Gimp users and mercator projections

[Beeblebrox ate my hamster]

I've been using the wonderful image editor Gimp to create some nice surface textures. But around the polar regions I'm getting a nasty distortion , as shown here..........



Can anyone help with getting the proper projection so I don't get the "pinched" effect?

[selden]

Celestia uses "simple cylindrical projection" also known as "Plate Caree" (with various spellings). It does not use Mercator.

There are at least two free packages that can translate images between various map projections.

Iris: http://astrosurf.com/buil/us/iris/iris.htm
by Christian Buil

and

MMPS: Matthew's Map Projection Software
http://www.users.globalnet.co.uk/~arcus/mmps/mmps.html
by Matthew Acrus

Even so, you won't be able to completely eliminate the "pinch" effect. Some distortions will still be visible because of the way 3D surface textures are mapped onto 3D objects. Consumer 3D hardware does not even try to smooth out the coloration of pieces of an image when it squeezes them between the vertices of a model. You can think of it as being similar to someone cutting out triangular and trapezoidal pieces from your picture and pasting them onto the sphere. Some parts get left out and the edges don't quite match. (Of course, this is an oversimplified description.)

Sorry.

[maxim]

Hm, some tool had a polar coordinate to rectangular coordinate conversion filter and produced quite good results. Was it photoshop? I don't really remember.

maxim

[Guest]

I've been using the wonderful image editor Gimp to create some nice surface textures. But around the polar regions I'm getting a nasty distortion , as shown here..........



Can anyone help with getting the proper projection so I don't get the "pinched" effect?

Thanks Seldon, as always. Now I've downloaded both, what on earth do I do with them

[selden]

Unfortunately, I really have to recommend that you read their documentation carefully.

I've only used Iris a few times, and MMPS is new to me. I'm sure their authors' documentation is better than what I could write. Sorry.

Both programs can read in an image and distort its dimensions so that it is a better shape for projecting onto a sphere and then write out the new version of the image.

If you need a refresher on map projections, you might want to take a look at the Web page http://erg.usgs.gov/isb/pubs/MapProjections/projections.html

[maxim]

Yes, it was photoshop. You can find it even in older versions in the group of distortion filters. The 'polarcoordinate' filter transforms a picture from rectangular to polar coordinates and vice versa. Using Iris is a little bit tricky, and the result is (IMHO) only moderate. It took me quite some time to get behind the functionality. The Manual asumes a lot of pre-knowledge.

Roughly:
Write two transformation definition files, one for the ingoing and one for the outgoing transformation. Finding the right size conversion factor is cruical for a correct result. Split your pic into the three color channels and transform everyone independently. Remerge after splitting. Test the whole procedure with a simple grit first.

How to apply to textures:
Paint your polar region as if you where standing exactly over the axis (Pole in center of pic).
Transform from polar to rectangular coordinates.
Merge the polar part (top part of result) with the undistorted rest of the texture.
Make corrections.
Test result.

maxim

[ajtribick]

GIMP has a polar-to-rectangular conversion filter but its really fussy about input resolutions.

[Rassilon]

You can also use flexify from flaming pear...I dont think it will work with the gimp but I know it does with photoshop...

http://www.flamingpear.com

[John Van Vliet]

one way to solve the pinch distorstion is to use virtual tex
even a level two will remove most of the pinch

hear are some examples




and a link to the fullsize
http://johnscelestiapage.no-ip.com/ScreenShots/spole1.png
http://johnscelestiapage.no-ip.com/ScreenShots/spole2.png
http://johnscelestiapage.no-ip.com/ScreenShots/spole3.png
http://johnscelestiapage.no-ip.com/ScreenShots/spole4.png

[Beeblebrox ate my hamster]

cheers guys, I'll put my thinking cap on!