blog added 08/11/2006

Enblend Issues with Zenith and Nadir Photos

In a recent maillist posting panoramic photographer Hans Nyberg alerted me to the fact enblend doesn't actually blend the zenith or nadir photographs.

To quote the enblend web site about one of the command line options:

-w         Blend around the -180/+180 boundary. Useful for full-360 panoramas. Enblend currently does not blend the zenith or nadir, so you may still see some seams in these areas.

Now one of my previous posting about enblend testing (see left links) I spent a long time trying to fix enblend errors. The above fact tells me I was trying to fix the impossible.

Below are a few images that illustrated that indeed enblend does cause seams.

1) Here is a PTGui stitch using enblend

2) Here is one the seams it creates at the zenith edge
3) Here is a PTGui stitch not using enblend just PTGui's internal blending tool - clearly this is very very good since there are no seams in the image at all.

4) Here are the two images over-lapped in Adobe Photoshop and layers switched to show image layer differences, black regions means identical layers but other regions show differences between the layers. You can just about see that the blending at the nadir is very different.
5) If I zoom in on the zenith area you can the enblend and PTGui internal blend are identical in most areas apart from the seams problems.
Now this has a huge impact in what stitching software you use for the these fisheye 360 degree panoramas. Both PTMac and Realviz Stitcher Unlimited 5.5 require enblend to get any sort of blended panoramas. Sadly therefore both PTMac and Realviz Stitcher will always therefore have seams in the zenith and nadir.

Note. PTMac at the moment doesn't produce as automatically a good as result as PTGui because the routine to create control points are different. PTGui control point are automatically spread better and thus the lens distortion parameters are better (the FOV, a, b, c, and d values) for PTGui and thus enblend is not required to fix the overlaps. From what I can make out Realviz Stitcher only optimizes the FOV and a parameter of  a fisheye lens and thus enblend is required to fix all the overlap distortions, so for Realviz Stitcher the zenith and nadir seams will appear.