Willy, Frans, et.al., Willy suggested:
> ... Photograph a drawing of a vertical sundial >and compare the drawing with the corrected photo. .... . Might I suggest a test photo of a Cartesian grid (or a checkers-, or chess-, board? The analysis would then be simpler and more generalized. Also, this subject is covered in textbooks on use of aerial photography for mapmaking. In English, the topic is called 'photo-rectification'. Sciagraphically, Bill Maddux > Frans, > > You can answer your own question. > Photograph a drawing of a vertical sundial and compare the drawing with the > corrected photo. > > Kind regards. > > Willy > > > > "Frans W. MAES" wrote: > > > Hi All, > > > > John Carmichael wrote: > > > > > But I discovered that by using digital editing, you can stretch or > > > compress a photo so that it appears that camara was directly over the > > > dial! I discovered this while using the "perspective" and "distort" > > > features of Adobe Photo Delux. > > > > I sometimes apply the same trick, using Paint Shop Pro. Starting > > with a picture of a rectangular dial face taken at an arbitrary angle, > > this involves 4 steps: the horizontal and vertical perspective tools are > > used to make the sides parallel, then the horizontal and vertical > > skewing tools are used to make them parallel to the picture frame. > > > > My question, however, is: does this procedure guarantee to yield the > > correct result? That is: is the resulting height/width ratio equal to that > > of the original? If not, angles between hour lines would be distorted. > > As a consequence, it would be impossible to check the correctness > > of the hour line layout, or to calculate the latitude for which the dial > > was designed. > > > > Kind regards, > > Frans > > > > ===================================== > > Frans W. Maes > > Peize, The Netherlands > > 53.1 N, 6.5 E > > www.biol.rug.nl/maes/sundials/ > > ===================================== >
