Optical flow with bottom camera on aerial vehicle

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I'm interested in obtaining an estimation of the linear velocity of the vehicle using optical flow, so which are the best packages for optical flow using the bottom camera of an aerial vehicle? (the camera is a simple webcam).

Or the best approach is to implement optical flow using OpenCV?

Notice that I am using an Odroid X2 (ARM) on the aerial vehicle, so I am searching for something that wont be too demanding for the processor.