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building view matrix

Started by JohnnyCode Feb 4, 2009 at 8:59 AM 2 replies 2.2k views

Original Post

Feb 4, 2009 at 8:59 AM

hello, I am trying to build a view matrix from parameters eye,at,up. The problem is this, in documentation it says the matrix is build like this:


void grViewMatrixLookAt(float* eye,float* at,float* up,float* res)
{
	grIdentity(res);
	float f[3],s[3],u[3],upn[3];
	grSubVector3(at,eye,f);
	grNormalize3(f,f);
	grNormalize3(up,upn);
	grCross3(f,upn,s);
	grCross3(s,f,u);

	*(res) = s[0];
	*(res+1) = u[0];
	*(res+2) = -f[0];
	*(res+4) = s[1];
	*(res+5) = u[1];
	*(res+6) = -f[1];
	*(res+8) = s[2];
	*(res+9) = u[2];
	*(res+10) = -f[2];

}

now the probelm is that my at and eye vectors are allways the same distance from each other, they just get rotated or translated as a pair, so they allways keep the same distance. Thus in f I allways get the same vector for the f is their subtraction. Then , becouse the up vector is allways the same too , my grCross3(f,upn,s) and grCross3(s,f,u) are allways the same value. So my view matrix is never translated but only rotated. Also I get a strange viewing result. How do I build a correct view matrix by eye,at,up parameters?

sebbit

Feb 4, 2009 at 9:06 AM

In that formula, at seems to be the point to look at. But you want at to be the direction the camera is facing, right?
There meight be a better way to do this but a fast fix would be to set at to position + your view direction (i.e. eye + at (that is the vector you currently use as at)).

Edit: Sorry, didn't look at your code close enough. But, grSubVector3(at,eye,f) should actually do the reverse of what I said above. It takes the camera position and point to look at and builds a vector that just points in the direction you want the camera to face in. So you just have to remove this line (i.e. set f to at) and you should be good to go.

Edit2: Sorry again. Seem to be quit slow at comprehension today. The matrix you build doesn't do any translation at all. It only sets the rotation/scaling part of the matrix. To get propper translation, you would also have to translate by the inverse eye vector (i.e. -eye). So you also have to set the translation part of your matrix to that values (shoule be res + 3, 7, 11 or 12, 13, 14 if I got that confused again).

JohnnyCode

Feb 4, 2009 at 9:40 AM

I am also getting an ugly fish-eye effect. When I use opengl lookat function all is good, so my projection matrix and world matrix are correct.

songho

Feb 4, 2009 at 2:39 PM

Normalize the side and up vectors, after performing cross product.

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