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Photo image warps. Correction of geometric warps with Perspective Pilot.

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Photo image warps
Correction of geometric warps with Perspective Pilot
  1. Correction steps
  2. Correction of distortion
      2.1 Correction of distortion in automatic mode
      2.2 Manual correction of distortion
  3. Image rotation
  4. Adjustment mode
  5. Perspective correction
  6. Completion phase

 

In urban landscape photos, vertical lines of building walls often are inclined or curved. The Perspective Pilot program allows you to straighten the lines, making your photos more attractive.

Original image     Corrected image

Let us review why geometric warps appear on photo images. Basic geometric warps relate to distortion, perspective, and rotation.

 

Distortion

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When distortion takes place, edges of objects that are straight in nature appear curved.

Distortion does not affect sharpness; it just misrepresents the objects' shapes.

Distortion types:

Original image

Pincushion distortion

Barrel distortion

 

Image distortion is caused by a nonlinear radius mapping from the object to the image.
Photographic objectives exhibit barrel distortion at extremely short focal distances, and pincushion distortion at extremely long focal distances.
Specially constructed objectives with non-spherical lengths allow capturing an image without the distortion effect.
For distortion, axial symmetry is common. Horizontal and vertical axes, which going through the center of an image, are not distorted.

 

Rotation

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Humans can measure an object canted on a photo by eye quite exactly. Even a slight slope of the horizon line looks wrong. If the camera in an operator's hands was inclined from the vertical / horizontal line while taking the image, objects on the photo will be unnaturally canted; the image will require correction.

 

Perspective

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The perspective shown in a photo is related to the projection of three-dimensional objects onto an image plane. For instance, you are standing on the ground and taking a photo of a building. The low floors are nearer to you and so look wider than the high floors. So, when photographed, the walls of the building appear inclined. That is how perspective works.



Perspective correction on a photo image is the changing of inclined lines which are straight or vertical in reality (walls, columns). Note that the slope of such lines does not warp properly. So, if you correct those inclined lines to be absolutely straight, the image would look unnatural.


A light slope of the building’s walls or of other high vertical objects shows the space perspective; it is not a warp of the image.

 

Correction of geometric warps with Perspective Pilot

1. Correction steps

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Perspective Pilot allows you to align objects on your urban landscape images, thereby making the photos more attractive.

With Perspective Pilot, the following order of correction steps is recommended:

  • distortion
  • rotation
  • perspective
  • zoom
Distortion should be corrected first because the warping is objective-induced. Then the rotation should be processed to correct a balk slope of objects on the photo. Then it is perspective's turn.

Perspective transformations regarding vertical and horizontal objects can be processed in any order. However, if there has been rotation based on a vertical line, for correction of all vertical lines the perspective transformation regarding vertical objects is required. Alternatively, if there has been rotation based on a horizontal line, for correction of all horizontal lines the perspective transformation regarding horizontal objects should be processed.

Note that with Perspective Pilot, software distortion and perspective geometric features are corrected by the same tool. In this way the corrected image is a result of one-step instead of two-step processing. It leads to increased quality of the resulting image and allows decreasing of the cut edge areas.

2. Correction of distortion

2.1. Correction of distortion in automatic mode

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Distortion is the objective-induced warping; it depends on focal distance. Distortion can be corrected automatically. EXIF data is used to detect the manufacturer and type of the camera, as well as the focal distance used to take the photo.


After calibration, distortion correction can be processed automatically using the focal distance of another image and the correction parameter for the camera determined during calibration.

If the automatic mode of distortion correction is unavailable, you can calibrate your camera by yourself. You should print a test image and then take calibration photos using two extreme focal lengths — the shortest and the longest. Distance from the camera to the taken image should allow the image to be fit into the photo frame.

Run Perspective Pilot and process the distortion correction. Then press Alt+F2 and the distortion coefficient for the camera and focal length indicated in the EXIF data will be saved in a PerspectivePilot?.dis file. For more details see the "Camera calibration” demonstration example.

2.2. Manual correction of distortion

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When processing an image with Perspective Pilot software, you should trace control lines along curved objects. The farther straight objects are placed from the center, the more they are curved. Therefore, when correcting distortion, you should choose the longest straight lines at the edge areas of the image.

Original image     Corrected image


Steps for distortion correction:

  1. Turn on the manual mode of correction.
  2. Set the distortion coefficient so that all curved lines become straight while processing the image.
  3. Process the rotation and perspective transformations.
  4. In the final step of correction, you should press the Refinement button. Then, using the control lines, check if all straight lines on a photo are really straight. If not, process the distortion correction again.

3. Image rotation

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For rotation processing you should trace a line along a vertical or a horizontal object.

Original image     Corrected image

To determine whether the image requires canting correction, check the slope of objects in the central area of the photo. Objects placed in the central area of the photo are less distorted.

If the central vertical/horizontal objects are not canted, then canting correction is not required. Slope of the vertical or horizontal objects in the edge areas of the image can be caused by perspective and/or distortion warping.

If the central vertical/horizontal objects are canted with the same slope angle, you should process the canting correction. Before rotation processing, we recommend measuring the angles of the horizon canting and of the slope of a central vertical object. For correction you should choose the object with the least slope angle. Trace the control line along the object and the program will rotate the image so that the line will be straight (vertical or horizontal). For canting correction, it is enough to trace a line on the photo that should be horizontal or vertical.

There can be several horizontal and vertical lines with various canting angles on a photo. For the canting correction you should measure the angles of horizon canting and of a vertical object in the center. Trace the line with the lesser variation off the vertical or horizontal value as a control line for correction.

If after rotation processing the lines indicated as controls have not become straight (vertical or horizontal), you can cancel the processing and then repeat the correction.

In the example photo (below), the image is warped by perspective distortion and canting. Corrections to the photo should be applied in the following sequence.

  1. Trace control lines along the horizon line and the vertical object. Measure their canting angles. Canting of the vertical object is less than that of the horizon line.
  2. Apply the rotation correction, indicating a line along the vertical object.
  3. Apply the perspective correction, indicating the horizon line.

Original image     Corrected image

4. Adjustment mode

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While drawing a control line, press the Ctrl button and the line will be adjusted on the image automatically. The program detects the most contrasting edge of the object and the drawn line is adjusted as if it sticks to the object. Using the Adjustment mode makes the user's work easier and image processing faster.

5. Perspective correction

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If the photo image of a high building was taken from the ground near the building, walls can appears too inclined. In this case the perspective correction of the image on the base of vertical lines of the building edges is required. For perspective correction, you should trace control lines along the object you want to correct the slope of. For better result, trace lines in the edge areas of the photo. In general, for perspective correction it is enough to trace two vertical or two horizontal lines.

Original image     Corrected image

While tracing the control lines, use the Ctrl button to adjust the lines along edges of objects.

Sometimes it is better not to correct inclined lines to straight (horizontal or vertical). A perspective coefficient allows you to adjust the degree of straightening of the inclined lines to make your photos more natural.

Note that if you trace the control lines on a photo along the object that is inclined in reality (Pisa tower, for example), the processing of perspective correction on the basis of your control lines will cause image warping.

The perspective correction may be canceled and then processed over again.

6. Completion phase

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The resulting image may be cut and zoomed. After pressing Refinement, the program processes the color adjustment. When you save the corrected image, the color adjustment is processed automatically.

 

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