Excellent question! To know how Threshold and Sensibility works is very important to make motion detection a success or a failure. So here comes a short story of them both. Maybe a little bit longer that I expected, but I found it interesting to write about ;). If you do not understand anything please, reply that it is crap and I will try again ;).
As the name reveals it is about detection of an object in motion. So the system need first to verify that it is an object in the field of view of the camera and second if the object is moving. Since it´s all mathematics the system need two parameters; let´s name them threshold value for the detection of an object and sensibility value for the detection of movement.
As you mentioned the threshold value is a number in % and is the amount of pixels that is covered by the object relative the total amount of pixels in the picture. Let’s say that a person cover 15% of the total amount of pixels. Then the threshold value is 15%. For the system to detect that person the threshold level must be set to a lower value than 15%, let´s say 10%. Now, if something covers more than 10% of the pixels in the field of view the system will acknowledge that as an object of interest. But, that information will solely not trigger the system for motion detection… Therefore, we need a second parameter called Sensibility value.
Sensibility is a parameter connected to if the object is moving slowly or fast. If the object is moving at all or if it is moving slowly or fast is determined from more mathematical calculations during a certain time interval. Let´s say that at time=1 the calculation gives the first threshold value. A specific time later at time=2 the second calculation gives the next threshold value and after time=3 it gives the third threshold value. If all these threshold values are the same the object is not moving and the system will not generate a motion trigg to start a recording of a video. But, if it is a difference between the threshold values something is happening with the object and it gets interesting. If the differences between the threshold values are small the object is probably moving slow. If the difference is big the object is probably moving fast. Let´s say that the system calculates a sensibility value that is between 0 and 100. A value of 0 means no change in motion, completely still. A value of 100 means t that the object is moving very fast. In Netcam system the Sensibility can be set to; very high – normal – very low. If you want to detect an object that moves very slowly you need to set the Sensibility to very high. Small changes in the sensibility value should be recognized as important and tell the system that here is something important going on. If the object is moving fast the sensibility value is high and it will also be recognized by the system as important. Well, why not always set the Sensibility to very high because then we will never miss a moving object? That is the million dollar question. If you set the Sensibility to very high it will detect everything and you will have a lot of false alarms! If you set the Sensibility to very low you will most likely never get a false alarm, but you will never get the moving object of interest either. Setting the Sensibility correct needs quite often some testing since it is very dependent of the situation.
To summarize setting the Threshold Level to a correct value let the system know that the object is of interest. Setting the Sensibility correct let the system know that the object of interest is moving. So the slower the object is moving the higher needs the Sensibility to be. These two parameters together trigger the motion detection system and it starts to record a video.
This was in very general terms how this works. There are many different mathematical algorithms depending on what object you want to detect and so on. In high-end application they also use Artificial Intelligence. Finally, if you know what object you want to detect and also a little bit how fast it will move the more likely it is that you get very good results from the system. But, usually some testing is needed to get it right.
From this comes also an interesting result why the cams of today that have megapixel sensors need rather powerful computers for motion detection, but that is another story ;).