Skridlov
Posts: 104
Location: United Kingdom
Registered: 12 Nov 2007
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Line 3 cp's - clarification required.
Posted: 27 Mar 2008 at 18:25 GMT
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I have had some minor success in rectifying a couple of bad stitches by assigning "new line" cp's however I'm baffled by how these work.
I have tried two methods.
Method 1)
Assigning a cp "n" on the left frame and simply accepting the auto-location of "n" on the right. Then assigning another line 3, "n+1" at another point on the same notional line, in the same way, leaving both auto-positioned.
Method 2)
I assign point "n" in the left frame, then moving its auto located point "n" in the right frame further along the line, so that they are on different pixels. I then repeated the same process with "n+1".
I cannot find any indication which method is correct, and the results of the two methods when tested simply add to my confusion. With either method the optimiser frequently gives errors in the range 50 - 100 for these cp's - but not always. In the case of method 2, the technique rectified the stitching error in one location but not in another.
Could someone please let me know how these cp's should be used as I am practically at my wits end after days of trying to get decent stitching.
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John Houghton
Posts: 2018
Location: Hitchin, United Kingdom
Registered: 17 Jan 2005
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Re: Line 3 cp's - clarification required.
Posted: 27 Mar 2008 at 19:03 GMT
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Method 2 is the correct one. The line feature that you put all the line 3 points on should be one that is expected to be straight in the output panorama image when correctly rendered. In an equirectangular image, that would be a vertical edge or the horizon (when the panorama is levelled). Only use PTOptimizer for this purpose as t3, t4... points are not supported by PTGui's own optimizer.
John
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Skridlov
Posts: 104
Location: United Kingdom
Registered: 12 Nov 2007
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Re: Line 3 cp's - clarification required.
Posted: 27 Mar 2008 at 19:28 GMT
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Thank you John
Hmm. Not sure that I understand this. I know about the h and v cp's for alignment of h and v but I thought that the t3/4 points were simply for aligning connected points on a line - irrespective of whether it's horizontal or not. I am trying to eliminate persistent stitching errors by forcing alignment. Is this an incorrect use? It has actually worked but not consistently.
I feel as though I am going backward as I seem to get no closer to achieving stitches that don't require lots of hacking about with Photoshop.
I seem to recall that there is a thread on which you described the correct sequence for use of the optimiser. At present I'm simply using defaults. I am puzzled by the way that re-optimising sometimes causes horrible regression to worse results than on previous passes despite minor changes.
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John Houghton
Posts: 2018
Location: Hitchin, United Kingdom
Registered: 17 Jan 2005
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Re: Line 3 cp's - clarification required.
Posted: 27 Mar 2008 at 20:13 GMT
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The rule for line control points is exactly as I stated. You can only attempt to align straight line features when they are straight in the output panorama. In a rectilinear image, all straight line features are preserved. In cylindrical and equirectangulars, only verticals and the horizon should remain straight; all other straight lines become curved, so tn points should not be used on them. Very small lengths of a curved line might be considered straight so you might reasonably use tn points there, with caution, to align those features in two images.
Using the optimizer is more of an art than a science. Remember that it works by trying to improve the current state, whatever that is. Always give the optimizer as much help as possible. If you know approximate values of various parameters - give this information to the optimizer. In particular, produce a template with calibrated lens parameters together with image parameters for your standard shooting setup. Stitching will then be much easier, quicker, and more reliable.
Some threads that might help: (You may need different starting fov and crop for your particular lens)
www.panoguide.com/forums/qna/3004/
www.panoguide.com/forums/qna/4299/
John
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