Fixing keystone effect with projection editor
A tall building leaning backwards on a photo is a prime example of the keystone effect. Sometimes the only way to capture both top and bottom of a tall building from the ground is to tilt camera back. This causes parallel wall sides to converge on the photo similar to the sides of a road converge on the horizon. With a rectilinear lens, parallel lines in the world remain parallel on the photo only if their plane is perpendicular to the lens optical axis.
One way to avoid the keystone effect is to use a perspective control (also called tilt-shift) lens. Another is to perform a software correction.
Perfectly rectangular buildings become trapezoidal on photo when the camera is tilted back. It is the shape of the keystone, hence the name of the effect. This example is a vertical two frame stitch.
One of the frequent use cases for photo stitching is architectural photography, because a very wide field of view may be required to fit the whole building into one image. The keystone effect is likely to be present on a such photo. Fortunately there is an easy way to to perform a software correction in a stitched image with Panorama Stitcher — the projection editor. To perform the correction:
- Open the projection editor.
- Drag the vertical handle until the parallel (in world) lines stop to converge on photo.
- Optionally drag the horizontal handle to improve the horizontal lines.
The same two frame stitch as above, with keystone correction applied in projection editor.
When the rectilinear projection is selected the horizontal and the vertical handles have an effect similar to the tilt-shift lens movements, reproduced in software. It is important that it will be much harder to perform the correction with a similar accuracy after the export of a panorama. Firstly it requires knowledge of intrinsic image parameters (field of view of a panorama is larger than that of a single image; exported EXIF focal length is the same). Also performing the correction after the export adds another image resampling pass, which always degrades the image quality. That is one of the reasons why a projection editor is an essential, must have feature for image stitching application.
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