working screen capture in /remote.html

go.mod

 require (
 	github.com/BurntSushi/toml v0.4.1
 	github.com/go-vgo/robotgo v0.99.0
+	github.com/kbinani/screenshot v0.0.0-20210720154843-7d3a670d8329
 )

go.sum

 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/gen2brain/shm v0.0.0-20200228170931-49f9650110c5 h1:Y5Q2mEwfzjMt5+3u70Gtw93ZOu2UuPeeeTBDntF7FoY=
+github.com/gen2brain/shm v0.0.0-20200228170931-49f9650110c5/go.mod h1:uF6rMu/1nvu+5DpiRLwusA6xB8zlkNoGzKn8lmYONUo=
 github.com/go-ole/go-ole v1.2.5 h1:t4MGB5xEDZvXI+0rMjjsfBsD7yAgp/s9ZDkL1JndXwY=
 github.com/go-ole/go-ole v1.2.5/go.mod h1:pprOEPIfldk/42T2oK7lQ4v4JSDwmV0As9GaiUsvbm0=
 github.com/go-vgo/robotgo v0.99.0 h1:Lp6kvic1/LxkSShf6KNjcrcE21FCFlonrCguQN96Ay4=
 github.com/go-vgo/robotgo v0.99.0/go.mod h1:0+i2QWRmZtbIF02RwmiGfFj33Judprukd8ls5J6Eajg=
+github.com/jezek/xgb v0.0.0-20210312150743-0e0f116e1240 h1:dy+DS31tGEGCsZzB45HmJJNHjur8GDgtRNX9U7HnSX4=
+github.com/jezek/xgb v0.0.0-20210312150743-0e0f116e1240/go.mod h1:3P4UH/k22rXyHIJD2w4h2XMqPX4Of/eySEZq9L6wqc4=
+github.com/kbinani/screenshot v0.0.0-20210720154843-7d3a670d8329 h1:qq2nCpSrXrmvDGRxW0ruW9BVEV1CN2a9YDOExdt+U0o=
+github.com/kbinani/screenshot v0.0.0-20210720154843-7d3a670d8329/go.mod h1:2VPVQDR4wO7KXHwP+DAypEy67rXf+okUx2zjgpCxZw4=
 github.com/lxn/win v0.0.0-20210218163916-a377121e959e h1:H+t6A/QJMbhCSEH5rAuRxh+CtW96g0Or0Fxa9IKr4uc=
 github.com/lxn/win v0.0.0-20210218163916-a377121e959e/go.mod h1:KxxjdtRkfNoYDCUP5ryK7XJJNTnpC8atvtmTheChOtk=
 github.com/otiai10/curr v0.0.0-20150429015615-9b4961190c95/go.mod h1:9qAhocn7zKJG+0mI8eUu6xqkFDYS2kb2saOteoSB3cE=

main.go

 import "errors"
 import "log"
 import "os"
+import "fmt"
 import "path"
 import "net/http"
 import "io/ioutil"
 import "encoding/json"
 import "github.com/go-vgo/robotgo"
 import "github.com/BurntSushi/toml"
+import "coffee.mort.mediator/screencap"
 
 type Config struct {
 	BasePath string `toml:"base_path"`
 	fs := http.FileServer(http.Dir("./web"))
 	http.Handle("/", fs)
 
-	http.HandleFunc("/api/screen-size", handler(func(w RW, req *Req) error {
+	http.HandleFunc("/api/remote/screen-size", handler(func(w RW, req *Req) error {
 		if req.Method == "GET" {
 			var size ScreenSizeData
 			size.Width, size.Height = robotgo.GetScreenSize()
 		}
 	}))
 
-	http.HandleFunc("/api/mouse-pos", handler(func(w RW, req *Req) error {
+	http.HandleFunc("/api/remote/mouse-pos", handler(func(w RW, req *Req) error {
 		if req.Method == "GET" {
 			var pos MousePosData
 			pos.X, pos.Y = robotgo.GetMousePos()
 		}
 	}))
 
