Next: Background Estimation
Up: Background estimation and removal
Previous: Background estimation and removal
Separating dynamic objects, such as people, from a relatively static background scene is a very important preprocessing step in many computer vision applications. Accurate and efficient background removal is critical for interactive games[7], person detection and tracking[1,4], and graphical special effects. One of the most common approaches to this problem is color or greyscale background subtraction. Typical problems with this technique include foreground objects with some of the same colors as the background (produce holes in the computed foreground), and shadows or other variable lighting conditions (cause inclusion of background elements in the computed foreground).
In this paper we present a passive method for background estimation and removal based on the joint use of range and color which produces superior results than can be achieved with either data source alone. This approach is now practical for general applications as inexpensive real-time passive range data is becoming more accessible through novel hardware[10] and increased CPU processing speeds. The joint use of color and range produces cleaner segmentation of the foreground scene in comparison to the commonly used color-based background subtraction or range-based segmentation.
Range has also been used for background removal[2,5,6]. The main issue in this approach is that depth computation via stereo, which relies on finding correspondences between two images, does not produce valid results in low contrast regions or in regions which can not be seen in both views. In our stereo implementation (described in section 2.1), these low confidence cases are detected and marked with a special value we will refer to as invalid . It is rare that all pixels in the scene will have valid range on which to base a segmentation decision. It is also difficult to use range data to segment foreground objects which are at approximately the same distance as the background. Figure 2 shows an example of range based segmentation failure.
We present a scheme which takes advantage of the strengths of each data source for background modeling and segmentation. Background estimation is based on a multidimensional (range and color) mixture of Gaussians which can be performed for sequences containing substantial foreground elements. Segmentation of the foreground is performed via background comparison in range and normalized color. For optimal performance, we find we must explicitly take into account low confidence values in range and color, as well as shadow conditions. The background estimation is described in section 2, followed by the segmentation method in section 3.
G. Gordon, T. Darrell, M. Harville, J. Woodfill."Background estimation and removal based on range and color,"Proceedings of the IEEE Computer Society Conference on Computer Vision and Pattern Recognition, (Fort Collins, CO), June 1999.