Another Dumb Question

[Bob Hegwood]

Sorry, I tried to research the answer for this question both here on the forum, and
elsewhere on the web. Thus far with no luck...

I'd like to use one of Bjorn Jonsson's outstanding planet textures on
my system, rather than a texture which I already have.

I've noticed, however, that Bjorn's texture is positioned differently than my current
texture. In other words, If I can see the same big crater on the right-hand side
of my current texture, but it exists in the CENTER of Bjorn's new texture, how do I
adjust my system (or do I even have to?) in order to use the new texture correctly?

Sorry for the dumb question, but I really am trying to understand this concept.

Thanks, Bob

[granthutchison]

Bob:
Celestia has a standard central meridian for textures, to ensure that planets and moons appear correctly orientated relative to each other - the textures all have zero degrees longitude in the centre.
If you have a texture that is differently orientated from that, and you want it to display correctly in Celestia, then you need to trim a bit off one edge and paste it on to the other edge, so that the new version matches the Celestia version. There has been some recent discussion on this topic in this thread, towards the end.

Grant

[Guest]

Yes; you can see the meridian on the surface of the planets- I have to edit it out in the images I have been making for Orion's Arm- can you switch it off?

[eburacum45]

Yes; you can see the meridian on the surface of the planets- I have to edit it out in the images I have been making for Orion's Arm- can you switch it off?

Sorry- not logged in (again)...
here is a rough half-terraformed Mars I put together, by the way.
http://www.siderealgames.com/uploads/mars_301_a.t.jpg

[Bob Hegwood]

Bob:
Celestia has a standard central meridian for textures, to ensure that planets and moons appear correctly orientated relative to each other - the textures all have zero degrees longitude in the centre.
If you have a texture that is differently orientated from that, and you want it to display correctly in Celestia, then you need to trim a bit off one edge and paste it on to the other edge, so that the new version matches the Celestia version.

Okay, I *think* I undertsand what your'e trying to tell me, Grant. I'll play around with it a bit to see if I can do what you've described. How do I know if the longitude is set at zero in the center of the image though? Do I simply use the texture in Celestia and then implement the gotolonglat command to see if I'm at the center?

Sorry, but I'm just a little confused by this topic.

Thanks VERY much for the advice.

Take care, Bob

[Bob Hegwood]

Yes; you can see the meridian on the surface of the planets- I have to edit it out in the images I have been making for Orion's Arm- can you switch it off?

Sorry eburacum45, but I have *no* idea what the hell you're talking about.

I can see the meridian on the surface of the planet? News to me... Can
you elucidate on this topic in terms that my poor brain can understand?

Thanks, Bob

[Guest]

The selection of meridian systems for some other planets seems to be not entirely settled within the scientific community, you'll sometimes see images and maps given geographic coordinates in reference to some recently established standard - which presumably means that there are a number of standards in contentionThere's nothing like the Greenwich Observatory on Charon.

Anyway, the rotational state of the planet will only be correctly displayed if your texture is in alignment with the default texture used by Celestia. One way of accomplishing this to high accuracy is to import the default Celestia texture into the GIMP image editor, scale it to the size of the texture you'd like to use, and then paste the new texture over the old one till they roughly match. There is a 'difference' image convolution tool that can be used working with layers in GIMP, so that the image becomes almost completely black when two similar images are aligned - so by incremental (1-pixel) moves, your new texture can be as well-aligned as possible. Then you just delete the 'old' texture, anchor layers, and paste the 'new' texture again into the frame to patch the gap (east or west) that arose while aligning with the old texture. Once aligned and patched, you can save in your favored texture format.

Most of this effort has probably already been done for you if you look around.

[Bob Hegwood]

Anyway, the rotational state of the planet will only be correctly displayed if your texture is in alignment with the default texture used by Celestia.

Well, at least I understood that part of your message.

I think I see what you mean though, and I'm going to have to use Gimp to do this? I have the program, but I'm really not well-versed in its use yet.

Ah well, might as well learn something new .

Thanks very much for the explanation.

Take care, Bob

[selden]

Bob,

If you're comfortable with command line interfaces, you could use something like either NetPBM or ImageMagick to "rotate" the surface maps. Personally, I've found the Gimp GUI more than slightly confusing and, shell we say, less than intuitive.

For example, to rotate a map 180 (the most common requirement) you cut it in half and exchange the pieces. Here's what I'd use for a 1Kx512 jpeg texture:

Code: Select all

jpegtopnm map1.jpg >tmp1.ppm
pnmcut -left 0 -width 512 tmp1.ppm >tmp2.ppm
pnmcut -left 512 -width 512 tmp1.ppm >tmp3.ppm
pnmcat -leftright tmp3.ppm tmp2.ppm >tmp4.ppm
pnmtojpeg tmp4.ppm >map2.jpg

Does this help?

[granthutchison]

Yes; you can see the meridian on the surface of the planets- I have to edit it out in the images I have been making for Orion's Arm- can you switch it off?

No, you shouldn't be able to see any meridian on your planet, unless there was one plotted on the original texture. If you're seeing a seam, it's most likely an edge effect where the left and right sides of your texture aren't coming together smoothly. Try zooming in along the texture margins in your image editor to see if you have a line of dodgy pixels there.

Grant

[granthutchison]

How do I know if the longitude is set at zero in the center of the image though? Do I simply use the texture in Celestia and then implement the gotolonglat command to see if I'm at the center?

