Planetocentric vs Planetographic maps

Description: Tips for creating and manipulating planet textures for Celestia.

FarGetaNik M
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FarGetaNik M
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#1by FarGetaNik » 17.04.2017, 10:27

Recently, I am working on a set of realisticly-colored maps of Jupiter. One issue of Jupiter maps is to get the latitude-structure of the cloud patterns right. For an oblate planet like Jupiter, this is especially difficult. As far as I know, Celestia just squeezes Jupiter into an oblate spheriod, without considering the consequences on the 3D body. For an extremely squeeced object, the terminator is clearly wrong. This is less obvious at Jupiter.

Also the coordinate system just gets sqeezed. One could overcome this problem by using a correctly projected map.
This Cassini map of Jupiter is projected planetocentric. When using it in Celestia, I get a discrepancy when comparing it to actual Cassini images. So this projection is not suited for Celestia.
In contrast, this map by Björn Jónsson is projected planetographicly. This projection is consistent when comparing to Cassini images.

Is there an easy way to reproject textures from planetocentric to planetographic? The better solution would be if Celestia used an accurate 3D-model of oblate (or even triaxial) objects, so the problems mentioned above wouldn't exist. We could use planetocentric maps then also. Or how about a parameter determining the coordinate system of the planet and thus the map projection.

John Van Vliet
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#2by John Van Vliet » 17.04.2017, 16:33

isis3 dose this very easily

just edit the custom "simplecylindrical.map" file you are using

form
LatitudeType = Planetocentric
to
LatitudeType = Planetographic


qgis and grass ( using gdal ) should also be able to do that

scalbers
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#3by scalbers » 17.04.2017, 18:39

Perhaps two sets of parameters would be useful. One could describe the projection of the texture, and another for the actual displayed shape of the object? I have coded (in IDL) some equations for (at least approximately) doing some of these conversions. 'gdal' sounds good as well as it could be easier to install.
http://stevealbers.net

FarGetaNik M
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#4by FarGetaNik » 17.04.2017, 20:19

John Van Vliet wrote:isis3 dose this very easily

You mean this? https://isis.astrogeology.usgs.gov/documents/InstallGuide/
Doesn't run on Windows. Even if my OS were compatible, this seems to require a good amount of work to get it running. I'd rather experiment with streching my texture in Gimp.

scalbers wrote:Perhaps two sets of parameters would be useful. One could describe the projection of the texture, and another for the actual displayed shape of the object?

Yes that's basically what I meant. Currently there is a parameter called "Oblateness" that determines Jupiter's shape. We would need another one defining map projection.

scalbers
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#5by scalbers » 17.04.2017, 23:00

Possibly GDAL can be downloaded for Windows here:

http://trac.osgeo.org/gdal/wiki/DownloadingGdalBinaries
http://stevealbers.net

John Van Vliet
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#6by John Van Vliet » 20.04.2017, 03:36

odd i posted a reply to this last night -- it's gone ????

gdal can remap using a dem of the oblatness

FarGetaNik M
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#7by FarGetaNik » 20.04.2017, 08:57

I'm trying to install it now.

Hm how do I create a dem of the oblateness? Is the contrast of the dem proportional to the oblateness then?

scalbers
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#8by scalbers » 20.04.2017, 17:46

Perhaps a dem wouldn't really be needed with 'gdal'. More a specification of map parameters. This would also help for converting and displaying my planetocentric map of Mimas.

http://stevealbers.net/albers/sos/sos.html#MIMAS

There is a planetographic version of Mimas from Paul Schenck, so this means it would display correctly on the triaxial ellipsoid without any modification?
http://stevealbers.net

FarGetaNik M
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#9by FarGetaNik » 20.04.2017, 20:51

For Mimas, I am currently using a modified version of this map (I only adjusted color and orientation, not projection). But I am using a mesh for Mimas. There is some misalignement, for example the Herschel crater extends more vertically on the texture than on the mesh. The opposite is true for a planetocentric projection of Jupiter on an oblate spheroid. I don't know what Celesia is doing here. For another body I am using a mesh, Vesta displays a slight horizontal misalignement. I have no idea how the texture was projected. Eros is just a mess, I assume the body center is wrong.

scalbers wrote:There is a planetographic version of Mimas from Paul Schenck, so this means it would display correctly on the triaxial ellipsoid without any modification?

Yes, this should work if Mimas is defined as a triaxial ellipsoid in Celestia (not as a spherical object displayed by a mesh as I have it). I would love to see this map projection.

scalbers
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#10by scalbers » 24.04.2017, 17:15

I think the link the show is the one by Paul Schenk on the planetographic projection. Indeed Hershel looks too elongated. We'll know if things are projected right in Celestia if the appearance is more round. I would also tone down the color on that map if visual realism is desired.
http://stevealbers.net

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#11by FarGetaNik » 24.04.2017, 21:45

Yes I decreased saturation of the texture. I'm thinking of reprocessing this texture appropriately. One important step would be to find the right color balance of the channels. A quick google search lead me to this paper: http://www.aanda.org/articles/aa/pdf/2005/19/aa2482-04.pdf, it gives a color index of 0.65 for the inner major satellites of Saturn. This would give a pretty neutral colored surface. Color indeces for minor planets are widely avaliable, bu it's hard to find color data for the planets and moons. Most spectra show infrared or UV wavelenghts also.

Back on topic, I found that in the case of Jupiter, most Hubble textures seem to be in planetographic projection. At least I found good correspondence of stable vortices at different latitudes on different maps. Only a poor resolution Voyager 1 texture I had to fit manually.


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