Dave Kennard
Posts: 91
Location: United Kingdom
Registered: 26 Mar 2006
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Problems stitching nadir and zenith
Posted: 7 May 2008 at 18:58 GMT
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My panohead got a bit beat up last year, and the lower rail is slightly bent, so I checked today to see if would still work okay. I determined lower rail position by shooting straight down at the bolt until the center of the bolt was at the center of the image. I determined upper rail position by shooting through a window and moving the camera forward and backwards until there was no parallax.
After this I did a test shoot of a messy bedroom. First I stitched just the 6 around shots. PTGUI gave avg. cp distance of 1.4px and max cp distance 4.2px. Not great, but I couldn't see any errors in the stitched image.
Then I added the nadir (shot straight down on the panohead, not handheld) and the zenith, manually assigned cps and re-optimised. This gave avg cp distance of 5.0px and max distance of 18.6px. After stitching I didn't see any errors round the center or near the zenith (the ceiling was pretty plain so probably masked any errors there), but there were errors at the nadir.
So, is it likely that these errors are due to the lower rail being bent?
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John Houghton
Posts: 2011
Location: Hitchin, United Kingdom
Registered: 17 Jan 2005
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Re: Problems stitching nadir and zenith
Posted: 7 May 2008 at 20:57 GMT
updated: 7 May 2008 at 20:58 GMT
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Dave, Does the bend in the lower rail cause the vertical arm to be no longer vertical? If so, it would probably not be appropriate to centre the bolt in the image frame.
John
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michael medina
Posts: 142
Location: United States
Registered: 27 Jan 2008
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Re: Problems stitching nadir and zenith
Posted: 7 May 2008 at 23:28 GMT
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shoot just tilted down enough to catch the rotator in the photos, if it lines up without optimizing for shift, then you are on the spot, if it looks like a star, then you are off a bit.
if you are not vertical by quite a bit, then doing it this way could give you a slight error, but it would have to be quite large.
if you bent it, i don't think it should make much difference when optimized properly
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Dave Kennard
Posts: 91
Location: United Kingdom
Registered: 26 Mar 2006
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Re: Problems stitching nadir and zenith
Posted: 7 hours ago
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The vertical arm appears to be pretty straight, but the horizontal arm is a bit twisted:
Taking 6 shots around at -30°, the rotator is slightly star shaped, here's an extract from the stitched pano:
I'm not sure if the parallax is due to the bent lower rail or me doing something wrong.
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John Houghton
Posts: 2011
Location: Hitchin, United Kingdom
Registered: 17 Jan 2005
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Re: Problems stitching nadir and zenith
Posted: 6 hours ago
updated: 6 hours ago
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Dave, A rectilinear nadir view shows better what's going on, but it looks like you have a circular saw effect, with teeth that cut in an anticlockwise direction. This means you need to move the vertical arm in towards the pivot by a small amount - a mm or two. If you go to far, the sawtooth will change cutting direction. If you have eliminated parallax, and the sawtooth is minimal, then you should get a good stitch. If you don't, then your stitching technique might be found wanting.
John
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