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Thread: Culling Math

  1. #1
    Junior Member Newbie
    Join Date
    Dec 2002

    Culling Math

    I need to take some math classes I think. In the meantime... I have a 'how do I...' question.

    I have a player position (P), and a player direction vector (D).

    I want to make 4 planes, that are view frustum planes (excluding near and far planes). Left (L), Right (R), Top (T), and Bottom (B).

    Now, all I have to do, is set the normal of the clipping planes to (D). Then, I rotate the plane how I want it. My problem is, the player is not at coordinate 0, 0, 0. So, I have to set the 'D' (distance from origin) attribute of the planes. But, how do I figure this?

    Code :
        P      |
        |      |
    So in the crappy diagram above, the player is P and the line from P points in the direction he is facing. How do I get planes set up so it's like an upside down 'V' with the planes meeting at the player's position?

    Keith Jackson

  2. #2
    Senior Member Regular Contributor
    Join Date
    Mar 2002
    Melbourne, Australia

    Re: Culling Math

    Here's how I do it:
    As input it takes player's origin and his (her ) view direction, so this should be what you need.
    Code :
    void Frustum_c::Build(const Vec3_c &Origin, const Vec3_c &View)
    	Matrix_c M1, M2;
    	this->Origin = Origin;
    	glGetFloatv(GL_PROJECTION_MATRIX, M1);
    	glGetFloatv(GL_MODELVIEW_MATRIX, M2);
    	// Near Plane
    	Planes[0].Normal = View;
    	Planes[0].Distance = this->Origin.Dot( View );
    	// Left Plane 
    	Planes[1].Normal[0] = M1[3] + M1[0];
    	Planes[1].Normal[1] = M1[7] + M1[4];
    	Planes[1].Normal[2] = M1[11] + M1[8];
    	Planes[1].Distance = -M1[15] - M1[12];
    	// Bottom Plane 
    	Planes[2].Normal[0] = M1[3] + M1[1];
    	Planes[2].Normal[1] = M1[7] + M1[5];
    	Planes[2].Normal[2] = M1[11] + M1[9];
    	Planes[2].Distance = -M1[15] - M1[13];
    	// Top Plane 
    	Planes[3].Normal[0] = M1[3] - M1[1];
    	Planes[3].Normal[1] = M1[7] - M1[5];
    	Planes[3].Normal[2] = M1[11] - M1[9];
    	Planes[3].Distance = -M1[15] + M1[13];
    	// Right Plane 
    	Planes[4].Normal[0] = M1[3] - M1[0];
    	Planes[4].Normal[1] = M1[7] - M1[4];
    	Planes[4].Normal[2] = M1[11] - M1[8];
    	Planes[4].Distance = M1[12] - M1[15];
    	// Normalize them
    	for (int i = 0; i < 5; i++)
    Hope it helps.

  3. #3
    Senior Member Regular Contributor
    Join Date
    Mar 2002
    Melbourne, Australia

    Re: Culling Math

    Ah, I need to add something - it assumes you have all of your translations set through OpenGL, so you can read in matrices.

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