Leave part of the picture out with an ignore area

Draw an ignore area so detections in part of the picture never reach any count, and compare two runs of the same clip.

Time: 10 minutes
You need: The street scene from guide 07
Pages of the app: Scene Builder, Experiments
Topics: Traffic

What you will be able to do

Draw an ignore area
Know which detections an ignore area drops
Understand scene versions
Compare two runs of one experiment

Some things in view should never be counted: people on a television, a reflection in a window, a poster, or the street seen through a window. An ignore area removes every detection whose position lies inside it, before tracking, so it reaches no line, zone, route or rule.

This guide continues with the Street counts experiment from guide 07. Pretend the parking bays at the top of the picture belong to a neighbour and must stay out of the study.

Step 1. Draw the ignore area

Open the Scene Builder of the Street camera with Street counts selected in the top bar.
Press I, or click Ignore area in the tool column.
Click the top-left corner of the picture, then the top-right corner. Then click on the right edge about a third of the way down, and on the left edge at the same height. Double-click the last corner to finish.
In the inspector, set Name to Parking bays.
Click Save in the top bar, or press Ctrl+S.

The scene version in the top bar went up by one. A run had used the previous version, so saving created a new version instead of changing the old one. The first run’s results keep their meaning.

Step 2. Run again and compare

Click Start and wait for the clip to finish.
Click Edit experiment. The Runs table lists every run of the experiment, and its Scene column shows the scene version each run used. Compare the newest run with the one before it.
Open Results for each run and compare Line crossings and Object classes (tracks).

With the recommended detector the sample changes like this:

Measure	Without the ignore area	With it
East edge	5: person 3, bicycle 2	1: person 1
West edge	3: car 2, person 1	2: car 2
Tracked objects	person 4, car 2, bicycle 2	person 1, car 2

The cyclists and a pedestrian moved only through the top strip, so they are gone from every count. The cars and the other pedestrian cross the lines below the ignore area and still count.

Which detections are dropped

CV-Scope places each object at the bottom centre of its box. A detection is dropped when that point lies inside an ignore area.

A television or a poster. Cover the screen or the poster itself. The bottom of a person shown on a screen is on the screen.
A window to the street. Cover the window pane.
A reflection on a shiny floor. Cover the reflection. Leave the floor where real people walk uncovered.

While its position is inside an ignore area, an object is not tracked. When it comes out, it is tracked again. If it was tracked just before it went in and comes out within the lost-object time, 30 processed frames by default, it keeps its old track number. Otherwise it starts as a new object.

Guide 09 measures how many people are in an area at the same time.