Input video frames are encoded into a sequence of output video frames arranged in groups of pictures (GOP). Each input video frame is encoded into an output video frame using a respective compression value. Each GOP in the sequence of output video frames has a GOP length defined by a number of frames contained in the GOP. A level of motion and a level of light in a first input video frame are determined. If level of light is below a predetermined threshold value, encode the first input video frame using a predetermined constant GOP length and a compression value which is a decreasing function of the level of light. If level of light is above, encode the first input video frame using a GOP length which is a decreasing function of the level of motion, and a compression value which is a decreasing function of the level of light.