On 8 April 2011 09:38, Yclept Nemo <[email protected]> wrote: > > Yes I did manage to borrow a panoramic head; even without out, since > the lightpost is about 50-75 feet distance I doubt parallax would > cause any problems.
It should be straight-forward to calculate the maximum parallax error (in pixels) based on an estimated upper bound in the change of position of the no-perspective point and the angular distance between pixels at 28mm. But yes: I would have thought that that would be sufficient, even if the control points were quite a lot further away and the pano head wasn't exactly calibrated (i.e., not rotating precisely around the NPP). > I use a cheap Canon EFS 18-55mm lens which I calibrated specifically for 28mm I think I would be worried about not being able to re-set the lens to exactly the focal length it was calibrated at. On my (relatively cheap) lenses, even if I were to tape the zoom ring to the body so it couldn't rotate between calibration and shooting (or equivalently make sure it is hard against the 18mm stop), there is enough play in the mechanism that I would not expect to get good results without optimizing (unlinked) v. I don't generally find it necessary to unlink a, b, and c, though, although I do include these in the optimisation. Aside: I admit that I'm not really sure I see the point of being able to save/load lens parameters when I can just include these in the optimisation. Is there any advantage, other than perhaps saving some CPU time? I would think think that, except for prime lenses, any reduction in optimisation time would be at the risk of the lens not actually being at the same focal length that was used for the calibration. > Anyway, is it possible to produce a perfectly aligned 360 degree > panorama? I've been trying really hard - many different strategies - > and am unable to get rid of artifacts. I have shot and aligned only one full 360-degree panorama, and it was from handheld, and despite this I did not have much trouble getting things to align sufficiently for there to be no visibile artifacts. Here is the strategy I used, which I've used before on several other less-than-360 panoramas with good success: - Shoot three bracketed exposures of each of 21 stacks (9 stacks around the equator) (from the middle of a large, cobbled public square). - Load 63 images into Hugin. - In the image list and in the preview, select the middle-exposed image from each stack (#1, 4, 7, ... 61) and use cpfind to create control points. - Optimize (y, p, r) to start with, with "Only use control points between image selected in preview window" ticked. - Manual control point editing: fine-tune all; delete all control points in sky and most on the cobblestones in the foreground (large parallax errors due to hand-holding); delete a few more down an alley (again, parallax errors); add quite a few manually between overlapping images where cpfind did not do a good job (probably about 25cps on each of a dozen different overlap pairs). - Optimize (y, p, r, v (unlinked), a, b, c (linked), d, e (unlinked)) and continue to examine and adjust or delete poorly-aligned control points until errors are relatively small (average error = 0.85, maximum error < 3.8). At this point almost all the control points are on the fronts of buildings around the square. - Do test stitch to check alignment. No problems - even with cobbles in foreground enblend has done a good job hiding misalignments. - Now, select first stack (images 0, 1 & 2) and use Align_image_stack linear to create control points. Repeat for each additional image stack. - Create control points manually for images where Align_image_stack did not do a good job (usually due to my having allowed the camera to roll slightly between exposure, which usually results in control points being clustered around centre of image with few at edges). - Select all images in preview window, and optimize the same parameters as before (y, p, r, v (unlinked), a, b, c (linked), d, e (unlinked)) to bring everything into alignment. - Stitch. Add masks to deal with moving people, birds, etc. Repeat. There are probably several ways I could improve this process (e.g.: I'm not sure it's necessary or particularly useful to select images in groups of three when using Align_image_stack). In general, I find the strategy of having only the middle-range exposure from each stack linked to adjacent images, with lots of auto-generated control points between the images in each stack provides good results while keeping the total number of control points to a reasonable value (~8k, for the panorama described above). Christopher -- You received this message because you are subscribed to the Google Groups "Hugin and other free panoramic software" group. A list of frequently asked questions is available at: http://wiki.panotools.org/Hugin_FAQ To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/hugin-ptx
