Cameras with bellows give photographers flexibility for controlling perspective, but once the picture is taken, its perspective is set. We introduce `virtual bellows' to provide control over perspective after a picture has been taken. Virtual bellows can be used to align images taken from different viewpoints, an important initial step in applications such as creating a high-resolution still image from video. We show how the virtual bellows, which implements the projective group, is an exact model fit to both pan and tilt. Specifically, we identify two important classes of image sequences accomodated by the virtual bellows. Examples of constructing high-quality stills are shown for the two cases: multiple frames taken of a flat object, and multiple frames taken from a fixed point.
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