Converging lines
As converging lines are in central projection illustrated vertical lines that run parallel in the object, respectively. With the vanishing point at the top , they seem to "crash" towards each other in the representation.
The term is often used in photography - especially architectural photography. In the latter one would like to u. a. show the frontal view of a surface (building front, wall, etc.) for documentation purposes or for a geometric evaluation. You are prevented from doing this because the appropriate pick-up point z. B. in front of a tall building is difficult or impossible to reach. In photo technology, the recording method with a transversely movable lens ( shift ) was developed, which allows instead of distorted recordings from unfavorable viewpoints from images without converging lines (e.g. recordings of a high-rise building from the ground and at a short distance).
There are now correspondingly adjustable lenses for 35mm cameras for use in amateur photography on the market. In this context it should be emphasized that shifting does not serve to reduce or eliminate an imaging error . On the contrary: the perspective in which the object appears to the photographer is faked. Apart from a specific use as in architecture photography, the photographer wishes i. d. Usually alienating effects achieved.
Emergence
A photographic camera - like the human eye - is subject to the laws of central perspective . Parallel lines in the subject will therefore only run parallel in the image if they lie in a plane in the subject that is parallel to the film plane / sensor surface. So if the camera z. B. pivoted upwards in front of a house, falling lines appear, the house seems to tilt backwards. The same effect arises with the opposite sign, for example in the case of material shots, if the camera is pointed downwards at an angle.
Cause of the unnatural effect
If the objects are very high and you cannot take a great distance, these objects can only be completely recorded with a wide-angle lens. If you look at the finished print or monitor image from such a short distance that the diagonal corners of the image correspond to the angle of view of the lens, the image impression would be natural and no longer disturbing converging lines. This distance would often only be a fraction of the image diagonal (for example, with a 90 ° image angle, half the diagonal). Usually a picture is viewed from a distance of almost twice the picture diagonal. The viewing perspective thus corresponds to a light telephoto lens. If you photograph the object with this telephoto focal length from a greater distance (assumption: enough free space), you would photograph less steeply upwards. This means that the perpendiculars are much less inclined to each other (e.g. only a quarter compared to a 90 ° exposure). This inclination is the degree that is perceived as natural.
correction
Since an object often cannot be photographed from a greater distance, the inclination of the perpendiculars to each other should be reduced to what is perceived as natural. Often one would like to make the verticals completely parallel, since an inclination towards the edge of the picture is also perceived as annoying. The measure required for this is called shifting . This can preferably be done using suitable cameras and lenses during the recording; alternatively, if such options are not available, by means of digital post-processing.
In the case of large format cameras or the tilt-and-shift lens of small and medium format cameras, converging lines are avoided by not pivoting the camera to select the image section, but shifting the image section. Corresponding functions are available for digital post-processing in most image processing programs, but also in specialized programs, some of which also offer automatic functions.
Limits of correction
Before “rectifying” an image, it should be checked whether the respective unprocessed original is at all suitable or whether the result of the processing contradicts the viewer's normal viewing habits too much. If unsuitable recordings with converging lines are rectified by processing with the appropriate software, the photographed object can be severely distorted. These edits, which depict vertical lines in parallel under all circumstances, can then appear unnatural because they no longer correspond to people's viewing habits (in particular, cuboid or cylindrical objects appear to diverge upwards, i.e. to become larger). This effect can often be avoided by only moderately correcting the vertical lines, i.e. correcting the vertical lines. H. not to the point of complete parallelism.
Areas at great heights that are offset or inclined from the facade are distorted despite every correction. The back surfaces of recesses slide down, sloping roof surfaces tilt back or disappear completely.
Examples:
Image rectified with digital image processing ShiftN
Photo of the work of art color conductor by Peter Lacroix ...
... straightened out with the DigitalPhotoShifter (today to FixFoto )
gallery
See also
Web links
- PanoTools Panorama Tools, also enables the correction of distortions of the optics and the alignment of the image (removal of converging lines) ( free software ), see also: Hugin (software) or PTOpenGUI
- ShiftN , automatic correction of converging lines by evaluating the object edges detected in the image (freeware)
- rectif , plane rectification with four point correspondences (Win32, Linux, Freeware )
- Digital Photo Shifter - Instructions and examples on digitalfotokurs.de. (Today the tool belongs to FixFoto .)
Individual evidence
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