Extracting Rectilinear Images from Panoramas Helmut Dersch der@fh-furtwangen.de

Panoramic images employing cylindrical projection can be generated in a variety of ways:

Direct capture using scanning slit cameras (Roundshot, PanoScan, etc.).
Direct capture using Swing Lens camera (Horizon, Widelux, Noblex).
Warping and stitching of normal images using software (eg Apple QTVRAS).
Rendering using 3d modeling software (eg Povray).

Often, perspectively corrected rectilinear partial views of these panoramas are wanted. This article describes how these can be extracted using free software (Panorama Tools). Image extraction uses high quality interpolators preserving the quality of the original photograph and making them suitable for printing.

An Example

The following image is a 360° panorama generated using the Roundshot camera equipped with a 14mm lens. (original photograph by Julio Moya, j_moya@idecnet.com)

To extract partial views, select a viewing direction by choosing proper yaw and pitch angles, in this case we use yaw=-115° and pitch=0°:

Then decide which field of view you wish to show: 50° corresponds to normal vision, whereas 104° is rather wide. Finally, you have to choose pixel dimensions for the extracted image, ie width=500 and height=300.

Now open the panoramic image in Photoshop, choose 'Panorama Tools' and 'adjust'. Push 'Prefs' and select options (b) and (c). Then, back in 'adjust', select 'extract' and 'use options', push 'set' and fill the dialog as shown below:

Clicking 'OK' twice initiates the transformation and results in the upper left image below. The other images were extracted using different yaw angles, and the last image corresponds to a camera tilted down by a pitch angle of -45°.

Notice that the lower left image runs across the seam from left to right, and some retouching may be required before printing. Finally, other image dimensions and fields of view are equally possible as shown by the "portrait"-images below:

Some Additional Comments:

If the lens used for image capture shows distortions, or if the scanning speed is not properly synchronized to the focal length of the lens, then the extracted images will exhibit barrel or pincussion type distortions. These can be corrected using the 'correct'-option in the 'adjust' submenu, see above. You have to find suitable correction parameters as outlined in this article.
For Swing-lens cameras field of view is smaller than 360°. Typical values are 120-140°. Enter these values in the 'Panorama->HFOV' field of the 'adjust' submenu.
Partical views can be extracted from other formats also, eg rectilinear, fisheye and equirectangular projections. Choose the appropriate options in the 'adjust' submenu above.
Using the interactive Panorama editing feature of 'Panorama Tools', you can shift the viewport across the Panorama similar to an interactive VR-viewer with the options to zoom in and out. At any time you can save or print the current viewport. See the Readme file of 'Panorama Tools' about this feature.