Texture coordinates for triangle fans

Question

I am trying to "fill" a surface of some geometry that I drew. I am using GL_TRIANGLE_FAN primitive. (for example : 1 hub (center) point and 12 other points). I have calculated texture coordinates for each vertex in the interval 0-1. But as a result I get this **image** , its a little bit confused. I desire to get result like that image . Please help, what is wrong here? How can I calculate correct texture coordinates in such Triangulation ( GL_TRIANGLE_FAN ) in the image red dots is my points

*Code - Snippet :

 assert(("CROSS type intersection needs only 5 vertices : center point and "
         "rest points in anticlockwise order", (lp->size() > 5) && (lp->size() < 5)));
 osg::Vec3 vAlong_1,vAlong_2;
 vAlong_1 = (*lp)[1] - (*lp)[4];
 vAlong_2 = (*lp)[1] - (*lp)[2];
 eps = ((*lp)[2] - (*lp)[4]).length() * 0.2 / 2;
 vAlong_1.normalize();
 vAlong_2.normalize();

 _edgeCoords->push_back((*lp)[0]);
 _edgeCoords->push_back((*lp)[1]);
 if (CMF::euclidDistance((*lp)[0],(*lp)[1]) <= CMF::euclidDistance((*lp)[0],(*lp)[2])) {
    float cosAlpha = -(vAlong_1 * vAlong_2);
    float extraLength = ((*lp)[2] - (*lp)[1]).length() * cosAlpha;
     _edgeCoords->push_back((*lp)[1] + vAlong_1 * (eps + extraLength));
     _edgeCoords->push_back((*lp)[2] + vAlong_1 * eps);
 } else {
     float cosAlpha = (vAlong_1 * vAlong_2);
     float extraLength = ((*lp)[2] - (*lp)[1]).length() * cosAlpha;
     _edgeCoords->push_back((*lp)[1] + vAlong_1 * eps);
     _edgeCoords->push_back((*lp)[2] + vAlong_1 * (eps + extraLength));
 }
_edgeCoords->push_back((*lp)[2]);
if (CMF::euclidDistance((*lp)[0],(*lp)[2]) <= CMF::euclidDistance((*lp)[0],(*lp)[3])) {
   float cosAlpha = -(vAlong_1 * vAlong_2);
   float extraLength = ((*lp)[3] - (*lp)[2]).length() * cosAlpha;
    _edgeCoords->push_back((*lp)[2] - vAlong_2 * (eps + extraLength));
    _edgeCoords->push_back((*lp)[3] - vAlong_2 * eps);
} else {
    float cosAlpha = (vAlong_1 * vAlong_2);
    float extraLength = ((*lp)[3] - (*lp)[2]).length() * cosAlpha;
    _edgeCoords->push_back((*lp)[2] - vAlong_2 * eps);
    _edgeCoords->push_back((*lp)[3] - vAlong_2 * (eps + extraLength));
}
_edgeCoords->push_back((*lp)[3]);
if (CMF::euclidDistance((*lp)[0],(*lp)[3]) <= CMF::euclidDistance((*lp)[0],(*lp)[4])) {
   float cosAlpha = -(vAlong_1 * vAlong_2);
   float extraLength = ((*lp)[4] - (*lp)[3]).length() * cosAlpha;
    _edgeCoords->push_back((*lp)[3] - vAlong_1 * (eps + extraLength));
    _edgeCoords->push_back((*lp)[4] - vAlong_1 * eps);
} else {
    float cosAlpha = (vAlong_1 * vAlong_2);
    float extraLength = ((*lp)[4] - (*lp)[3]).length() * cosAlpha;
    _edgeCoords->push_back((*lp)[3] - vAlong_1 * eps);
    _edgeCoords->push_back((*lp)[4] - vAlong_1 * (eps + extraLength));
}
_edgeCoords->push_back((*lp)[4]);
if (CMF::euclidDistance((*lp)[0],(*lp)[1]) <= CMF::euclidDistance((*lp)[0],(*lp)[4])) {
   float cosAlpha = -(vAlong_1 * vAlong_2);
   float extraLength = ((*lp)[4] - (*lp)[1]).length() * cosAlpha;
    _edgeCoords->push_back((*lp)[4] + vAlong_2 * eps);
    _edgeCoords->push_back((*lp)[1] + vAlong_2 * (eps + extraLength));
} else {
    float cosAlpha = (vAlong_1 * vAlong_2);
    float extraLength = ((*lp)[4] - (*lp)[1]).length() * cosAlpha;
    _edgeCoords->push_back((*lp)[4] + vAlong_2 * (eps + extraLength));
    _edgeCoords->push_back((*lp)[1] + vAlong_2 * eps);
}
_edgeCoords->push_back((*lp)[1]);

_tCoords->push_back(osg::Vec2(0.5,0.5));
_tCoords->push_back(osg::Vec2(0.666,0.666));
_tCoords->push_back(osg::Vec2(0.666,1.0));
_tCoords->push_back(osg::Vec2(0.333,1.0));
_tCoords->push_back(osg::Vec2(0.333,0.666));
_tCoords->push_back(osg::Vec2(0.0,0.666));
_tCoords->push_back(osg::Vec2(0.0,0.333));
_tCoords->push_back(osg::Vec2(0.333,0.333));
_tCoords->push_back(osg::Vec2(0.333,0.0));
_tCoords->push_back(osg::Vec2(0.666,0.0));
_tCoords->push_back(osg::Vec2(0.666,0.333));
_tCoords->push_back(osg::Vec2(1.0,0.333));
_tCoords->push_back(osg::Vec2(1.0,0.666));
_tCoords->push_back(osg::Vec2(0.666,0.666));

@CherkesgillerTural i'm confused - what's the final result you want, when fitted on the fan? you've posted the original texture; do you want to fit each number on to the fan with equal size, and discard the yellow bits? — user3235832
Yep looks like texture image shape does not resemble your geometry so it gets distorted .... — Spektre
@willywonkadailyblah yes absolutely I want result which you mention in post. I already changed image of desired result. How can I do it — Cherkesgiller
I believe your geometry is wrong - if you want uniformly sized numbers then the geometry must match that, otherwise the stretching of the texture is non-uniform, hence the artifacts. unless you dont? — user3235832

Quinchilion Quinchilion · Accepted Answer · 2016-06-07T23:51:51

Try keeping the 2d positions always equal to the texture coordinates for each vertex. That will ensure your geometry appears as an undistorted cutout of your texture. You can then rescale and center the mesh as you like without distorting the texture by applying transforms to the vertex positions.

One way to do this would be to create a function that pushes a single vertex, accepting the 2d coordinates of the vertex and any transforms you want to apply. The function would then push the 2d coordinates as texcoords, then transform them and push the result as positions.

Texture coordinates for triangle fans

1 Answers