This tutorial covers the final stage in the preparation of a full 360x180 degree VR panorama: the patching of the "hole" at the nadir with either a hand held shot, or one taken with the camera still on the tripod after shifting it sideways to give an unobstructed view.  If you want to play along, there's an accompanying download containing all the images together with a project file saved at the point when the optimization of all the images apart from the hand held nadir has been completed. Download from:

http://strana360/ahtml/downloadsptgvpt.zip(14MB)

PROJECT SETUP

Unzip all the files to a folder of your choice.  Apart from the project file (ptgvpt.pts) there are 10 fullframe fisheye image files:

6 around at pitch = 0
1 up (the zenith) at pitch = 90
2 down (nadir shots at pitch = -90 with the camera on the pano head, taken at right angles)
1 down taken with the camera hand held and the tripod moved out of the way

There's also a fully completed project file - ptgvpt-final.pts.

(Another shooting possibility is to tilt the horizontal row down by 10 degrees to reduce the size of the nadir hole and dispense with the two down shots at right angles).

In order that the blending will work properly, it is necessary to add alpha channel masks to the three down shots.   The masks will exclude unwanted parts of the images - like the tripod/pano head and my feet.  As the jpeg file file format does not support alpha channels, please add them yourself and save to tiff format, as described in this tutorial: http://www.johnhpanos.com/alphatut.htm

The masks should look like this:

Note that in the case of the handheld nadir (on the right) that will be used with the viewpoint option, it is necessary to mask anything that is not part of the flat plane of the paved area (as these parts are likely to be badly affected by parallax effects after the viewpoint transformation). Alternatively, if using the PTGui blender, you can adjust the blend priority of the nadir image via the Image Parameters tab to restrict the blending to a circular area in the middle of the frame, thereby excluding the outer parts of the image.

The supplied project file assumes that this masking has been done, and will expect the nadir images as tiff files.  If you want to proceed without doing the masking, edit the project file with a text editor such as Wordpad to change the three .tif image file names to .jpg.  You can then perform the project optimization with the viewpoint option to see how that works, but stitching will probably not produce a well blended result.

LOAD THE PROJECT

Launch PTGui Pro and open the project file ptgvpt.pts (not the final version) with File->Open

Note that the hand held nadir shot is not initally included.

Select Advanced mode on the Project Assistant tab and select the Optimizer tab. Use the Advanced mode there too:

Click on the Run Optimizer button to confirm the current optimization state:

Now select the numerical transform option (the 123 button) to bring the nadir to centre stage:  set the pitch to -90 and click Apply.  The Panorama Editor display (equirectangular projection, 360x180) will look like this:

Note that this transformation is mainly necessary in order to work around a problem with versions of PTGui Pro up to V7.8.  Viewpoint optimization works more reliably with the nadir at the centre of the output area.  This is not necessary in Version 8.

ADD THE HAND HELD NADIR IMAGE

Select the Source Images tab and use the Add button at the bottom to add the hand held nadir image.

Select the Control points tab and add five points between images 7 and 9, like this:

Select the Optimizer tab and uncheck all the present lens and y,p,r settings, and then check y,p,r for the nadir image as well as all boxes in the "Use control points of"  table.  Like this:

Click on the run button to optimize the nadir roughly into the right position.  A poor result will be reported because it wasn't taken on the pano head with the others:

This is to be expected and is nothing to worry about.

THE VIEWPOINT OPTIMIZATION

Now additionally check the viewpoint box for the nadir image.

Repeat the optimization (you must use the PTGui optimizer for this, BTW, not the Panorama Tools optimizer). Magically, a good result is now reported!

Occasionally, the image might not snap into good alignment at the first attempt.   If that should happen, try selecting and manually aligning the nadir image on the Panorama Editor window, and then try another optimization.

With a good result, you now need to restore the panorama to its normal orientation by applying pitch=90 with the numerical transform option.

STITCHING

All that remains is to stitch the panorama.  Blending with Smartblend gives an excellent result, with no visible joins in the nadir area.

John Houghton
25/02/09