Great idea , but It is rather a complex logic if you want to detect the human only presence. Flex provides activity level which can through an event when you receive a defined amount of activity in webcam. Using activity level in webcam you can detect the presence of a living organism or some moving object. Its not specific to human. The logic of human detection can be worked around taking bitmap from webcam periodically and analyzing it with a specific logic. As far as I know the human presence logics work around the face detection and try to trace a human face pattern inside the image.
The logics are slow and also fail in certain cases
When the person is masked, low light, side pose etc

Thank you Andrid for your fast response
yes a very interesting idea. I want to improve my algorithm for motion detection by the recognition of human forms.
I started some research I found that bin is complicated algorithm as they use neural networks !!!
I have no idea how it works !
cordially