A system or method for automatic calculation of scene geometry may include receiving, by a processor, a video stream including a plurality of video frames. A video frame may be divided into blocks, and a vertical vanishing point of the video stream may be determined based on a dominant orientation of detected edges in the blocks. A horizon and scale of the video stream may be estimated based on optical flow of pixels between two consecutive video frames. A set of human heads may be detected in a video frame of the video stream based on the estimated horizon and scale of the video stream.