Bob, a conscientious texture-builder will tell you somewhere (maybe in a readme?) what the central meridian is for their texture. Most commonly, it's either zero or 180 degrees. If it's 180 degrees, then all you need to do is switch around the left and right halves of the texture to make it Celestia compatible. If it were, say, 90 degrees west, then you're going to need to slide the texture 90 degrees to the left - so you cut off the left-hand quarter, slide the remaining part of the texture across, and then paste the cut-off portion on to the right hand side. In general, if the central meridian is x degrees west, you have to shift the left-hand x/360 portion of the texture to the right-hand side.
But if you're still having trouble, let us know which specific texture is creating the problem, and perhaps we can advise.

Grant

[Brendan]

An easy way to use the gimp to rotate textures is with the Offset tool in the Image->Transforms menu, or Shift+Crtl+O. There is a button that figures half of the length and the width of the image. Use that and then make the Y offset zero to use just the offset for X.

Brendan

[eburacum45]

No, you shouldn't be able to see any meridian on your planet, unless there was one plotted on the original texture. If you're seeing a seam, it's most likely an edge effect where the left and right sides of your texture aren't coming together smoothly. Try zooming in along the texture margins in your image editor to see if you have a line of dodgy pixels there.

Grant

No- that's not it; this mysterious meridian is at 0 degrees, and the edge of the texture is at 180 degrees; it must be a peculiarity of my graphics card if no-one else gets it.

[granthutchison]

No- that's not it; this mysterious meridian is at 0 degrees, and the edge of the texture is at 180 degrees; it must be a peculiarity of my graphics card if no-one else gets it.

Strange. Are you able to post a screenshot?

Grant

[Bob Hegwood]

Bob,

Personally, I've found the Gimp GUI more than slightly confusing and, shell we say, less than intuitive.

For example, to rotate a map 180 (the most common requirement) you cut it in half and exchange the pieces.

Does this help?

Selden,

I don't have any trouble with rotations, mirroring or any of the common graphics features. It's just that I don't understand whether a map is based on a longitude of 0 or 180. That's okay though, Grant has given me some insight into this problem, and I'll get it sooner or later.

By the way, why would you cut a map in half to rotate it 180 degrees? There are *plenty* of free programs which will allow easy rotation of an image.

At any rate, thanks for the advice.

Take care, Bob

[Bob Hegwood]

Bob, a conscientious texture-builder will tell you somewhere (maybe in a readme?) what the central meridian is for their texture. Most commonly, it's either zero or 180 degrees. If it's 180 degrees, then all you need to do is switch around the left and right halves of the texture to make it Celestia compatible. If it were, say, 90 degrees west, then you're going to need to slide the texture 90 degrees to the left - so you cut off the left-hand quarter, slide the remaining part of the texture across, and then paste the cut-off portion on to the right hand side.

Okay, now this explanation I understand. Thanks Grant. So Selden is not really referring to rotation, but a redefinition of the map's central meridian? Sorry Selden, I mis-understood you again.

Thanks again, Grant... I'm starting to understand.

Take care, Bob

[Bob Hegwood]

An easy way to use the gimp to rotate textures is with the Offset tool in the Image->Transforms menu, or Shift+Crtl+O. There is a button that figures half of the length and the width of the image. Use that and then make the Y offset zero to use just the offset for X.

Thanks for the advice, Brendan. I'll give it a try. A point in all this confusion though... You guys keep talking about "Rotation" when in fact, you're really talking about moving the central meridian. Ain't no wonder I'm confused by all this.

Take care, Bob

[jim]

An easy way to use the gimp to rotate textures is with the Offset tool in the Image->Transforms menu, or Shift+Crtl+O. There is a button that figures half of the length and the width of the image. Use that and then make the Y offset zero to use just the offset for X.

A similar filter is in Photoshop available 'filter->other filter->shifting effect' (don't know if it's correct translated from german version).

Bye Jens

[granthutchison]

A point in all this confusion though... You guys keep talking about "Rotation" when in fact, you're really talking about moving the central meridian.

Bob, by shifting the central meridian of the texture you're effectively rotating it in longitude - it appears to have twisted around the planet when you view it in Celestia. But I guess you were taking this to mean that you rotated the texture itself (which would just appear to turn the planet upside down in Celestia).
Sorry!

Grant

[Bob Hegwood]

Grant,

You got it... I'm slow, but I will eventually get there.

I now understand what you're saying though. It IS a rotation effect I guess, but if I don't understand what you're talking about to begin with, I'm left with the exact impression that you described.

i.e. - "What's so difficult about rotating the image?"

Thanks muchly kind sir... I understand a LOT more about it now - thanks mainly to your understandable, PLAIN English explanations.

For your further edification, I'll tell you that the texture I had a problem with was the Callisto.jpg map I apparently got from Bjorn's site some time ago. As I am currently working on a Tour of Jupiter script, this did not come to my attention until I got around to visiting Callisto. Then, I noticed that prominent features like Valhalla were not lined up correctly with the displayed locations labels.

I had a go at correcting Bjorn's image, using all of the advice you people have given to me, but with not much luck. I *did* learn how to use the Gimp program a lot better though. (And I did get close to correcting the image, but my guesses at where the 0 degrees Central Meridian line should be were not precise enough.

Tell me, how does one KNOW where the Central Meridian is in a planet map? I mean, where would I find this information? Besides looking from another image in which the location was already known?

At any rate, to make a longer story shorter, I finally just located the Callisto DDS file in Praesepe's site, I re-sized it using IrfanView, and now my locations line up to where they should be.

Thanks again VERY much for your explanations. I think I have a real handle on this problem now.

Take care, Bob