Hi everybody, I got curious about how radial correction is computed and did a small test.
>From Helmut's description (remember, he implemented the code): http://www.all-in-one.ee/~dersch/barrel/barrel.html I learned that the correction is dd + cr + br^2 + ar^3 (found in the function radial). Noticed the first parameter dd. In the page above is referred as d, but d in panotools has a different meaning. As he explains, to avoid scaling the sum of dd + c + b + a = 1. This is supported by the code. dd is computed from a, b and c to satisfy the invariant. Now, look at the following test: http://turingmachine.org/~dmg/temp/test_grid.zip It looks as if, in the case of the 2 rectangular images, there is scaling, but not in the squarish image. In the code, the scale of the image is normalized as 1/2 the smallest of dimensions. This means the image is not scaled with respect to the longest dimension, but it is scaled with respect to the shortest dimension. Now, this example is a bit extreme because the ration width to length is 2. But even if it was 1.5 (as with 36mm cameras) the different is significant. I suspect helmut did this because most people shoot panoramas with the camera in portrait mode. This way the "horizontal" field of view, remains unchanged after the correction has been applied. With this information we should be able to computer a, b and c parameters from imatest data (such as the ones published at photozone: http://www.photozone.de/canon-eos/336-canon-ef-35mm-f14-test-report--review?start=1) What about the samyang 8mm lens? This radial correction is made before the photo is remapped to its native projection. One would need to either: calibrate the lens using the typical methods, or do some math and approximations to come to the estimation of these parameters based on the formulas above. Remember, the lens is closer to stereographic (which we have implemented in panotools), do not use for this one a typical equidistant. Samyang should donate us a couple of lenses to be able to properly test the code. It has never been used. --dmg --- Daniel M. German http://turingmachine.org -- 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
