how to detect face occluded image??

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I wouldn't try to detect the different features of all types of hoods. What I would do is following these steps:

1. Use a robust person detection algorithm. The latentSVM detector of Felzenszwalb or the Integral Channel detector of Dollar would probably do best here. A simpler approach could be the HOG+SVM combination suggested by Dallal and Triggs.
2. Once a person is found, apply a face detector, preferably the Viola and Jones face detection.
3. If a detection for a face is found, make sure that it is located in a correct region, so the upper part of the person detection.
4. If no face is detected, raise an alarm, because this probably means someone is hiding its identity.

Since Viola and Jones is quite influenced by masking and stuff, it will fail quickly if no actual face features are present.

I think the problem you give here is very broad and you need to start by narrowing it to some small problems. For example, are you talking about images or live video?

To find some known object in image you can try to implement some kind of features matching algorithm. The main idea is: - Extract useful features from images of people with helmet/mask - Learn what are the main features unique to person with helmet/mask - Find the same features in new image

Have a look at the feature2d and objdetect modules tutorials, there is a lot of information about features there.