Cubemapping and gluLookAt()

OpenGL OpenGL: Advanced Coding
1

I’ve got a sphere in the middle of a skybox, and I’m using cubemapping to reflect the skybox on the sphere. Because the environment is static i don’t use dynamic cubemapping, instead I just create the cubemap once at startup to speed things up.

I was using glRotatef to spin around the sphere on the y axis, and to make the sphere reflect correctly I just performed the opposite rotation on the texture matrix. However now I want to use gluLookAt to fly all around the sphere and I’m having difficulty making the cubemapped sphere reflect properly. I think creating the cubemap dynamically will work, but this would definitely cause slowdown. Is there anyway to manipulate the texture matrix and avoid dynamic cubemap creation?

2

If the environment is not changing, you definitely don’t need to update the cube map. These functions (in glh_convenience.h) can be used to achieve the results you’re looking for. You need the texture matrix to be the inverse of the camera’s lookat.

// inverse of camera_lookat
inline matrix4f object_lookat(const vec3f & from, const vec3f & to, const vec3f & Up)
{
vec3f look = to - from;
look.normalize();
vec3f up(Up);
up -= look * look.dot(up);
up.normalize();

quaternionf r(vec3f(0,0,-1), vec3f(0,1,0), look, up);
matrix4f m;
r.get_value(m);
m.set_translate(from);
return m;

}

// inverse of object_lookat
inline matrix4f camera_lookat(const vec3f & eye, const vec3f & lookpoint, const vec3f & Up)
{
vec3f look = lookpoint - eye;
look.normalize();
vec3f up(Up);
up -= look * look.dot(up);
up.normalize();

matrix4f t;
t.set_translate(-eye);

quaternionf r(vec3f(0,0,-1), vec3f(0,1,0), look, up);
r.invert();
matrix4f rm;
r.get_value(rm);
return rm*t;

}

Thanks -
Cass

3

ahhh… was having the same problem. Thanks for solving it, Cass.

btw, are you Cass Everitt from nvidia?

4

Thanks alot, cass.
It’s late tongiht, I’ll try implementing it tomorrow

5

I’m glad that helped.

Dakeyras - yes, I’m Cass Everitt from NVIDIA.

Thanks -
Cass

6

Ah cool. Double thanks then, for providing glh to us needy souls =). BTW, is there any documentation available for glh?

7

Sorry - no documentation. I know that limits its usefulness to others.

Cass