j

12-06-2000, 04:33 PM

I've been working on figuring out cube maps and I've managed to bind, enable, and display them. Now I'm trying to get the Reflection TexGen functionality working. In the extension specification it says:

An application note: When using cube mapping with dynamic cube

maps (meaning the cube map texture is re-rendered every frame),

by keeping the cube map’s orientation pointing at the eye position,

the texgen-computed reflection or normal vector texture coordinates

can be always properly oriented for the cube map. However if the

cube map is static (meaning that when view changes, the cube map

texture is not updated), the texture matrix must be used to rotate

the texgen-computed reflection or normal vector texture coordinates

to match the orientation of the cube map. The rotation can be

computed based on two vectors: 1) the direction vector from the cube

map center to the eye position (both in world coordinates), and 2)

the cube map orientation in world coordinates. The axis of rotation

is the cross product of these two vectors; the angle of rotation is

the arcsin of the dot product of these two vectors.

What I think this means is that to get the reflection to look correct with a static cube map, you need to rotate the texture matrix. My code to do this is:

void setupCubeTextureMatrix(float cmo1, float cmo2, float cmo3, float camp1, float camp2, float camp3, float cmor1, float cmor2, float cmor3)

{

float v1 = camp1 - cmo1;

float v2 = camp2 - cmo2;

float v3 = camp3 - cmo3;

float rotaxisx = v2 * cmor3 - cmor2 * v3;

float rotaxisy = cmor1 * v3 - cmor3 * v1;

float rotaxisz = v1 * cmor2 - cmor1 * v2;

float angle = asin(v1 * cmor1 + v2 * cmor2 + v3 * cmor3);

glRotatef(angle, rotaxisx, rotaxisy, rotaxisz);

}

The cmo variables are the x, y, and z coordinates of the cube map orgin, the camp variables are the x, y, and z coordinates of the camera position, and the cmor variables are the x, y, and z coordinates for the cube map orientation.

In my actual drawing code, I do this

glMatrixMode(GL_TEXTURE);

glLoadIdentity();

setupCubeTextureMatrix(0, 0, 0, camerax, cameray, cameraz, 0, 0, 1);

glMatrixMode(GL_MODELVIEW);

gluSphere(sphere, 0.5f, 64, 64);

I have enabled normal generation for the sphere, so that is not the reason it isn't working.

The problem is that this does not actually rotate the texture properly. It always looks like I am looking at the sphere from the same viewpoint, even when I view the sphere from different directions.

Does anybody have any idea what I am doing wrong, or have any example code for doing this?

Thanks in advance.

j

An application note: When using cube mapping with dynamic cube

maps (meaning the cube map texture is re-rendered every frame),

by keeping the cube map’s orientation pointing at the eye position,

the texgen-computed reflection or normal vector texture coordinates

can be always properly oriented for the cube map. However if the

cube map is static (meaning that when view changes, the cube map

texture is not updated), the texture matrix must be used to rotate

the texgen-computed reflection or normal vector texture coordinates

to match the orientation of the cube map. The rotation can be

computed based on two vectors: 1) the direction vector from the cube

map center to the eye position (both in world coordinates), and 2)

the cube map orientation in world coordinates. The axis of rotation

is the cross product of these two vectors; the angle of rotation is

the arcsin of the dot product of these two vectors.

What I think this means is that to get the reflection to look correct with a static cube map, you need to rotate the texture matrix. My code to do this is:

void setupCubeTextureMatrix(float cmo1, float cmo2, float cmo3, float camp1, float camp2, float camp3, float cmor1, float cmor2, float cmor3)

{

float v1 = camp1 - cmo1;

float v2 = camp2 - cmo2;

float v3 = camp3 - cmo3;

float rotaxisx = v2 * cmor3 - cmor2 * v3;

float rotaxisy = cmor1 * v3 - cmor3 * v1;

float rotaxisz = v1 * cmor2 - cmor1 * v2;

float angle = asin(v1 * cmor1 + v2 * cmor2 + v3 * cmor3);

glRotatef(angle, rotaxisx, rotaxisy, rotaxisz);

}

The cmo variables are the x, y, and z coordinates of the cube map orgin, the camp variables are the x, y, and z coordinates of the camera position, and the cmor variables are the x, y, and z coordinates for the cube map orientation.

In my actual drawing code, I do this

glMatrixMode(GL_TEXTURE);

glLoadIdentity();

setupCubeTextureMatrix(0, 0, 0, camerax, cameray, cameraz, 0, 0, 1);

glMatrixMode(GL_MODELVIEW);

gluSphere(sphere, 0.5f, 64, 64);

I have enabled normal generation for the sphere, so that is not the reason it isn't working.

The problem is that this does not actually rotate the texture properly. It always looks like I am looking at the sphere from the same viewpoint, even when I view the sphere from different directions.

Does anybody have any idea what I am doing wrong, or have any example code for doing this?

Thanks in advance.

j