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budipro

Float precision problem not over

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Hi all, This is the continuation from my post in http://www.gamedev.net/community/forums/topic.asp?topic_id=330097 After changing my function to convert from geo-coordinate into cartesian into this:
public static Vector3 GetCartesian(AirPoint p)
{
   Vector3 ret = new Vector3();

   double lat_rad = p.m_Lat * m_deg2rad;
   double rad = (m_eqRadius + p.m_Alt) * m_scale;
   double proj = rad * Math.Cos( lat_rad );
   double lon = (p.m_Lng - m_upperleftlon) * m_deg2rad;

   ret.X = (float)(rad * Math.Sin(-lat_rad));
   ret.Z = (float)(proj * Math.Sin(lon) - m_sclThreeQuarterRad);
   ret.Y = (float)(proj * Math.Cos(lon) - m_sclThreeQuarterRad);
		
   return ret;
}
My airplane can fly smoothly. But then I realize that this code works only in a certain area on earth (near the point specified by (0, m_sclThreeQuarterRad, m_sclThreeQuarterRad)). So I decided to change my reference coordinate based on the camera position, which is like this:
public static Vector3 GetCartesian(AirPoint p, VectorD translation)
{
   Vector3 ret = new Vector3();

   double lat_rad = p.m_Lat * m_deg2rad;
   double rad = (m_eqRadius + p.m_Alt) * m_scale;
   double proj = rad * Math.Cos( lat_rad );
   double lon = (p.m_Lng - m_upperleftlon) * m_deg2rad;

   ret.X = (float)(rad * Math.Sin(-lat_rad) + translation.X);
   ret.Z = (float)(proj * Math.Sin(lon) + translation.Z);
   ret.Y = (float)(proj * Math.Cos(lon) + translation.Y);
		
   return ret;
}
With the translation is obtained based on the camera position. Using this way, my airplane is vibrating again. I wonder why. Is there any other idea of changing the world reference coordinate? Thanks a lot.

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