Help with nan
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Thread: Help with nan

  1. #1
    Join Date
    Mar 2009
    Posts
    20

    Help with nan

    Hi all

    Yet another question from me and hope some one shoots the answer immediately.. very simple though

    I have been tryin to do a rigid body simulation ... things work fine until i hit a nan ..

    What I am trying to do is retrieve a angle from a quaternion for which i wrote this following function...

    Code:
    
    				void get_AxisAngle_from_Quat(float &angle, Xlib::vec3d &axis) 
    
    				{
    
    					float squareLength = get4d_X()*get4d_X() + get4d_Y()*get4d_Y() + get4d_Z()*get4d_Z();
    
    					
    						
    
    					if (squareLength>C_EPSILON)
    
    					{
    
    						angle = 2.0f * (float) acos(get4d_Z());
    						//assert (!XlibMath::is__nan(angle));
    
    						float inverseLength ;
    						inverseLength = 1.0f /  (float) pow(squareLength, 0.5f) ;
    
    						axis.x = get4d_X() * inverseLength;
    
    						axis.y = get4d_Y() * inverseLength;
    
    						axis.z = get4d_Z() * inverseLength;
    
    					}
    
    					else
    
    					{
    
    						angle = 0.0f;
    
    						axis.x = 1.0f;
    
    						axis.y = 0.0f;
    
    						axis.z = 0.0f;
    
    					}
    
    				}
    This is being refered in the code by this way
    Code:
            Xlib::vec3d axis(1,0,0);
    	float angle=0.0f;
    	
    	RBody[0].q.get_AxisAngle_from_Quat(angle,axis);
    	
    	Xlib::DevMsg("Quat  ",RBody[0].q);
    	Xlib::DevMsg("Angle ", 	angle);
    but This hits a nan and everytime it hits nan my object disappears from the screen.... (DevMsg is a overload for cout i use) When i try printing out the values at the point where i hit the nan this shows this

    Code:
    DevMsg: Quat   1.442195 0.698514 1.700220 0.403262
    Angle  nan
    I read online but it states nan is got by dividing by 0 .. but clearly i am not using any divisions at al...

    can someone clarify me on where i went wrong ????

    Cheers and regards

  2. #2
    Join Date
    May 2002
    Posts
    1,435

    Re: Help with nan

    You don't state where you are getting the NAN error, but your code indicates that it may be in the call to acos().

    Note that the domain for arccosine is -1 to +1 so you need to determine why you are asking for the arccosine of a number outside of its domain. In other words, assert on the NAN but then back up from there to determine the cause.

  3. #3
    Lindley is offline Elite Member Power Poster
    Join Date
    Oct 2007
    Location
    Fairfax, VA
    Posts
    10,893

    Re: Help with nan

    NaN isn't just division by 0---it may be used for any calculation which has an undefined result. Trying to take the acos of a vector of length 0 would do it.

  4. #4
    Join Date
    Oct 2000
    Location
    London, England
    Posts
    4,773

    Re: Help with nan

    acos of 0 is well defined to be pi/2

  5. #5
    Join Date
    May 2002
    Posts
    1,435

    Re: Help with nan

    angle = 2.0f * (float) acos(get4d_Z());
    //assert (!XlibMath::is__nan(angle));

    You show the values for Xlib::vec3d axis but not for get4d_Z(). What is get4d_Z()? If it's not the Z-coordinate of a unit vector then that could be your problem.

  6. #6
    Lindley is offline Elite Member Power Poster
    Join Date
    Oct 2007
    Location
    Fairfax, VA
    Posts
    10,893

    Re: Help with nan

    Quote Originally Posted by NMTop40 View Post
    acos of 0 is well defined to be pi/2
    Oh, right. I was thinking of atan2.

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