how can I detect human bodies in opencv

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There are several solutions to this problem, but lets start by telling you that this forum has a search option, which will lead you to 1000 of topics doing exactly the same. Techniques you could try:

1. If your camera is stable than you could start with background/foreground segmentation and could the amount of segmented areas/blobs to count people. Adding trackers will make sure you count each blob only once.
2. More advanced techniques try to model humans. Examples are HOG+SVM descriptor/detector approaches.
3. You could also try out the pedestrian detector for full bodys trained with the Viola and Jones approach inside OpenCV.
4. Or you could go even further and try more less documented techniques like LatentSVM or ICF detector, which are also inside the OpenCV repository.

I guess, start googling on this topics and start reading!