Improving encoding efficiency of endoscopic videos by using circle detection based border overlays

Videos of endoscopic procedures typically feature a circular content area in the image center. This area is surrounded by a dark border that carries no relevant information but is subject to noise. Thus, a considerable proportion of the available bitrate has to be wasted to encode the border regions. We propose to superimpose the border regions with a homogenous black mask so that it can be encoded efficiently with skipped macroblocks. To determine the exact position and size of the circular content area we use an efficient circle detection algorithm. Through an evaluation with 138 videos we show that the border overlay can significantly reduce the bitrate without degrading the visual quality of the content area.