Re: arc headlights and dynamos - A bit more research

Posted by John Greenly on Jan 30, 2020; 2:46am
URL: http://c-sng-discussion-forum.254.s1.nabble.com/arc-headlights-and-dynamos-tp15165p15211.html

Todd,

thank you for finding these photos!  Your #12 was a beautiful model.  Makes me wish I was working in 1/4" scale instead of HO.  I love the "intermediate" tender.  

As to deriving dimensions from an oblique photo view, I agree with what you said exactly, if you have no other information it's impossible to find the actual dimensions.  But we do have other information, various dimensions on the engine that are known.  I won't go into a big lecture here, but if you know the dimensional relationship among four coplanar but non-collinear points in an obliquely oriented plane in a photo then you can exactly determine the position of the camera with respect to the object, and then derive the positions of all other points in that plane.  And with additional information in orthogonal planes, you can in some cases unfold all positions in three dimensions.  We have such information, for instance the size and spacing of the drivers on the locomotive, and also a number of other dimensions,  so we have redundant checks on accuracy.  It's laborious, not always possible to carry through, and its accuracy depends on the specific configuration and how much dimensional information you have.  I didn't do a complete job on the tank position on #12.  I said the tank position was "about" 5:9 between the domes.  That's not just the direct measurement of those lines I drew, it includes a first-order correction for the oblique perspective, using the drivers for reference.   I know from other experience that this correction should give the ratio within better than 5% of the actual value.   With more work it would be possible to get those dimensions within an inch.  With a photograph of this quality, and knowing enough reference dimensions, it's even possible to measure and correct for lens aberrations in the camera.  If you have multiple photos of the same object from different angles, that's even better.  

anyway,  
Cheers,
John