Hi Raul, I would use Lowe's SIFT (Scale Invariant Feature Transform) for that, see Wikipedia. I just tried it on an "arbitrary" image: selected a part of it and scaled that down by 0.4321 using bilinear interpolation. Then used SIFT to extract key points from the original and the scaled-down selection and 'match' (a little program that comes with SIFT) those: perfect!
The only issue might be the 'not too slow'; SIFTing my 900 x 600 image takes almost 2s on an i6 laptop. If you want you can mail me one of your image pairs and I will try. Greetings from Sydney, Ben On Tue, Apr 30, 2019 at 8:03 AM Raul Miller <[email protected]> wrote: > I've got a situation where I've got some large images (maybe on the > order of 4000 pixels square - though many are not square) which have > had crops made from them which are quite a bit smaller. 1200 pixels > wide and 600 high is one fairly common example. > > These crops were taken from the original, and with a reduced pixel > count (that 1200x600 might represent a 3000x1500 pixel rectangle in > the original image). > > Is there a reasonably robust and not-too-slow technique to extract the > pixel coordinates representing the crop, given the two images? (I > think in all cases that matter, the size reduction was the same in > both height direction and width direction. Also, in all cases that > matter, the crop was reduced in size, not magnified.) > > I am also interested in verifying that the coordinates are correct, > but I expect that that can be done by creating a fresh crop using the > coordinates and dimensions and comparing that with the original crop. > > (I don't think fourier transforms would get me where I need to be. I > wonder if some sort of wavelet variation might?) > > Any clues appreciated... (executable code, if you have it,, would be > great, but this seems too specialized for that to be a likely > possibility). > > Thanks, > > -- > Raul > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
