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Pirosan

OpenGL particle not facing camera

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Hiya, i took the code from the http://www.lighthouse3d.com/opengl/tutorials.shtml tutorial on billboarding and implemented it in my engine... only problem is that the particle stands still until i pass by it, then it swivels quickly to face the other direction... i can't figure why its acting this way...
static void billboardSphericalBegin(
				float camX, float camY, float camZ,
				float objPosX, float objPosY, float objPosZ) {
	
		float[] lookAt = new float[3];
		float[] objToCamProj = new float[3];
		float[] upAux = new float[3];
		float[] modelview = new float[16];
		float angleCosine;
	
		GL11.glPushMatrix();
	
		GL11.glDisable(GL11.GL_LIGHTING);
	// objToCamProj is the vector in world coordinates from the 
	// local origin to the camera projected in the XZ plane
		objToCamProj[0] = camX - objPosX ;
		objToCamProj[1] = 0;
		objToCamProj[2] = camZ - objPosZ ;
	
	// This is the original lookAt vector for the object 
	// in world coordinates
		lookAt[0] = 0;
		lookAt[1] = 0;
		lookAt[2] = 1;
	
	
	// normalize both vectors to get the cosine directly afterwards
		Math3d.normalize(objToCamProj);
	
	// easy fix to determine wether the angle is negative or positive
	// for positive angles upAux will be a vector pointing in the 
	// positive y direction, otherwise upAux will point downwards
	// effectively reversing the rotation.
	
		Math3d.CrossProduct(upAux,lookAt,objToCamProj);
	
	// compute the angle
		angleCosine = Math3d.InnerProduct(lookAt,objToCamProj);
	
	// perform the rotation. The if statement is used for stability reasons
	// if the lookAt and objToCamProj vectors are too close together then 
	// |angleCosine| could be bigger than 1 due to lack of precision
	   if ((angleCosine < 0.99990) && (angleCosine > -0.9999))
	      GL11.glRotatef((float)(Math.acos(angleCosine)*180/3.14),upAux[0], upAux[1], upAux[2]);	
	      
	// so far it is just like the cylindrical billboard. The code for the 
	// second rotation comes now
	// The second part tilts the object so that it faces the camera
	
	// objToCam is the vector in world coordinates from 
	// the local origin to the camera
	   float[] objToCam = new float[3];
		objToCam[0] = camX - objPosX;
		objToCam[1] = camY - objPosY;
		objToCam[2] = camZ - objPosZ;
	
	// Normalize to get the cosine afterwards
		Math3d.normalize(objToCam);
	
	// Compute the angle between objToCamProj and objToCam, 
	//i.e. compute the required angle for the lookup vector
	
		angleCosine = Math3d.InnerProduct(objToCamProj,objToCam);
	
	
	// Tilt the object. The test is done to prevent instability 
	// when objToCam and objToCamProj have a very small
	// angle between them
	
		if ((angleCosine < 0.99990) && (angleCosine > -0.9999))
			if (objToCam[1] < 0)
				GL11.glRotatef((float)(Math.acos(angleCosine)*180/3.14),1f,0f,0f);	
			else
				GL11.glRotatef((float)(Math.acos(angleCosine)*180/3.14),-1f,0f,0f);	
	      
	}

oh, and the define macros that were in this tutorial had to be converted (im using lwjgl in java) and here is that class:

public class Math3d {

	static void normalize(float[] vector){
		float dist=(float)Math.sqrt(vector[0]*vector[0]+vector[1]*vector[1]+vector[2]*vector[2]);
		float[] vec= new float[]{vector[0] / dist, vector[1] / dist, vector[2] / dist};
	}
	static void CrossProduct(float[] a,float[] b,float[] c){
		(a)[0] = (b)[1] * (c)[2] - (c)[1] * (b)[2];
		(a)[1] = (b)[2] * (c)[0] - (c)[2] * (b)[0];
		(a)[2] = (b)[0] * (c)[1] - (c)[0] * (b)[1];
	}
	static float InnerProduct(float[] v,float[] q){
		return ((v)[0] * (q)[0] + 
				(v)[1] * (q)[1] + 
				(v)[2] * (q)[2]);
	}
}



any help would be greatly appreciated cheers

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If you are interested in particles you should be interested in Point Sprites; Point Sprite is an extesion that create sprite from point, largely used in particle effects and well supported.

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