Ebiten example - hsv

package main

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

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

const (
    screenWidth  = 320
    screenHeight = 240
)

var (
    hueInt        = 0
    saturationInt = 128
    valueInt      = 128
    gophersImage  *ebiten.Image
)

func clamp(v, min, max int) int {
    if min > max {
        panic("min must <= max")
    }
    if v < min {
        return min
    }
    if max < v {
        return max
    }
    return v
}

func update(screen *ebiten.Image) error {
    if ebiten.IsKeyPressed(ebiten.KeyQ) {
        hueInt--
    }
    if ebiten.IsKeyPressed(ebiten.KeyW) {
        hueInt++
    }
    if ebiten.IsKeyPressed(ebiten.KeyA) {
        saturationInt--
    }
    if ebiten.IsKeyPressed(ebiten.KeyS) {
        saturationInt++
    }
    if ebiten.IsKeyPressed(ebiten.KeyZ) {
        valueInt--
    }
    if ebiten.IsKeyPressed(ebiten.KeyX) {
        valueInt++
    }
    hueInt = clamp(hueInt, -256, 256)
    saturationInt = clamp(saturationInt, 0, 256)
    valueInt = clamp(valueInt, 0, 256)

    w, h := gophersImage.Size()
    op := &ebiten.DrawImageOptions{}
    op.GeoM.Translate(float64(screenWidth-w)/2, float64(screenHeight-h)/2)
    hue := float64(hueInt) * 2 * math.Pi / 128
    saturation := float64(saturationInt) / 128
    value := float64(valueInt) / 128
    op.ColorM.ChangeHSV(hue, saturation, value)
    if err := screen.DrawImage(gophersImage, op); err != nil {
        return err
    }

    msg := fmt.Sprintf(`Hue:        %0.2f [Q][W]
Saturation: %0.2f [A][S]
Value:      %0.2f [Z][X]`, hue, saturation, value)
    if err := ebitenutil.DebugPrint(screen, msg); err != nil {
        return err
    }
    return nil
}

func main() {
    var err error
    gophersImage, _, err = ebitenutil.NewImageFromFile("_resources/images/gophers.jpg", ebiten.FilterNearest)
    if err != nil {
        log.Fatal(err)
    }
    if err := ebiten.Run(update, screenWidth, screenHeight, 2, "HSV (Ebiten Demo)"); err != nil {
        log.Fatal(err)
    }
}