Selden's CMOD Billboard Meshes for Celestia

0.0: Introduction.
1.0: Flat Billboards.
2.0: Using Flat Billboards.

2.1: Using "billboard.png".
2.2: Making a Different Flat Billboard.
2.3: Creating a Flat Billboard for Carina.

3.0: Problems with the Flat Billboard.
4.0: Some Background.
5.0: Some Flat Examples.

5.1: Hoag's Object.
5.2: Eta Carina Nebula.
5.3: NGC 3372.
5.4: Horsehead Nebula Region.
5.5: Some Planetary Nebulae.

6.0: Aligning Pictures and Objects.

6.1: An Alignment Procedure.
6.2: Axis and Angle adjustment details

7.0: Some 3D Billboards.

7.1: A Pie-Plate Galaxy.
7.2: A Sphere to map the Sky.

7.2.1: The Infra-Red Milky Way
7.2.2: The H-alpha Sky

8.0: Acknowledgements.
Appendices.

A.1: Signature.
A.2: Admonishment.
A.3: Disclaimer.


---

See also:
• Selden's List of Resources for Celestia

From Webster's Revised Unabridged Dictionary (1913)

Billboard \Bill"board`\, n.
   1. (Naut.) A piece of thick plank, armed with iron plates,
      and fixed on the bow or fore channels of a vessel, for the
      bill or fluke of the anchor to rest on. --Totten.

   2. A flat surface, as of a panel or of a fence, on which
      bills are posted; a bulletin board.

Here are some simple CMOD object models. By changing the pictures associated with them, you can use them to display colorful images of deep space objects in Celestia. They're billboards in space!

Celestia v1.3.0 and later let you create Deep Space Catalogs (.DSC files). You can use them to define the locations of Nebulae that consist of 3D models with appropriate shapes and surface textures.

Unfortunately, making three dimensional Nebulae that look right from all directions can be rather difficult. We only know what most nebulae look like from here on Earth. Although radio and InfraRed telescopes have provided hints about some of their shapes, we really don't know what their other sides look like.

These "billboards" let you display a photograph instead of having to create a three dimensional shape. The pictures will only look right in Celestia when seen from the vicinity of the Earth, of course. If you look at them from a viewpoint that is far away from Solar System, they'll be distorted.

Celestia v1.3.2 and later support the use of a special 3D model format named CMOD: short for "Celestia Model". It's designed for efficient use with modern 3D graphics cards. Large, complex CMOD models load and run noticably faster than 3DS models. Celestia's framerate is improved by about 1.5x - 2x. This improvement won't be noticable with the simple models demonstrated here.

To repeat: CMOD models do not work with earlier versions of Celestia. In particular, CMOD models do not work with v1.3.0 or v1.3.1. See the Web page Selden's Billboard Meshes for Celestia for some 3DS billboard models which can be used with those older versions of the program.

• billboard1.cmod (1KB, 11June04)

  This CMOD model is a flat, square object that can hold any square picture you want to display in Celestia. This example specifies the surface texture name "billboard1.*". You'll probably want to change that. See Section 2.2 below.

• billboard2.cmod (1KB, 11June04)

  This is a rectangular, flat billboard, proportioned 2x1. You can use it with images that are twice as long as they are wide. This example specifies the surface texture name "billboard2.*".

• billboard-cmod.zip (730KB, 6Feb06)

  This zip archive below includes both the square and rectangular CMOD billboard objects and two sample billboard images. The CMOD object definitions are each less than 1KB. The ZIP file is much larger because of the size of the sample images. PNG images don't compress very well, and they include alpha transparency channels, which make them larger.

  ---

  Archive:  billboard-cmod.zip
    Length     Date   Time    Name
   --------    ----   ----    ----
        935  02-04-06 16:33   billboard-cmod/billboard-cmod.dsc
        801  02-04-06 16:37   billboard-cmod/billboard-cmod.html
       3418  02-04-06 16:47   billboard-cmod/Billboard-Readme.txt
          0  02-04-06 16:35   billboard-cmod/models/
        553  06-12-04 09:32   billboard-cmod/models/billboard1.cmod
        549  02-04-06 16:17   billboard-cmod/models/billboard2.cmod
          0  02-04-06 15:54   billboard-cmod/textures/
          0  02-04-06 16:35   billboard-cmod/textures/medres/
     545011  06-12-04 09:31   billboard-cmod/textures/medres/billboard1.png
     195771  06-12-04 09:27   billboard-cmod/textures/medres/billboard2.png
   --------                   -------
     747038                   10 files

  ---

2.1: Using Billboard1.

As an example,

1. billboard-cmod.dsc defines a nebula named "Billboard1".
2. The "Billboard1" Nebula definition specifies the 3D model "billboard1.cmod".
3. Inside the 3D model file billboard1.cmod it specifies the name of the surface texture map billboard1.png. This is the picture you want to see.

