How oddly wad...
I log in to gimpchat.com and I am randomly (I think, unless goolgol's ai has been perfected at last) offered an introductory link to: "Spherical Zoom."
Now--and here's where the intriguing mysteriousliness is--I was visiting preCISEly because I've been trying to do the nearly same thing, but with G'Mic Cartesian-transform deformation.
Now that's eery and I don't mean Pee-Ay either icantellyourightnow.
Anyway, I'm not interested in an "effect" as much as I am in a by-golly reversible tranformation. What I've got is a map of the lunar surface (from USGS), reportedly, and somewhat intelligibly in cylindrical coordinates. I want to cut the rect. pic into two square "hemisquares" and then transform each into a hemisphere.
I played with Filters->LensEffect, but it proved to be that, an effect.
Then I remembered G'MIC's underlying math engine, and after some piddly-diddling around, I found "Cartesian transform" under "Deformations." (It helped that the default already has an equation visible in it...)
After more p-d'ing around trying to construct something by experimentation, I finally drug out the trusty paper and pencil and did some trig. (remember trig?) I don't think I understand, and I even think I don't understand, what G'MIC is up to here. I thought it was going to use my transformation equations to calculate a new (xn,yn) position for the pixels located at old (xo,yo) positions.
However, I've been surprised. Finally, I created a spreadsheet to create the transforms there first, and from that to infer and copy what-must-be the transformations into G'MIC's Cartesian transform, but still no success. Whence I think I don't understand. Wince, I think I come ask you all what you think about it.
So I'll'mattach a) a square moonar image in cylindrical projection and b) ask you if these Cartesian transformation equations make sense to you:
newx=w/2 - ( cos( asin(( (h/2)-y ) / (h/2)) ) * (w/2-x) )
newy=y
I'm interpreting x,y to be coordinates in the orig. image, and w,h to be width and height of same. I am also expecting both images to have origins at the upper left.
I'llcan give the spreadsheet too if you want to see it.
(Yes, I have put the "Model Railroad Coffee Tables" on hold for the nonce...)
Best greetings, friends!
--
gramp
P.S. found the map at:
http://astrogeology.usgs.gov/search/det ... ar2014/cub