Dahua NVR21-4KS3 Series User Manual page 261

Step 2 Select the channel and AI type.
Step 3 Click Add to add a rule.
Step 4 On the Type list, select Tripwire.
Step 5 Draw the detection rule.
1) Click to draw a straight line or a curve on the surveillance video image. Right-click
the image to stop drawing.
Figure 5-132 IVS
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User's Manual