Here's billboard-cmod.dsc:

---

# a square "billboard" object 

# this example displays a picture of Hoag's Object
# s.ball 12jun04

Nebula "Billboard1" {
    Mesh     "billboard1.cmod"

Axis       [-0.2542 0.8936 0.3699]
Angle      152.350

    RA          15.3208  # 15h 17m 17s
    Dec         21.57917 # 21deg 34' 45"

    Distance   6.1e+8
    Radius     60000

    InfoURL   "http://heritage.stsci.edu/2002/21/table.html"
}

# a rectangular 2x1 "billboard" object

# this example displays a picture of ngc4013
# Minimum credit line: C. Howk (JHU), B. Savage (U. Wisconsin),
# N.A.Sharp (NOAO)/WIYN/NOAO/NSF

# s.ball 13jun04

Nebula "Billboard2" {
    Mesh     "billboard2.cmod"

Axis       [-0.4976 0.8506 0.1700]
Angle      108.693

# ICRS 2000.0 coordinates 11 58 31.4 +43 56 51 
    RA          11.97539
    Dec         43.94750

    Distance   55e6
    Radius     55600

    InfoURL   "http://www.noao.edu/image_gallery/html/im0222.html"
}

---

The name "Billboard" is just being used as an example. There is no real nebula with that name.

Here's billboard1.cmod:

---

#celmodel__ascii
# a square billboard using the cmod format by s.ball 21apr04

material
texture0 "billboard1.*" # modify this filename to display other images
diffuse 1 1 1 opacity 1 end_material
mesh
vertexdesc position f3 normal f3 texcoord0 f2 end_vertexdesc

vertices 4
 0 0 0  0 1 0  0 1
 0 0 1  0 1 0  0 0
 1 0 1  0 1 0  1 0
 1 0 0  0 1 0  1 1

# the "negative" side: facing the origin
trilist 0 6 0 3 2 2 1 0

# the "positive" side: facing away from the origin 
trilist 0 6 0 1 2 2 3 0
end_mesh

---

2.2: Making a Different Flat Billboard.

In order to use this CMOD model for different objects in Celestia, ones not called "Billboard", you have to copy the model file, creating a new .cmod file in a models directory. Each copy of the model must be given a different name.

You also have to edit the new CMOD file to change the name of the texture image file that it looks for. Currently it specifies billboard1.png. This is an ASCII CMOD file, so you can use your favorite text editor to make this change.

You don't have to use or specify a PNG image file. Celestia will happily render a .JPG or .DDS file just as well. When ".*" is specified as the type of the image file name, Celestia will look for any of PNG, JPG or DDS images, in that order. However, the image file does need to be in a directory named textures/medres, and it needs to be small enough to fit into your graphic card's texture buffer. Recent Nvidia cards can display images up to 4096 pixels on a side. Recent ATI cards can display images up to 2096 pixels on a side. Many other graphics chipsets are limited to images 1024 pixels on a side.

2.3: Creating a Flat Billboard for Carina.

For example, if you wanted to display an image of a portion of the Eta Carina nebula, do the following:

1. Acquire a picture of the Carina Nebula.
2. Using your favorite picture editor, create a new version of the picture that is square and a power of two on a side. For example, create a picture that is 1024x1024.
3. Save your new picture of the Carina Nebula in your directory textures/medres as carina.jpg
4. Create a DSC file named carina-cmod.dsc in your extras directory, containing the following Nebula definition:

   Here's carina-cmod.dsc:

   ---

   # Eta Carina Nebula (detail)
   #
   # the "billboard" picture used with this definition is derived from
   # a NASA/STScI image found on the Web (see below)
   #
   Nebula "Carina" {
       Mesh     "carina.cmod"
   	Axis [ 0.517248 -0.0448043 0.854662 ]
   	Angle 149.296
       Distance   8000
       RA        10.7454 # 10h 44m 46s
       Dec      -59.6487 #-59deg 38' 56"
       Radius     4.55
       InfoURL "http://hubblesite.org/newscenter/archive/2000/06/fastfacts"
   }
   

   ---

5. Copy the file billboard1.comd to carina.cmod in your models directory.
6. Using your favorite text editor, change the filename inside carina.cmod from billboard1.* to carina.*
7. Start Celestia and GoTo Carina
8. Enjoy the sight!

Bear in mind that this .DSC file provides the coordinates of a tiny portion of the Carina Nebula, corresponding to the picture taken by the Hubble telescope. You'll have to use different coordinates if you use a different picture. See below for the coordinates used for a picture of the whole nebula.

Note:

If you use both Carina Nebula definitions, combine both .DSC files into one. Make sure that NGC 3372 is defined first, before Carina. That will ensure that the larger image is drawn first, behind the smaller one.

1. CMOD billboard model files cannot be displayed by any 3D modelling program other than Celestia.

2. A problem in Celestia v1.3.0 (and later) is that Nebula models are not depth sorted. Instead, they are drawn on your screen in the order that you define them or in the order that Celestia happens to read their .DSC files. If overlapping Nebula models are declared in separate files. often they are drawn in the wrong order. When they overlap, you may not be able to see the one that is supposed to be closest to you. However, when you declare them in the same .DSC file, the model that is declared first in that .DSC file is drawn first, the second one is drawn on top of the first one, etc. At least that way you have some control over which ones are visible from a particular viewpoint.

