main.go

package main

import (
    "github.com/faiface/pixel"
    "github.com/faiface/pixel/imdraw"
    "github.com/faiface/pixel/pixelgl"
    "image/color"
)

func run() {
    monitorX, monitorY := pixelgl.PrimaryMonitor().Size()
    cfg := pixelgl.WindowConfig{
        Bounds: pixel.R(0, 0, monitorX, monitorY),
        VSync:  true,
        Undecorated:true,
        TransparentFramebuffer:true,
    }
    win, err := pixelgl.NewWindow(cfg)
    if err != nil {
        panic(err)
    }

    win.Clear(color.RGBA{
        R: 0,
        G: 0,
        B: 0,
        A: 10,
    })

    var u = pixel.V(0.0, 0.0)
    var v = pixel.V(0.0, 0.0)
    drawing := false
    imd := imdraw.New(nil)
    for !win.Closed() {
        if win.Pressed(pixelgl.MouseButtonLeft) {
            win.Clear(color.RGBA{
                R: 0,
                G: 0,
                B: 0,
                A: 10,
            })

            if !drawing {
                u = win.MousePosition()
                drawing = true
            } else {
                v = win.MousePosition()

                rect := pixel.R(u.X, u.Y, v.X, v.Y)

                imd.Clear()
                imd.Color = color.RGBA{
                    R: 225,
                    G: 225,
                    B: 225,
                    A: 10,
                }
                imd.Push(rect.Min, rect.Max)
                imd.Rectangle(1)

            }
        }

        if win.JustReleased(pixelgl.MouseButtonLeft) {
            drawing = false
        }
        imd.Draw(win)
        win.Update()
    }
}

func main() {
    pixelgl.Run(run)
}