Abstract: A controller interacts with an image processing chain configured to process a video stream captured by a video camera and including an object detection algorithm which outputs a frame-wise detection score for each image region, and a masking function which applies a privacy mask in dependence of the detection score. When the controller detects a sudden change in luminance of a scene which is being imaged the video camera, it maintains or expands, for a period while the video camera is imaging the scene, the privacy mask which was applied at the detected sudden change. The maintaining or expanding of the privacy mask may be achieved by locally reducing a masking threshold applied by the masking function. The expansion of the privacy mask may be gradual over time and related to an estimated speed of a moving object in the scene.