This paper proposes a realistic interpretation of the bluespill problem that affects blue-screen matting in digital image composition. This phenomenon consists in the contamination of the foreground object color by the background color. Based on the notion of mutual illumination introduced by Forsyth and Zisserman [5], we interpret the bluespill phenomenon and propose an extended composition equation explicitly taking this into consideration. Experiments confirm the accuracy of the proposed method. Comparisons are made with the classical composition equation.