The background and foreground are processed in the same way. If the background is not black then on the target screen there are always black regions(last row in Figure 9). This is visually unpleasing.

[JAMcNaughton]

I think a paragraph in the conclusion highlighting the limitations of the technique - specifically that non-uniform backgrounds will need to be cropped or scaled (leaving blank regions of the background). This may be where warping comes in handy or context-aware filling (like filling the blank spaces with the average colour of the background image).