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Robert D Feinman

Correcting Cylindrical Distortion

 

Images made from swing lens cameras such as the Noblex are subject to cylindrical distortion. This is caused by straight lines being projected onto a curved surface and is especially noticable if the camera is tilted up or down.

Straight lines below the horizon curve up in a "smile" and those above curve down in a "frown".

This image shows the effect of pointing the camera down and twisting it slightly counter-clockwise when taking the picture.

Notice the curve of the sidewalk.

Original image
Original Image

 

By using the panoramic tools plugins available at http://www.fh-furtwangen.de/~dersch/ we can correct the distortion and straighten the curved lines.

Select the Panorama Tools - Adjust filter. Select the extract option and then press the set option. This will bring up the dialog box shown.

Set the Image options as HF0V=127° and set the format to Rectilinear. Set the width and height to 1.8 times the size of the original image. This will produce a warped image which has the same vertical size as the original after cropping.

In the Panorama section set HFOV to 127° and the format to QTVR Pan.

Adjust Filter Settings
Adjust Filter Settings

 

After running the image a new file is created which is stored in the folder you specify in the prefs dialog box of the filter setup.

Since we set the size to be 1.8 times the original this image is almost twice as large.

Because we took the original picture with the camera tilted and twisted we corrected for this by using the pitch and roll settings in the previous step. Pitch controls the amount the camera was pointing up or down while roll controls the rotation from horizontal. You can also change the apparent direction that the camera was pointed at by means of the yaw setting, but this may increase distortion in the corrected image.

Since it is hard to determine how much correction is needed in these settings without trial and error it may be easier to leave them set to 0 and fix the resulting image with the Photoshop Transform - Perspective control.

Straightened Image
Straightened Image

 

The final image with the black areas cropped.

The image is the same height as the original, but has been stretched horizontally.

Because the center of the image was kept the same size the sides have expanded vertically reducing apparent sharpness and producing "wide angle" distortion.

There is also a loss of image due to the cropping, but the buildings and the sidewalk are now almost straight. If you plan to use this technique it is best to keep important parts of the image away from the corners.

There is, after all, no free lunch.

Straightened and Cropped
Straightened and Cropped

 

If we wish to present the image as an almost conventional one we can simply remove the less interesting detail at the sides and restore it to a more typical aspect ratio.

The image has been reduced by 50% to make it easier to see all at once.

I find it is better to straighten the complete original image using the settings shown rather than cropping the original and trying to estimate the HFOV and scaling factor.

Be advised that this filter can take a very long time to run. If a 35mm image is scanned in at 1600dpi for printing at 300dpi the image will be about 5 inches tall by almost 28 inches long after correcting. This is a file of about 40Mbytes. On my machine the filter takes almost an hour for this size image.

Final Cropped
Trimmed and Resized for Display
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© 2001 Robert D Feinman