3. When you GoTo a flat Billboard object, Celestia sometimes overshoots. I suspect the extreme thinness of the model may be confusing it. If it thinks you are inside an object, various parts of the model get clipped off. In some cases, the whole thing will be invisible. Typing a "C" (center) or "T" (track) command and then backing away from it usually makes the billboard visible again.

4. In the past, another difficulty for me has been getting the correct values for the Axis and Angle directives so the billboard is properly oriented and face-on to the line-of-sight from Earth. Grant Hutchison was kind enough to create a spreadsheet which calculates the correct orientation angles for most models. Also, a recent insight made it much easier to do positioning manually. The details are a little too complicated to go into right here. See 6.0: Aligning Pictures and Objects below.

The CMOD model that I've created for use as a billboard is a simple square, 1 unit on its X and Y sides, with no depth in the Z dimension. The actual size the billboard object doesn't matter, only its proportions. Celestia scales it to match the radius you specify in the .DSC file. The model could just as well have been 1000x1000.

When seen from the front (the side facing the Earth), the image on the Z face is upright. The image is reversed on the back of the billboard object. As a result, the picture seems to be embedded inside the billboard. The billboard object has no depth, so it becomes invisible when seen from the sides.

The example surface texture file billboard1.png that I've supplied happens to be an image of Hoag's Object. The image that Celestia renders is somewhat fainter than it should be. The greyscale alpha channel is transparent where there should be dark obscuring dust clouds, since the conversion software cannot distinguish between darkness due to obscuration and darkness due to the lack of stars. Producing a proper alpha channel for Hoag's Object is left as an exercise for the artist. If someone does produce a good transparency channel, I'll be glad to include it here.

Below are pictures of the flat square billboard in use. Each of the "thumbnail" pictures is a link to much larger image.

5.1: Hoag's Object

Image	Comments
Billboard Image: Hoag's Object	Note that this image is slightly fainter than the primary layer of the texture below. Cel:// URL for this viewpoint Hoag's Object
Billboard Surface Texture: Hoag's Object	These texture maps were derived from a Hubble picture. See the acknowledgements below. billboard.png (1024x1024, 2.4MB) billboard-1k.jpg (1024x1024, 100KB) hoag-1k.dds (1024x1024, 1.4MB, DXT3) hoag-2k.dds (2048x2048, 5.5MB, DXT3, requires recent ATI or Nvidia card) These PNG and DDS files include an Alpha transparency channel, but the JPG file does not.

• billboard1.cmod (1KB, 11June04)
  This is the 1x1 "billboard1" mesh described above. Its texture has not been changed. It specifies the texture map billboard.png. Rename an appropriate PNG image or edit this mesh file to use a different picture.

• billboard1.dsc

  ---

  #Hoag's Object
  Nebula "Billboard1" {
      Mesh     "billboard1.cmod"
  
      Axis       [-0.2542 0.8936 0.3699]
      Angle      152.350
  
      RA          15.3208  # 15h 17m 17s
      Dec         21.57917 # 21deg 34' 45"
  
      Distance   6.0e+8
      Radius     60000
      InfoURL   "http://heritage.stsci.edu/2002/21/table.html"
  }
  

  ---

  After you've installed these, your directory structure should look something like this:

  ---

  [-] Celestia
      --- data
      [-] extras
       |  [-] addons
       |      [-] billboard
       |           -- billboard.dsc
       |          --- models
       |               -- billboard1.cmod
       |          [-] textures
       |              --- medres
       |                   -- billboard1.png
      --- models
      [+] textures
  

  ---

  For an explanation of why your directories should be organized like this, please see
• A (not so) Brief Introduction to Add-Ons (3rd edition)

---

5.2: Eta Carina Nebula (detail)

• carina-cmod.zip (4.4MB, 19June'04, updated 4Apr06)

Image	Comments
Earth in the Carina Nebula	This image was not generated by cheating, as a previous version was. To see this and similar dramatic views, one only has to find the right combination of viewpoint and telescopic magnification. Aye, there's the rub. Unfortunately, adjusting Celestia's viewpoint is extremely difficult at high magnifications. This image was finally acquired by first aligning the Earth's orbit and then letting the planet drift across the field of view. Cel:// URLs for Earth and the Nebula Each is progressively closer to the detailed Hubble picture. This looks best if you also install the definition below for NGC 3372. Earth in the constellation of Carina NGC 3372 in Carina Keyhole Eta Car Carina Nebula (detail) Earth in the Carina Nebula
Carina Nebula Detail	This texture map was derived from a Hubble image showing details of a portion of the nebula. See the acknowledgements below. Using a 2K texture map requires a recent ATI or Nvidia card. carina-1k.jpg (1024x1024, 84KB) carina-2k.jpg (2048x2048, 333KB) carina-2k.dds (2048x2048, 5.5MB, DXT3) The DXT3 file includes an alpha channel to trim off the black borders.

• carina.cmod (1KB)
  This is the same 1x1 "billboard" mesh described above. Only the name of its texture image file has been changed. It specifies the texture map carina.*. Rename an appropriate image or edit this mesh file to use a different picture.

