fix random timestamp offset in examples (#628)

examples/client-record-format-mjpeg-from-image/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"crypto/rand"
 	"image"
 	"image/color"
 	"image/jpeg"
@@ -25,6 +26,15 @@ func multiplyAndDivide(v, m, d int64) int64 {
 	return (secs*m + dec*m/d)
 }
 
+func randUint32() (uint32, error) {
+	var b [4]byte
+	_, err := rand.Read(b[:])
+	if err != nil {
+		return 0, err
+	}
+	return uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3]), nil
+}
+
 func createRandomImage(i int) *image.RGBA {
 	img := image.NewRGBA(image.Rect(0, 0, 640, 480))
 
@@ -73,6 +83,11 @@ func main() {
 
 	start := time.Now()
 
+	randomStart, err := randUint32()
+	if err != nil {
+		panic(err)
+	}
+
 	// setup a ticker to sleep between frames
 	ticker := time.NewTicker(200 * time.Millisecond)
 	defer ticker.Stop()
@@ -102,7 +117,7 @@ func main() {
 
 		// write packets to the server
 		for _, pkt := range pkts {
-			pkt.Timestamp = pts
+			pkt.Timestamp = randomStart + pts
 
 			err = c.WritePacketRTP(desc.Medias[0], pkt)
 			if err != nil {