Hello,

I try to set a good distance for the full view of a particles disk and in the same time have a valid line scale which represents the current value for the scale of the disk as a function of zooming with mouse.

The scene has the following parameters :

```
w_width = 600;
w_height = 600;
g_nearPlane = 0.1f;
g_farPlane = 1000.0f;
```

I do graphics initialization of 3D scene with the following code sample :

```
// Reset line scale value
lineScaleValue = 100.0f;
// Initialize View
glViewport(0, 0, w_width, w_height);
glMatrixMode(GL_PROJECTION); // Select The Projection Matrix
glLoadIdentity(); // Reset The Projection Matrix
// Perspective with angle set to 45 degrees
gluPerspective(45.0f, (float) w_width / w_height, g_nearPlane, g_farPlane);
glMatrixMode(GL_MODELVIEW); // Select The Modelview Matrix
glLoadIdentity(); // Reset The Modelview Matrixi
gluLookAt (0.0, 0.0, 3, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
glScalef(0.03f, 0.03f, 0.03f);
```

The above values for gluLookAt and glScalef have been chosen approximately, i.e manually without having an accurate calculation.

Here’s the result on this image :

[ATTACH=CONFIG]1261[/ATTACH]

I would like to have only a full view for the disk (white particles).

For this, I know the maximum and minimum in the (x,y) plane of the disk :

-25 < x <25 and -22 < y < 22. As you can see on above figure, the line scale has not a good value ( 100 kpc (kiloparsec) is too high if you consider that radius of disk is about 25 kpc).

from what I know, I could do (with Fov/2 = 45 degrees) :

```
z_distance = 25*tan(pi/4) + dist = 50
```

with dist = 25 to be in front of the disk.

Instead of

```
gluLookAt (0.0, 0.0, 3, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
glScalef(0.03f, 0.03f, 0.03f);
```

Can I do directly in my sample code :

```
gluLookAt (0.0, 0.0, 50, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
```

?

But this solution doesn’t work, the size of the disk seems to fill all the window but particles don’t display very well, as they were little points :

[ATTACH=CONFIG]2098[/ATTACH]

How can I fix this ?

If anyone sees where my error is in the code sample, can he report to me.

Thanks

PS : I have forgotten to say that I use the following gluLookAt function :

```
void GLWidget::gluLookAt(GLfloat eyex, GLfloat eyey, GLfloat eyez, GLfloat centerx, GLfloat centery, GLfloat centerz, GLfloat upx, GLfloat upy, GLfloat upz)
{
GLfloat m[16];
GLfloat x[3], y[3], z[3];
GLfloat mag;
/* Make rotation matrix */
/* Z vector */
z[0] = eyex - centerx;
z[1] = eyey - centery;
z[2] = eyez - centerz;
mag = sqrt(z[0] * z[0] + z[1] * z[1] + z[2] * z[2]);
if (mag) { /* mpichler, 19950515 */
z[0] /= mag;
z[1] /= mag;
z[2] /= mag;
}
/* Y vector */
y[0] = upx;
y[1] = upy;
y[2] = upz;
/* X vector = Y cross Z */
x[0] = y[1] * z[2] - y[2] * z[1];
x[1] = -y[0] * z[2] + y[2] * z[0];
x[2] = y[0] * z[1] - y[1] * z[0];
/* Recompute Y = Z cross X */
y[0] = z[1] * x[2] - z[2] * x[1];
y[1] = -z[0] * x[2] + z[2] * x[0];
y[2] = z[0] * x[1] - z[1] * x[0];
/* mpichler, 19950515 */
/* cross product gives area of parallelogram, which is < 1.0 for
* non-perpendicular unit-length vectors; so normalize x, y here
*/
mag = sqrt(x[0] * x[0] + x[1] * x[1] + x[2] * x[2]);
if (mag) {
x[0] /= mag;
x[1] /= mag;
x[2] /= mag;
}
mag = sqrt(y[0] * y[0] + y[1] * y[1] + y[2] * y[2]);
if (mag) {
y[0] /= mag;
y[1] /= mag;
y[2] /= mag;
}
#define M(row,col) m[col*4+row]
M(0, 0) = x[0];
M(0, 1) = x[1];
M(0, 2) = x[2];
M(0, 3) = 0.0;
M(1, 0) = y[0];
M(1, 1) = y[1];
M(1, 2) = y[2];
M(1, 3) = 0.0;
M(2, 0) = z[0];
M(2, 1) = z[1];
M(2, 2) = z[2];
M(2, 3) = 0.0;
M(3, 0) = 0.0;
M(3, 1) = 0.0;
M(3, 2) = 0.0;
M(3, 3) = 1.0;
#undef M
glMultMatrixf(m);
/* Translate Eye to Origin */
glTranslatef(-eyex, -eyey, -eyez);
}
```

and the following gluPerspective function ;

```
void GLWidget::gluPerspective( GLfloat fovY, GLfloat aspect, GLfloat zNear, GLfloat zFar )
{
const GLfloat pi = M_PI;
GLfloat fW, fH;
//fH = tan( (fovY / 2) / 180 * pi ) * zNear;
fH = tan( fovY / 360 * pi ) * zNear;
fW = fH * aspect;
glFrustum( -fW, fW, -fH, fH, zNear, zFar );
}
```