detecting whether a particular object is present or not

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I need to detect whether an object (leaf) is present or not in the live frame captured from the camera. I searched for object detection and i find face detection in openCV documentation using cascade classifier, where some image of the object is given to train the classifier.

In my case i have to detect whether the current frame contains any leaf(of any plant) or not.

1. whether i have to train the classifier with large set of data to detect the leaf?
2. is there any alternate way for my task?