  ---

  #celmodel__ascii
  # a square billboard using the cmod format
  # s.ball 21apr04
  
  material
  texture0 "carina.*" # modify this filename to display other images
  diffuse 1 1 1 opacity 1 end_material
  
  mesh
  vertexdesc position f3 normal f3 texcoord0 f2 end_vertexdesc
  vertices 4
   0 0 0  0 1 0  0 1
   0 0 1  0 1 0  0 0
   1 0 1  0 1 0  1 0
   1 0 0  0 1 0  1 1
  
  # the "negative" side: facing the origin
  trilist 0 6 
  0 3 2 2 1 0
  
  # the "positive" side: facing away from the origin 
  trilist 0 6 
  0 1 2 2 3 0
  
  end_mesh

  ---

• carina-cmod.dsc (1KB, 19Jun04)

  ---

  
  # Eta Carina Nebula (detail)
  #
  # the "billboard" picture used with this definition is derived from
  # a NASA/STScI image found on the Web (see below)
  #
  Nebula "Carina" {
      Mesh     "carina.cmod"
  	Axis [ 0.517248 -0.0448043 0.854662 ]
  	Angle 149.296
      Distance   8000
      RA        10.7454 # 10h 44m 46s
      Dec      -59.6487 #-59deg 38' 56"
      Radius     4.55
      InfoURL "http://hubblesite.org/newscenter/archive/2000/06/fastfacts"
  }
  

  ---

  After you've installed these, your directory structure should look something like this:

  ---

  [-] Celestia
      --- data
      [-] extras
       |  [-] addons
       |      [+] billboard
       |      [-] Carina
       |           -- carina-cmod.dsc
       |          --- models
       |               -- carina.cmod
       |          [-] textures
       |              --- medres
       |                   -- carina.jpg
      --- models
      [+] textures
  

  ---

  For an explanation of why your directories should be organized like this, please see
• A (not so) Brief Introduction to Add-Ons (3rd edition)

---

5.3: Eta Carina Nebula: NGC 3372

Image	Comments
Earth in NGC 3372	Cel:// URL for this viewpoint: Earth in NGC 3372
NGC 3372	This texture was derived from the image 02192.tiff. Credit: NOAO/AURA/NSF. For details, see the acknowledgements below. ngc3372-1k.jpg (241KB) ngc3372-2k.dds (5.5MB DXT3) Using a 2K texture map requires a recent ATI or Nvidia card. The dds texture includes an Alpha channel to trim off the black borders.

• ngc3372.cmod (1KB)
  This is the same 1x1 "billboard" mesh described above. Only its texture filename has been changed. It specifies the texture map ngc3372.*. Rename an appropriate image or edit this mesh file to use a different picture.

• ngc3372-cmod.dsc (1KB, 21jun04)

  ---

  # NGC 3372
  #
  # The accompanying images of NGC3372 were derived from 02192.TIFF
  # at http://www.noao.edu/image_gallery/html/im0061.html
  # Original tiff image credit:
  # "National Optical Astronomy Observatory/
  # Association of Universities for Research in Astronomy/
  # National Science Foundation" 
  #
  # According to http://www.noao.edu/image_gallery/copyright.html
  # their images may be used freely for
  # educational outreach activities composed by amateur astronomers.
  # 
  Nebula "NGC 3372" {
      Mesh     "ngc3372.cmod"
   	Axis [-0.206916 0.153726 0.966206]
  	Angle 154.5
  	Distance   8500
  	RA      10.7345 
  	Dec      -59.752
  	Radius     219
  	InfoURL "http://www.noao.edu/image_gallery/html/im0061.html"    
  }
  
  

  ---

  After you've installed these, your directory structure should look something like this:

  ---

  [-] Celestia
      --- data
      [-] extras
       |  [-] addons
       |      [+] billboard
       |      [+] Carina
       |      [-] NGC3372
       |           -- ngc3372.dsc
       |          --- models
       |               -- ngc3372.cmod
       |          [-] textures
       |              --- medres
       |                   -- ngc3372.jpg
      --- models
      [+] textures
  

  ---

  For an explanation of why your directories should be organized like this, please see
• A (not so) Brief Introduction to Add-Ons (3rd edition)

  Note:

  If you use both Carina Nebula definitions, combine both .DSC files into one. Make sure that NGC 3372 is defined first, before Carina. That will ensure that the larger image is drawn first, behind the smaller one.

• carina-cmod.zip (4.4MB, 19June'04, updated 4Apr06)

  The file carina-cmod.dsc in this zip archive includes the declarations for both NGC 3372 and the detailed Hubble picture of Carina.

