步履蹒跚

The GL_OBJECT_LINEAR function calculates the texture coordinates in the model's coordinate system. In Example 9-3 , where the contour lines are perpendicular to the base of the teapot, they would remain so, no matter how the teapot was rotated or viewed. Sometimes you'd like the contour lines to be calculated relative to the eye's coordinate system. In other words, you want to multiply the vector (p1p2p3p4) by the inverse of the modelview matrix before calculating the distance to the plane. If you specify GL_TEXTURE_GEN_MODE with GL_EYE_LINEAR, this is exactly what happens. The texture coordinate is generated with the following function:

generated coordinate = p1'xe + p2'ye + p3'ze + p4'we

where (p1' p2' p3' p4') = (p1p2p3p4)M-1

In this case, (xe, ye, ze, we) are the eye coordinates of the vertex, and p1, ..., p4 are supplied as the param argument to glTexGen*() with pname set to GL_EYE_PLANE. The primed values are calculated only once, at the time they're specified, so this operation isn't as computationally expensive as it looks. To see the effect of this function, in the example above, change sgenparams back to {1, 0, 0, 0}, and change GL_OBJECT_LINEAR to GL_EYE_LINEAR. The result is red stripes parallel to the y-z plane from the eye's point of view, as shown in Figure J-25

#ifdef AUTO_TEXTURE_COORD_GEN
    GLfloat eyePlaneS[] = { 1.0, 0.0, 0.0, 0.0 };
    GLfloat eyePlaneT[] = { 0.0, 1.0, 0.0, 0.0 };
    GLfloat eyePlaneR[] = { 0.0, 0.0, 1.0, 0.0 };
    GLfloat eyePlaneQ[] = { 0.0, 0.0, 0.0, 1.0 };
    glTexGeni(GL_S, GL_TEXTURE_GEN_MODE, GL_EYE_LINEAR);
    glTexGenfv(GL_S, GL_EYE_PLANE, eyePlaneS);
    glTexGeni(GL_T, GL_TEXTURE_GEN_MODE, GL_EYE_LINEAR);
    glTexGenfv(GL_T, GL_EYE_PLANE, eyePlaneT);
    glTexGeni(GL_R, GL_TEXTURE_GEN_MODE, GL_EYE_LINEAR);
    glTexGenfv(GL_R, GL_EYE_PLANE, eyePlaneR);
    glTexGeni(GL_Q, GL_TEXTURE_GEN_MODE, GL_EYE_LINEAR);
    glTexGenfv(GL_Q, GL_EYE_PLANE, eyePlaneQ);
#endif