Illustration of the circumstellar disk geometry and the components contributing to the channel maps. In case 1 the CPD phase angle θ is <90° (CPD as foreground object relative to the CSD). The CPD absorption map (ii) is used to extinct the CSD emission map (i). The CPD emission (iii) is then added to produce the final image (iv). When θ > 90° the CSD gap edge becomes the foreground object, and the process follows case 2. At θ = 90°, the extinction caused by either object on the other is negligible and the two channel maps are simply co-added.