  ---

  Archive:  CARINA-CMOD.ZIP
    Length     Date   Time    Name
   --------    ----   ----    ----
       1020  06-19-04 09:37   carina-cmod/carina-cmod.dsc
       3283  04-04-06 12:16   carina-cmod/carina-cmod.html
        236  04-04-06 10:56   carina-cmod/eta-car.stc
          0  04-04-06 10:45   carina-cmod/models/
        550  06-19-04 09:12   carina-cmod/models/carina.cmod
        551  06-19-04 09:33   carina-cmod/models/ngc3372.cmod
          0  04-04-06 10:45   carina-cmod/textures/
          0  04-04-06 10:45   carina-cmod/textures/medres/
    5592560  06-19-04 09:39   carina-cmod/textures/medres/carina.dds
      85914  06-19-04 09:15   carina-cmod/textures/medres/carina-1k.jpg
    5592560  06-19-04 09:42   carina-cmod/textures/medres/ngc3372.dds
     241637  06-19-04 09:13   carina-cmod/textures/medres/ngc3372-1k.jpg
   --------                   -------
   11518311                   12 files
  

  ---

---

5.4: Horsehead Nebula Region

Image	Comments
Seeing Barnard 33 past Pan	Cel:// URLs for Pan and the Nebula: Pan and Orion Barnard 33 (vertical: North up) Barnard 33 (horizontal: West up) Closeup of Barnard 33 Barnard 33 is also known as the Horsehead Nebula. The following are points-of-view to help align the other nebulae in Orion. UKS 23 M42
Horsehead Region	These textures were derived from an astrophotograph taken by Walter Koprolin. They are provided here with his kind permission for use with Celestia. For details, see the acknowledgements below. horsehead_region-1k.jpg (1024x1204, 120KB) horsehead_region-2k.dds (2048x2048, 2.5MB) Using a 2K texture map requires a recent ATI or Nvidia card. horsehead_region-4k.dds (4096x4096, 11MB) Using a 4K texture map requires a recent Nvidia card. These DDS files are in DXT1c format. They do not contain an Alpha channel.

• b33.cmod (1KB)

  This is the same 1x1 "billboard" mesh described above. Only its texture has been changed. It specifies the texture map b33.jpg. Rename an appropriate JPEG image or edit this mesh file to use a different picture.

• b33-cmod.dsc (1KB; updated 22May03 to improve its orientation.)

  ---

  #Horsehead Nebula in Orion
  #
  # the "billboard" picture used with this definition is derived from
  #a photograph of the Horsehead Nebula Region taken by Walter Koprolin,
  # Austria, Europe: http://www.nightsky.at/
  
  #
  Nebula "Barnard 33" {
      Mesh     "b33.cmod"
  	Axis [ -0.641856 0.38352 -0.664029 ]
  	Angle 132.636
      Distance   1600
      RA          5.69
      Dec        -2.35
      Radius     40.3
  }
  

  ---

  After you've installed these, your directory structure should look something like this:

  ---

  [-] Celestia
      --- data
      [-] extras
       |  [-] addons
       |      [+] billboard
       |      [+] Carina
       |      [+] NGC3372
       |      [-] B33
       |           -- b33.dsc
       |          --- models
       |               -- b33.cmod
       |          [-] textures
       |              --- medres
       |                   -- b33.jpg
      --- models
      [+] textures
  

  ---

  For an explanation of why your directories should be organized like this, please see
• A (not so) Brief Introduction to Add-Ons (3rd edition)

---

5.5: Some Planetary Nebulae.

Image	Comments
Four Planetary Nebulae	Cel:// URLs for these viewpoints: NGC 2346 NGC 2392 NGC 2440 NGC 6369

pn-cmod.zip (288KB, update: 19June04)

This ZIP archive contains pn-cmod.dsc and four billboard models and their associated pictures. They provide the planetary nebulae images shown above. It was updated on May 25th, 2003, to precisely align the images of the nebulae, and then in June, 2004, to use CMOD models.

After you've installed this archive, your directory structure should look something like this:

---

[-] Celestia
    --- data
    [-] extras
     |  [-] addons
     |      [+] billboard
     |      [+] Carina
     |      [+] NGC3372
     |      [+] B33
     |      [-] pn
     |           -- pn-readme.txt
     |           -- pn.dsc
     |          --- models
     |               -- ngc2346.cmod
     |               -- ngc2392.cmod
     |               -- ngc2440.cmod
     |               -- ngc6369.cmod
     |          [-] textures
     |              --- medres
     |               -- ngc2346.jpg
     |               -- ngc2392.jpg
     |               -- ngc2440.jpg
     |               -- ngc6369.jpg
    --- models
    [+] textures

---

For an explanation of why your directories should be organized like this, please see
• A (not so) Brief Introduction to Add-Ons (3rd edition)

Consider using Grant Hutchison's spreadsheet which calculates the correct orientation angles for most models instead using of the technique described here.

These alignment techniques only work for DSC objects, not for objects defined in an SSC file.

One way to get the orientation and position correct for a remote astronomical object being represented by a Nebula is to properly orient a photograph and then match your model to the picture.

These adjustments are needed because the accepted astronomical coordinates of an object usually are nowhere near the centroid either of its photograph or of the model that you've designed.

6.1: An Alignment Procedure.

In summary:

1. Place a photograph of the nebula, including the surrounding stars, within Celestia.
2. Observe the photograph from a viewpoint near the Earth
3. Align the picture face-on to the viewpoint.
4. Adjust your object's size and position to match the starry background
5. Repeat steps 3 and 4 as necessary.

