Lessons learned and conclusion
The bad side :
The two following statements which could be extracted from a course to make panorama by stitching several fisheye pictures are contradictionary :
- Two adjacent images are shot with constant settings (including focal length) from a fixed point and toward two different directions such that the two pictures shall overlap. The camera shall be rotated between the shots around the Nodal Point in order to avoid parallax errors.
- The Nodal Point is located along the optical axis at a position which depends on the angle of rotation (2 * ß).
Only one set of conditions fulfill completely both requirements :
- Two hemispherical (or more than hemispherical i.e. FOV >180deg) taken exactly in opposite direction.
- The rotation of exactly 180deg is performed by turning around the nodal point corresponding to ß = 90deg.
- The two pictures are then stitched at their common limit of the two hemispheres.
This is a method which has been patented in the US and furiously protected by a commercial venture....
This method limits the resolution of the panorama and the overlapping area is rather small and therefore requires some skill to make good panos.
Approximation of this method is presented there for 2004 DSLR...
The bright side :
Everybody now knows where this elusive Nodal Point is located for a given rotation (2 * ß) of the camera and better control against parallax errors is possible.
Parallax errors have possible effect
- When an object is closed to the camera lens and
- when in the region of overlap, one part of this object is on one picture and another part on the adjacent picture and
- when the junction of the two parts is away from ß angle for both pictures.
When such an occurence is unavoidable during a shooting sequence even when it is rekognized, then take another picture centered on the object to be used as a patch.
In some cases the only solution is Creative Alteration in a suitable picture editing software :
- The object is long enough as to extend on more than two adjacent pictures,
- The patching technique is only partially successfull,
- The masks from PTSticher do not finally produce expected seam areas (in the case of a multi-row and complex arrangement or composite assembly of pictures with different formats for exemple).