiveGroundImage.Clear()
        gw, _ := ground.Size()
        pw, ph := perspectiveGroundImage.Size()
        for j := 0; j < ph; j++ {
                // z is in [1, -1]
                rate := float64(j) / float64(ph)
                z := (1-rate)*1 + rate*-1
                if z <= 0 {
                        break
                }
                op := &ebiten.DrawImageOptions{}
                op.GeoM.Translate(-float64(pw)/2, 0)
                op.GeoM.Scale(1/z, 8) // 8 is an arbitrary number not to make empty lines.
                op.GeoM.Translate(float64(pw)/2, float64(j)/z)

                src := image.Rect(0, j, gw, j+1)
                op.SourceRect = &src
                perspectiveGroundImage.DrawImage(ground, op)
        }

        perspectiveGroundImage.DrawImage(fogImage, nil)

        op := &ebiten.DrawImageOptions{}
        op.GeoM.Translate(-float64(pw)/2, 0)
        op.GeoM.Rotate(-1 * float64(thePlayer.lean) / maxLean * math.Pi / 8)
        op.GeoM.Translate(float64(screenWidth)/2, screenHeight/3)
        screen.DrawImage(perspectiveGroundImage, op)
}

func update(screen *ebiten.Image) error {
        // Manipulate the player by the input.
        if ebiten.IsKeyPressed(ebiten.KeySpace) {
                thePlayer.MoveForward()
        }
        rotated := false
        if ebiten.IsKeyPressed(ebiten.KeyRight) {
                thePlayer.RotateRight()
                rotated = true
        }
        if ebiten.IsKeyPressed(ebiten.KeyLeft) {
                thePlayer.RotateLeft()
                rotated = true
        }
        if !rotated {
                thePlayer.Stabilize()
        }

        if ebiten.IsRunningSlowly() {
                return nil
        }

        // Draw the ground image.
        screen.Fill(skyColor)
        updateGroundImage(groundImage)
        drawGroundImage(screen, groundImage)

        // Draw the message.
        tutrial := "Space: Move forward\nLeft/Right: Rotate"
        msg := fmt.Sprintf("FPS: %0.2f\n%s", ebiten.CurrentFPS(), tutrial)
        ebitenutil.DebugPrint(screen, msg)
        return nil
}

func main() {
        if err := ebiten.Run(update, screenWidth, screenHeight, 2, "Air Ship (Ebiten Demo)"); err != nil {
                log.Fatal(err)
        }
}