ShiftN

Automatic correction of converging lines by evaluation of vertical edges

ShiftN  Review

Converging lines in photographs, particularly of buildings, forests and so on, can often detract from the desired effect. Architectural elements in photographs often have diagonal lines, but the mind thinks of buildings as having vertical and horizontal lines. Converging lines are caused by the film-plane being angled relative to the object of the photo, and that effect can be avoided optically by the use of either a shift lens (quite expensive) or the use of a medium or large-format view camera having swing features. With a shift lens, the lens itself is shifted perpendicularly to the z-axis, so that the film level can remain parallel to the subject. Shift lenses are mechanically and optically more complex than standard lenses, which is why they are comparatively expensive. Often, one is shooting with a "point and shoot" camera, where the option of changing lenses does not exist. The advent of digital photography or scanning existing photos from prints opens up new possibilities for manipulating the picture. Here is where ShiftN shows its strength. ShiftN permits correction of converging lines; a majority of the correction work is taken over automatically by the program. Using the "automatic correction" item in the menu is in most cases sufficient to produce a satisfying result. Both the effects of converging lines and poor camera angle are corrected automatically. The process in ShiftN is to search the image first for straight lines and edges, and to consider those which are sufficiently vertical to be likely architectural elements. On the basis of these straight line segments the program runs an optimization process that attempts to determine perspective, correcting the image so the lines are made parallel to each other.