got a area display working with overlapping contours highlighted

stream.go

@@ -3,6 +3,7 @@ package main
 import (
 	"encoding/json"
 	"image"
+	"image/color"
 	"io/ioutil"
 	"log"
 	"net/http"
@@ -63,8 +64,32 @@ func (s Stream) Update() {
 	diff := gocv.NewMat()
 	previousGrey := s.IMReadPrevious()
 	gocv.AbsDiff(previousGrey, currentGrey, &diff)
-	gocv.Threshold(diff, &diff, 50, 255, gocv.ThresholdBinary)
-	go gocv.IMWrite(s.GetDebugInstantPath(), diff)
+	gocv.Threshold(diff, &diff, 100, 255, gocv.ThresholdBinary)
+
+	dilateKernel := gocv.GetStructuringElement(gocv.MorphRect, image.Pt(20, 20))
+	gocv.Dilate(diff, &diff, dilateKernel)
+
+	debug := gocv.NewMat()
+	diff.CopyTo(&debug)
+	gocv.CvtColor(debug, &debug, gocv.ColorGrayToBGRA)
+
+	area := image.Rectangle{
+		Min: image.Point{X: 50, Y: 50},
+		Max: image.Point{X: 300, Y: 300},
+	}
+
+	gocv.Rectangle(&debug, area, color.RGBA{255, 0, 255, 255}, 10)
+
+	contours := gocv.FindContours(diff, gocv.RetrievalExternal, gocv.ChainApproxSimple)
+	for i, contour := range contours {
+		for _, point := range contour {
+			if point.In(area) {
+				gocv.DrawContours(&debug, contours, i, color.RGBA{50, 250, 50, 150}, 5)
+			}
+		}
+	}
+
+	go gocv.IMWrite(s.GetDebugInstantPath(), debug)
 }
 
 // UpdateInterval calls Update() every interval