In detail:

1. Place a photograph of the nebula, including the surrounding stars, within Celestia.

   My "Billboard" object can be used for this, or you can construct one of your own. See above.

2. Observe the photograph from a viewpoint near the Earth

   Use a telescopic viewpoint from the vicinity of the Solar System to look at your picture. E.g.: Goto Earth, Track object, increase magnification with the "," command until the object fills your field of view appropriately.

3. Align the picture face-on to the viewpoint.

   You'll need to set the Axis and Angle of the picture's DSC definition so the image is face-on to the solar viewpoint. Then you'll have to rotate it into the correct position against the background stars. You can use Celestia's builtin positioning tool to do the initial alignment or you can use a spreadsheet provided by Grant Hutchison. This spreadsheet is much more accurate for setting the initial orientation, but you'll probably need to use Celestia's builtin positioning tool to set the final rotation angle. Too many astronomical pictures are not aligned north-south.

   Here's the procedure using Celestia's builtin positioning tool:

   • Run Celestia so it creates a log file:
     .\celestia --verbose > align.log
     • Type the key pair "Shift-~" to enable edit mode (Celestia v1.3.1pre9 or older)
       or
     • Type the key pair "Shift-2" (@) to enable edit mode (Celestia v1.3.1pre10 or newer)
     • Hold down Control-Shift Left-Mouse-Button and use the mouse to adjust the object's x-y orientation.
     • Hold down Control-Shift Right-Mouse-Button and use the mouse to adjust the object's Z-axis rotation orientation.
     • Type the key pair "Shift-!" to save your final Axis and Angle values to align.log
   • Exit Celestia
   • Type align.log
   • Put the Axis and Angle values you see there into your object's .DSC file.

   If these instructions are too cryptic, please take a look at the 6.2: Axis and Angle adjustment details below.

4. Adjust your object's size and position to match the starry background

   Edit the object's DSC file. Restart Celestia after each edit. Change the RA, Dec and Radius values until the stars of the photograph precisely match those which Celestia draws in that region. There's no reason to adjust the distance. That's the least well known of any of the parameters. Once the distance has been set to a reasonable value, changing the radius of the object is all that's needed to make the object's size appropriate.

   You have to exit and restart Celestia each time you modify the DSC file. I usually use a Cel://URL to restart Celestia with the appropriate viewpoint, with the object selected and centered.

5. Repeat steps 3 and 4 as necessary.

   Sometimes moving the object will change its location enough that it no longer directly faces the solar system. This makes it harder to line up with the stars around it. Realigning the object's Z axis may help.

Sometimes there simply aren't enough stars in Celestia near the object you're trying to align. In some cases I've had to install Pascal's 2 million star database in order to have enough stars nearby. This extended database is available at http://perso.wanadoo.fr/celestia.stars/index.html.

Another way to get enough alignment stars is to use a larger picture of the area which includes many stars.

One source of pictures of large or small areas of the northern sky is http://virtualsky.org/. You can center the sky map on the object you're working with and then zoom in until you get a reference image of the right size.

Align the larger picture to Celestia's stars, then align the smaller picture on the larger one. This is easiest if you use a translucent image: a PNG or DDS image with an alpha channel.

I've found that using an alpha channel that's a greyscale version of the image being aligned works quite weil. The stars are white, so they're opaque, while the darker spaces between the stars become more or less transparent. As a result, it's easy to see the stars of the background picture through the picture being aligned.

When working with more than one picture billboard, put the ones that cover a larger area first in the DSC file. Pictures that cover a smaller area should be later in the same DSC file. This ensures that larger pictures are drawn before smaller ones. The ones drawn last will seem to be in front of the earlier ones. This is necessary because Celestia does not yet depth-sort Nebula objects. If it did, you could just declare them at appropriate distances and the order wouldn't matter.

Once you've managed to get your photograph(s) oriented correctly, add your 3D model as the final object in that same DSC file. This makes Celesia draw your Nebula model last, in front of the pictures.

Then use the same procedures as above to change the model's orientation and position (Axis, Angle, RA, Dec and Radius) until it precisely matches the photograph.

Finally, you can comment out the DSC entry for the photograph and your model will be shown at the right location in space.

6.2 Axis and Angle adjustment details

Although mouse movements can be used to adjust an object's orientation, trying to align the object to properly face the earth can be quite frustrating. Not only does the object often rotate in quite unexpected directions, but finding exactly the right direction for the object to face sometimes can be difficult.

If you've created your models similarly to the way I created the Billboard, then the Z face is the one you want facing toward the Solar System. Celestia's rotational adjustment assumes this is the case.

To orient your object properly, you need to point the normal vector of the positive Z surface toward the viewpoint.

Although this jargon sounds complicated, thinking about it in these terms can be used to simplify how one does the alignment. Below is a solution that I came up with after thinking about it this way.

You can avoid this this manual Z-axis adjustment, at least for setting the initial orientation of your object, if you use the spreadsheet provided by Grant Hutchison.