+	http.HandleFunc("/api/remote/screencast", handler(func(w RW, req *Req) error {
+		if req.Method == "GET" {
+			w.Header().Add("Content-Type", "multipart/x-mixed-replace;boundary=MEDIATOR_FRAME_BOUNDARY")
+			w.WriteHeader(200)
+
+			for {
+				img := <-screencap.Capture()
+				log.Printf("Got image, %v bytes", img.Length)
+
+				var err error
+				_, err = w.Write([]byte(fmt.Sprintf(
+					"--MEDIATOR_FRAME_BOUNDARY\r\n" +
+					"Content-Type: image/jpeg\r\n" +
+					"Content-Length: %d\r\n" +
+					"\r\n", img.Length)))
+				if err != nil {
+					log.Printf("Write error: %v", err)
+					return nil
+				}
+
+				_, err = w.Write(img.Data[0:img.Length])
+				if err != nil {
+					log.Printf("Write error: %v", err)
+					return nil
+				}
+
+				_, err = w.Write([]byte("\r\n"))
+				if err != nil {
+					log.Printf("Write error: %v", err)
+					return nil
+				}
+			}
+		} else {
+			return errors.New("Invalid method: " + req.Method)
+		}
+	}));
+
 	http.HandleFunc("/api/dir/", handler(func(w RW, req *Req) error {
 		if req.Method == "GET" {
 			subPath := req.URL.Path[len("/api/dir/"):]
 		}
 	}))
 
+	go screencap.Run()
+
 	log.Println("Listening on :3000...")
 	err = http.ListenAndServe("localhost:3000", nil)
 	if err != nil {

screencap/screencap.go

+package screencap
+
+import "github.com/kbinani/screenshot"
+import "image/jpeg"
+import "sync"
+import "time"
+import "log"
+
+type Buffer struct {
+	Data []byte
+	Length int
+}
+
+func (buf *Buffer) Write(data []byte) (int, error) {
+	if len(buf.Data) == 0 {
+		l := len(data)
+		if l < 1024 {
+			l = 2024
+		}
+		buf.Data = make([]byte, l)
+	} else if buf.Length + len(data) > len(buf.Data) {
+		newSize := len(buf.Data) * 2
+		for buf.Length + len(data) >  newSize {
+			newSize *= 2
+		}
+
+		newBuf := make([]byte, newSize)
+		copy(newBuf, buf.Data[0:buf.Length])
+		buf.Data = newBuf
+	}
+
+	copy(buf.Data[buf.Length:], data)
+	buf.Length += len(data)
+	return len(data), nil
+}
+
+var (
+	mut = sync.Mutex{}
+	chans = make([]chan *Buffer, 0)
+	startChan = make(chan struct{}, 1)
+
+	buffers = make([]Buffer, 4)
+	currentBuffer = 0
+)
+
+func Capture() chan *Buffer {
+	ch := make(chan *Buffer)
+	mut.Lock()
+	chans = append(chans, ch)
+	mut.Unlock()
+
+	select {
+	case startChan <- struct{}{}:
+	default:
+	}
+
+	return ch
+}
+
+func Run() {
+	for {
+		<-startChan
+
+		img, err := screenshot.CaptureDisplay(0)
+		if err != nil {
+			log.Printf("Failed to capture screenshot: %v", err)
+			time.Sleep(2 * time.Second)
+			continue
+		}
+
+		buf := &buffers[currentBuffer]
+		currentBuffer = (currentBuffer + 1) % len(buffers)
+		buf.Length = 0
+		err = jpeg.Encode(buf, img, &jpeg.Options{Quality: 80})
+		if err != nil {
+			log.Printf("Failed to encode jpeg: %v", err)
+			time.Sleep(2 * time.Second)
+			continue
+		}
+
+		mut.Lock()
+		for _, ch := range chans {
+			ch <- buf
+		}
+		chans = make([]chan *Buffer, 0)
+		mut.Unlock()
+	}
+}

web/files.html

 <html>
 	<head>
 		<meta charset="utf-8">
-		<title>Mediator</title>
+		<title>Mediator - Files</title>
 		<link rel="stylesheet" href="style.css">
 		<link rel="icon" type="image/png" href="favicon.png">
 

web/remote.html

+<!DOCTYPE html>
+<html>
+	<head>
+		<meta charset="utf-8">
+		<title>Mediator - Remote</title>
+		<link rel="stylesheet" href="style.css">
+		<link rel="icon" type="image/png" href="favicon.png">
+	</head>
+
+	<body>
+		<img src="/api/remote/screencast">
+
+		<script src="util.js"></script>
+		<script>
+async function main() {
+	let screenSize = api("GET", "remote/screen-size");
+	let mousePos = api("GET", "remote/mouse-pos");
+}
+
+main();
+		</script>
+	</body>
+</html>