Illumination independent marker tracking using cross-ratio invariance

Marker tracking is used in numerous applications. Depending on the context and its constraints, tracking accuracy can be a crucial component of the application. In this paper, we firstly highlight that the tracking accuracy depends on the illumination, which is usually not controlled in most applications. Particularly, we show how corner detection can shift of several pixels when light power or background context change, even if the camera and the marker are static in the scene. Then, we propose a method, based on the cross ratio invariance, that allows to re-estimate the corner extraction so that the cross ratio of the marker model corresponds to the one computed from the extracted corners in the image. Finally, we show on real data that our approach improves the tracking accuracy, particularly along the camera depth axis, up to several millimeters, depending on the marker depth.