Hello!

Just, a short question

Is there a Knot-count-limitation in “gluNurbsSurface”?

I can’t render more then 20 knots and i can’t figure out why!?

If not, i have to check my code once more… and once more…once more…

Thank you!

Hello!

Just, a short question

Is there a Knot-count-limitation in “gluNurbsSurface”?

I can’t render more then 20 knots and i can’t figure out why!?

If not, i have to check my code once more… and once more…once more…

Thank you!

If I am not wrong, the number of knots for each s and t directions in the array depends on the numbers of control points + the order of the NURBS curve for each s and t directions. Maybe the count or the size of your arrays don’t match. I did not find any limitations in documentation like GLU_MAX_KNOT_… . Can you post some code just to verify it.

Thank you trinitrotoluene!

I found nothing about it, too!

My goal is to generate a plane up to 640x480 points (video - one corresponding point per videopixel)

Some Infos:

Mac OSX 10.4.11 / NVIDIA GeForce 7300 GT

XCode

Here is a simple code Example i found on INet and i used for tests.

Everything is working fine till Knots>20; ???

hmmm… is there another fast way, genrating a 640x480 dots plane with connected Quads?

Code:

```
GLfloat ctlpoints[10][10][3];
GLUnurbsObj *theNurb;
void nurbsinit_surface(void)
{
int u, v;
for (u = 0; u < (10); u++) {
for (v = 0; v < (10); v++) {
ctlpoints[u][v][0] = ((GLfloat)u - 1.5);
ctlpoints[u][v][1] = ((GLfloat)v - 1.5);
ctlpoints[u][v][2] = 0.0;
if ( u == 2 && v == 2){
ctlpoints[u][v][2] = 20.0;
}
if ( u == 3 && v == 3){
ctlpoints[u][v][2] = -200.0;
}
}
}
}
void nurbsknotmyinit(void)
{
GLfloat mat_diffuse[] = { 0.7, 0.7, 0.7, 1.0 };
GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat mat_shininess[] = { 100.0 };
glClearColor (0.0, 0.0, 0.0, 1.0);
glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
glMaterialfv(GL_FRONT, GL_SHININESS, mat_shininess);
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
glDepthFunc(GL_LEQUAL);
glEnable(GL_DEPTH_TEST);
glEnable(GL_AUTO_NORMAL);
glEnable(GL_NORMALIZE);
nurbsinit_surface();
theNurb = gluNewNurbsRenderer();
gluNurbsProperty(theNurb, GLU_SAMPLING_TOLERANCE, 25.0);
gluNurbsProperty(theNurb, GLU_DISPLAY_MODE, GLU_FILL);
}
void nurbsdisplay(void)
{
GLfloat knots[20] = {0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0};
GLfloat knotsb[26] = { 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0 };
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glPushMatrix();
glRotatef(330.0, 1.,0.,0.);
glScalef (0.5, 0.5, 0.5);
gluBeginSurface(theNurb);
gluNurbsSurface(theNurb,
20, knots,
20, knots,
10 * 3,
3,
&ctlpoints[0][0][0],
10, 10,
GL_MAP2_VERTEX_3);
gluEndSurface(theNurb);
glPopMatrix();
glFlush();
}
```

GLfloat knots[26] = { 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0 };

```
gluBeginSurface(theNurb);
gluNurbsSurface(theNurb,
26, knots,
26, knots,
13 * 3,
3,
&ctlpoints[0][0][0],
13, 13,
GL_MAP2_VERTEX_3);
gluEndSurface(theNurb);
```

26 knots do not work… but it should!! ? Empty, black Viewport!

The count and the size of the arrays match… or am i blind?

I think the order 10 for a bezier curve may introduce floating point error due to the high exponent for each basis functions. I try the same set of control points with the knot vector

```
void nurbsinit_surface(void)
{
int u, v;
for (u = 0; u < (10); u++) {
for (v = 0; v < (10); v++) {
ctlpoints[u][v][0] = ((GLfloat)u - 1.5);
ctlpoints[u][v][1] = ((GLfloat)v - 1.5);
ctlpoints[u][v][2] = 0.0;
if ((u == 4 || u == 5) && (v == 4 || v == 5))
{
ctlpoints[u][v][2] = 3.0;
}
else
{
ctlpoints[u][v][2] = -3.0;
}
}
}
}
GLfloat knots2[14] = {0.0, 0.0, 0.0, 0.0, 0.25, 0.25,0.25,0.5, 0.5, 0.5, 1.0, 1.0, 1.0, 1.0};
glPushMatrix();
glRotatef(45.0, 1.0,0.0,0.0);
glRotatef(45.0,0.0,1.0,0.0);
gluBeginSurface(theNurb);
gluNurbsSurface(theNurb,
14, knots2,
14, knots2,
10 * 3,
3,
&ctlpoints[0][0][0],
4, 4,
GL_MAP2_VERTEX_3);
gluEndSurface(theNurb);
glPopMatrix();
```

which reduce the order of basis functions and it work. The result is a plane with a “mountain” in the middle.

Thank you very much!

Your example is working. But none of my attempts to get a larger number of Knots

I think, i misunderstand here something completely!!

Do you know a good explanation of “gluNurbsSurface”, made for fools in the Inet?!