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Index › Programming Questions & Help › Syntax Questions › Mapping points on an imaginary sphere
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Mapping points on an imaginary sphere (Read 459 times)
zooto68
Mapping points on an imaginary sphere
Oct 29th, 2008, 9:35am  
I am trying to write a planetarium software and I want to be able to map the positions of stars onto a sphere. The view would be based at the centre of the invisible sphere and stars would be mapped onto the relevant position on the sphere using their Right Ascension and Declination co-ordinates. I then want to be able to roll that sphere around and see the stars and constellations move around.

Can anyone help with how I may go about doing this please?
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Amateur Astronomer/Astrophotographer and Electronics Hobbyist.
 
WebDext
Re: Mapping points on an imaginary sphere
Reply #1 - Nov 3rd, 2008, 8:49pm  
This sounds like a typical problem of converting between different projections.

If you can get your star map stored in a format where x and y positions are between 0 and 1, you can pipe them into a spherical mapping that will place them in 3D world space and then just provide processing with X and Y rotations to change your view.

This is probably the simplest way to do this:
Quote:
float starMapX[] = null;
float starMapY[] = null;

void setup()
{
  size(640,480,P3D);
  
  final int starCount = 1000;
  starMapX = new float[starCount];
  starMapY = new float[starCount];
  for( int i=0; i<starCount; ++i )
  {
    starMapX[i] = random(0,1);
    starMapY[i] = random(0,1);
  }
}

float rot;
void draw()
{
  background (0);
  stroke     (255);

  // Some basic settings
  final float TWOPI            = 2.0f * PI;
  final float radius           = 150.0f;
  final float sphere_origin_x  =  width/2;
  final float sphere_origin_y  =  height/2;
  final float sphere_origin_z  =  width/2;
  
  rot += 0.01f;

  pushMatrix();
  
    translate ( sphere_origin_x,
                sphere_origin_y,
                sphere_origin_z );
    rotateX   ( rot );
    rotateY   ( rot * 3.0 );

    // For each star position in (x,y) cylindrical format, convert to world coordinates
    for( int i=0; i<starMapX.length; ++i )
    {
      float lat          = starMapX[i] * TWOPI;
      float lon          = starMapY[i] * TWOPI - PI;
    
      float star_world_x = sin(lat) * cos(lon) * radius;
      float star_world_y = sin(lon)            * radius;
      float star_world_z = cos(lat) * cos(lon) * radius;
    
      point ( star_world_x,
              star_world_y,
              star_world_z );
    }
    
  popMatrix();    
}
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fjen
Re: Mapping points on an imaginary sphere
Reply #2 - Nov 4th, 2008, 10:20am  
no, actually processing can do all the math for you:
Code:

float starMapX[] = null; 
float starMapY[] = null;
float radius = 150;
float rot; 
float sphere_origin_x; 
float sphere_origin_y; 
float sphere_origin_z; 

void setup() 
{ 
    size(640,480,P3D); 

    final int starCount = 1000; 
    starMapX = new float[starCount]; 
    starMapY = new float[starCount]; 
    for( int i=0; i<starCount; ++i ) 
    { 
	  starMapX[i] = random(0,TWO_PI); 
	  starMapY[i] = random(0,TWO_PI); 
    } 
    
    sphere_origin_x  =  width/2; 
    sphere_origin_y  =  height/2; 
    sphere_origin_z  =  width/2; 
} 

void draw() 
{ 
    background (0); 
    stroke     (255);

    rot += 0.01f; 

    pushMatrix(); 

    translate ( sphere_origin_x, sphere_origin_y, sphere_origin_z ); 
    rotateX   ( rot ); 
    rotateY   ( rot * 3.0 ); 

    for( int i=0; i<starMapX.length; ++i ) 
    {
	  pushMatrix();
	  
	  rotateX( starMapX[i] );
	  rotateY( starMapY[i] );
	  translate( radius, 0, 0 );
	  
	  point ( 0,0,0 );
	  
	  popMatrix();
    } 

    popMatrix();     
}


F
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zooto68
Re: Mapping points on an imaginary sphere
Reply #3 - Nov 5th, 2008, 10:31am  
That will put random stars around a sphere but will not put the position of a star on that sphere in the correct place according to its RA and DEC co-ordinates.
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Amateur Astronomer/Astrophotographer and Electronics Hobbyist.
 
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