Extract and insert rectilinear views
Intro
One of the most problematic points when shooting spherical panoramas is the view straight down also called the nadir view. Either the tripod is visible or there may be alignment or color mismatches if shooting handheld.
Unfortunately neither the nadir nor the zenith (straight above) area can not be edited directly in the equirectangular image since it is very distorted. For this taske we need to extract a partial view - best in rectilinear projection.
Possibilities
There are basically three possibilities to extract partial rectilinear views:
- PTEditor
- Panotools Plugins
- PTStitcher
PTEditor
PTeditor is a Java application provided with the standard Panotools distribution. It should run on any OS where Java and the panotools runtime (pano12) is installed. There is a nice tutorial on How to use PTEditor. This is the most convenient way to extract and insert any arbitrary rectilinear view. The disadvantage is that it doesn't save 16 bit images.
Panotools plugins
The Photoshop compatible Panorama Tools Plugins are part of the standard Panotools distribution as well. There is an upgraded version available from Thomas Niemann that corrects some bugs and works for 16 bit.
The plugins contain the Adjust filter, which is capable of extracting and inserting any view in almost any projection. Any data (pan/tilt angle, size of view etc.) has to be inserted manually. Yaw, roll and pitch value have to be calculated or estimated which makes the usage a bit inconvenient.
(More text of an experienced plugin user needed here)
PTStitcher
PTStitcher can be used to extract and insert partial views in different ways. Either by one of it's GUIs (PTGui, PTAssembler, hugin, PTMac...) or by directly scripting it. PTStitcher supports 16 bit images.
Scripts
There are sevaral scripts or batch files available that extract 6 cube faces from an equirectangular image. One of the most convenient windows batch file is written by Eric Gerds and part of his DOS Utilities for Panoramas. A less convenient batch file is found on Erik Krause's page. PTStitcher scripts are found on Ben Kreunen's page
GUI
All of the GUIs can be used to extract an arbitrary rectilinear view from a spherical pano. Use the equirect image as input image with FoV of 360�x180� and specify a rectilinear image with 90� as output "panorama". Set all lens correction parameters to 0.0. Then change yaw and pitch of the input image until you get the desired view. Note that the values work the other way round than when inserting an image.
PTGui
PTGui offers an additional advantage: The panorama editor and numerical transform. To use it load your image as described above, open panorama editor and choose 'Numerical Transform' from the edit menu.
To pan along the horizon in 20� steps insert 20 into the 'Yaw' field and press 'Apply' repeatedly until you get the desired view. Positive values pan right, negatives left.
Do the corresponding for 'Pitch' and 'Roll' if needed. This way you can extract any partial view. Once your are done create the "Panorama" (which is the desired partial view) using output format TIFF. Note down or save the exact values for Yaw, Roll and Pitch as shown on 'Image Parameters" tab page.
Now edit the partial view to your need. Then add it to the project with your original panorama and on 'Panorama settings' tab change to equirectangular and 360�x180�. Choose individual lens parameters for the new image on 'Lens settings' tab and set 'Rectilinear' and 90� on 'Image parameters' tab. The image should fit now perfectly in the original panorama.
In order to move the panorama back to it's original position go to panorama editor again and choose 'Numerical transform'. Now Enter the exact values for Yaw, Roll and Pitch you previously saved or noted down. The panorama should move back to it's original position now.
You can create either the complete panorama or the partial view only. Choose the size of the original panorama for output.