Fix gocv-receive example

Error build: cannot range over contours (type gocv.PointsVector).

Error run on macOS: NSWindow drag regions should only
be invalidated on the Main Thread!

Author: cgojin

gocv-receive/README.md

@@ -11,6 +11,8 @@ This example requires you have GoCV and ffmpeg installed, these are the supporte
 #### Debian/Ubuntu
 * Follow the setup instructions for [GoCV](https://github.com/hybridgroup/gocv)
 * `sudo apt-get install ffmpeg`
+#### macOS
+* `brew install ffmpeg opencv`
 
 ### Build gocv-receive
 ```

gocv-receive/main.go

@@ -9,6 +9,7 @@ import (
 	"image/color"
 	"io"
 	"os/exec"
+	"runtime"
 	"strconv"
 	"time"
 
@@ -26,6 +27,11 @@ const (
 	minimumArea = 3000
 )
 
+func init() {
+	// On macOS, NSWindow drag regions should only be invalidated on the Main Thread!
+	runtime.LockOSThread()
+}
+
 func main() {
 	ffmpeg := exec.Command("ffmpeg", "-i", "pipe:0", "-pix_fmt", "bgr24", "-s", strconv.Itoa(frameX)+"x"+strconv.Itoa(frameY), "-f", "rawvideo", "pipe:1") //nolint
 	ffmpegIn, _ := ffmpeg.StdinPipe()
@@ -93,8 +99,9 @@ func startGoCVMotionDetect(ffmpegOut io.Reader) {
 
 		// now find contours
 		contours := gocv.FindContours(imgThresh, gocv.RetrievalExternal, gocv.ChainApproxSimple)
-		for i, c := range contours {
-			area := gocv.ContourArea(c)
+
+		for i := 0; i < contours.Size(); i++ {
+			area := gocv.ContourArea(contours.At(i))
 			if area < minimumArea {
 				continue
 			}
@@ -103,7 +110,7 @@ func startGoCVMotionDetect(ffmpegOut io.Reader) {
 			statusColor = color.RGBA{255, 0, 0, 0}
 			gocv.DrawContours(&img, contours, i, statusColor, 2)
 
-			rect := gocv.BoundingRect(c)
+			rect := gocv.BoundingRect(contours.At(i))
 			gocv.Rectangle(&img, rect, color.RGBA{0, 0, 255, 0}, 2)
 		}