Briefly, you should align a long, skinny object instead of the relatively flat or round object that you've designed. It will be properly aligned when you've positioned the the object's Marker in the center of the relatively small front (Z) surface of that long, skinny object while you're looking at it from the solar system. The resulting Axis and Angle values can then be used to properly orient the real object.

The alignz.zip archive below contains a CMOD model of a long, skinny box which has a small Z surface covered by an alignment pattern.

• alignz.zip (23KB, updated 8Jun03: new alignment pattern)

  ---

  Archive: alignz.zip
    Length     Date   Time    Name
   --------    ----   ----    ----
          0  06-08-03 14:53   alignz/
        425  06-08-03 17:22   alignz/alignz.dsc
          0  06-08-03 14:53   alignz/models/
        710  05-20-03 05:40   alignz/models/alignz.cmod
          0  06-08-03 14:53   alignz/textures/
          0  06-08-03 14:53   alignz/textures/medres/
      18382  06-08-03 15:16   alignz/textures/medres/12345678.jpg
       8392  06-08-03 17:32   alignz/alignz-readme.txt
   --------                   -------
      27909                   8 files
  

  ---

You can follow the following steps to simplify positioning of your object:

• Modify the file alignz.dsc to contain the desired position of the object you're creating for Celestia.
• Run Celestia so it creates a log file:
  .\celestia --verbose > align.log
  In other words:
  • Open a command window (sometimes misnamed a DOS window)
  • cd to the directory where you put Celestia
  • type the command
    .\celestia --verbose > align.log

    .\celestia
    means "run the program in this directory that's named celestia"

    --verbose
    means "create a log of debugging messages"

    >
    means "send the log output to the file name which follows"

    align.log
    is the name of the log file that you want Celestia to create. You can use any file name you want. I chose "align.log"

  ---

  Here's an example of what I see in the command window when doing this for one of the add-ons I'm working on. For clarity, what Windows types is shown in green, what I type is in black. Windows doesn't actually use different colors.

  ---

  Microsoft Windows XP [Version 5.1.2600] 
  (C) Copyright 1985-2001 Microsoft Corp. 
  
  C:\Documents and Settings\Selden>e: 
  
  E:\>cd "Program Files\Celestia131p5" 
  
  E:\Program Files\Celestia131p5>.\celestia --verbose > align.log 
  
  E:\Program Files\Celestia131p5> 
  

  ---

  ---

• Wait for Celestia to open its window
• Enable Markers with the Control-K command.
• Select alignz and view it telescopically from the solar system
• Use Celestia's builtin alignment controls to center alignz's Marker on its alignment grid.
  • Type the key pair "Shift-~" to enable edit mode (Celestia v1.3.1pre9 or older)
    or
  • Type the key pair "Shift-2" (@) to enable edit mode (Celestia v1.3.1pre10 or newer)
  • Hold down Control-Shift Left-Mouse-Button and use the mouse to adjust the object's x-y orientation until the Marker is centered on the grid.
  • Hold down Control-Shift Right-Mouse-Button and use the mouse to adjust the object's rotation orientation until the object is correctly aligned with the Celestial coordinate system. (Requires Celestia v1.3.1pre3 or later.)
  • Type the key pair "Shift-!" to save your final Axis and Angle values to align.log
• Exit Celestia
• Type align.log
• Put the Axis and Angle values you see there into your object's .DSC file.

  ---

  ---

  E:\Program Files\Celestia131p5>type align.log 
  Opened registry key 
  Using joystick: Microsoft PC-joystick driver 
  nStars: 112523 
  . 
  . 
  Parent body 'TYC 5503-946-1' of 'b' not found. 
  . 
  . 
  Initializing ARB vertex programs . . . 
  Loading ARB vertex program: shaders/diffuse_arb.vp 
  Loading ARB vertex program: shaders/specular_arb.vp 
  Loading ARB vertex program: shaders/haze_arb.vp 
  Loading ARB vertex program: shaders/bumpdiffuse_arb.vp 
  Loading ARB vertex program: shaders/bumphaze_arb.vp 
  Loading ARB vertex program: shaders/shadowtex_arb.vp 
  Loading ARB vertex program: shaders/diffuse_texoff_arb.vp 
  Loading ARB vertex program: shaders/rings_arb.vp 
  Loading ARB vertex program: shaders/ringshadow_arb.vp 
  Loading ARB vertex program: shaders/night_arb.vp 
  Loading ARB vertex program: shaders/glossmap_arb.vp 
  All ARB vertex programs loaded successfully. 
  render path: 5 
  Trying DDS: ./extras/seb/lmc/textures/medres/lmc-2123.dds 
  Trying DDS: ./extras/seb/lmc/textures/medres/phot-05a.dds 
  Trying DDS: ./extras/seb/lmc/textures/medres/30dor_hst_big.dds 
  alignz 
  Orientation: [0.0625254,0.0255321,0.997717], 182.961 
  Saving preferences . . . 
  Opened registry key 
  
  E:\Program Files\Celestia131p5> 
  

  ---

  The line starting with "Orientation" is the one to be inserted into the .DSC file. The line above it shows the name of the object. The Orientation has to be edited to become

  ---

  Axis  [0.0625254 0.0255321 0.997717] 
  Angle 182.961 

  ---

  In words: replace "Orientation" by "Axis", replace the commas "," by spaces " ", and insert "Angle" before the last number. Putting the "Angle" on a separate line is optional.

Here are pictures of Celestia while aligning the object.

Alignz before alignment	no Orientation or Angle settings
Alignz during alignment	A red arrow points to the "edit mode" indication. A yellow arrow points to the high magnification.
Alignz after alignment	Orientation or Angle settings have been set

7.1: A Pie-Plate Galaxy.

This model is not yet available in CMOD format.

---

7.2: A Sphere to map the Sky.

Reminder: these CMOD add-ons require Celestia v1.3.2 prerelease 3 or later. They don't work with earlier versions of the program. In particular, they do not work with v1.3.0 or v1.3.1. Sorry.

Here is a faceted spherical model that displays images on its interior. It can be used as a screen on which to project maps of the sky. It's like having a planetarium within Celestia.

The individual facets of this emodel are 10 degrees on a side. The distortions aren't very noticable from the inside, but when viewed from the outside, the model's edges can be seen. Reducing the size of the facets would make the model smoother, but would greatly increase the size of the file. For example, 3 degree facets would increase the size by about a factor of 10, from about 50KB to half a megabyte.

• mw-msx.cmod (46KB, 20June04)
  This model specifies the surface texture image file mw-msx.*. You can edit it to change the name.

  Celestia only draws surfaces if their surface normal vectors point toward the observer. Usually a sphere's surface normals point outward, so you see an image of the outside of the sphere. When its normals point inward, you see the inside of the sphere instead. The side closest to you, which normally would be opaque, is now transparent because its normals are pointed away from you. Celestia doesn't draw those facets of the sphere. The far side of the interior, which normally would not be drawn, both because it would have been hidden by the front surface and because its normals pointed away from you, is now visible.

  7.2.1: The Infra-Red Milky Way

  Here's an example showing a map of galactic Infrared emissions against the Milky Way.

  Image	Comments
  The MSX IR Mosaic Map of the Milky Way	Cel:// URL: The MSX IR Mosaic Map of the Milky Way
  The fire this time: MSX IR map of the Milky Way	Cel:// URL: The fire this time: MSX IR map of the Milky Way
  The MSX IR Mosaic Map surface texture	mw-msx-2k.png (360KB, 20Jun04) The PNG texture map used for this image was derived from a mosaic of the infrared sky produced by the Midcourse Space Experiment. Using a 2K texture map requires a recent ATI or Nvidia card. See the acknowledgements below. There are several things to note about this model and its surface texture. Unlike the previous (3DS) implementation of this spherical model, the surface texture map does not have to be reversed. Pictures of the sky can be "projected" directly onto the inside of this model. The "simple cylindrical" map that you want to display only has to be scaled (and perhaps padded) so that its dimensions are a power of two on a side. This is a PNG file with an alpha channel. It's transparent everywhere you see black in the thumbnail image. It's square in order to preserve resolution in the vertical direction. The original mosaic was 4Kx120. 120 pixels don't provide much resolution. Surface texture maps used with this model should have 0 degrees of longitude in the center in order to align with the center of the Milky Way.

• mw-msx.cmod (46KB, 20June04)
  (this is a sphere which specifies the surface texture mw-msx.png)

• mw-msx-cmod.zip (KB, 20jun04)

  ---

  Archive:  mw-msx-cmod.zip
    Length     Date   Time    Name
   --------    ----   ----    ----
          0  06-20-04 10:51   mw-msx-cmod/models/
      46401  06-20-04 11:31   mw-msx-cmod/models/mw-msx.cmod
        924  06-20-04 12:27   mw-msx-cmod/mw-msx.dsc
       2401  06-20-04 12:40   mw-msx-cmod/mw-msx-readme.txt
  

  Here's the contents of mw-msx.dsc:

  ---

  Nebula "mw-msx" {
  	Mesh "mw-msx.cmod"
   	Axis [-0.409897 0.623875 0.665406]
  	Angle 217.369
  	RA 17.76033
  	Dec -28.936172
  	Distance 2
  	Radius 10000
  	InfoURL "http://irsa.ipac.caltech.edu/applications/MSX/galplane.html"
  }

  ---

  These Axis, Angle and position values precisely align the sphere with the galactic plane. The center of the sphere is slightly offset from the solar system in order to avoid numeric problems. 2 LY out of 10,000 isn't noticable at all.

---

[back to Contents]

A.0 Appendices.

A.1: Signature.

This Web page is maintained by Selden Ball at Wilson Lab.

A.2: Admonishment.

Please send any questions, comments or corrections to seb@lepp.cornell.edu
Alternatively, you can post a message (anonymously or otherwise) on Celestia's Web Forum or use its Private Message page.

If you don't tell me that something's missing, unclear or wrong, I can't improve it.

A.3: Disclaimer.

The author disclaims all liability and endorsement.
Copyrights may apply.
For details, please read the full text of his disclaimer.