Hi! In transforming 2D screen coordinaes to a 3D space coordinates I have read, is a good chance to use the function gluUnProject. But the doc tell to make to calls to this function, one with screen z value to 0.0f, and the second call with screen z value to 1.0f. Then I ask, with 2 calls, I get 2 results. Which is the good one, or if the 2 ar valid, who then I use them to obtain the 3D space coordiante? Thanks, to this forums funs. Bye Hi. My mind is similar to yours, but I know where you are.

Hi Sneftel.

I''m coding a very basic 3D editor, to make maps and later show thems in first person.
Think about a 3D editor, 3D Studio MAX for example, and then think about the 3D perspective viewport, where you can move the mouse and you can see on a status bar the 3D world space coordinate you are pointing to, just with z value of 3D, truncated to 0.0f.

That''s what I''m looking for. Just without painting (perhaps), and only moving over the viewport I can se the 3D space coordinate.

But now, as you say, if nothing is drawn, openGL can''t know how far is the point i''m aiming to. So I think I could draw some kind of invisible big plane, with z=0 (z is vertical axis for me, like 3D Studio MAX) and over that plane calculate the intersection of the line that I get with the 2 calls to gluUnProject(), just to know exactly the famous 3D point.

Jeje, do you know, how to render that kind of invisible plane I refer to? Or you think that''s not a good chance for my porpuse.

Thanks.

Hi.
My mind is similar to yours, but I know where you are.

Simple as ........ nothing, jajaja. I did it!!!

I have taken my institute math book and look for:

- Line equation having to points.
- Plane and Lines interseccion.

Then I did some general ecuations and the result is this:

GLdouble x_obj1,y_obj1,z_obj1,
x_obj2,y_obj2,z_obj2;

GLdouble x_AB, y_AB, z_AB; // Algebra variables
GLdouble factor_t; // for parametric line
// equation
GLdouble space_x,space_y,space_z;

gluUnProject(screenPoint.x,view_height-screenPoint.y,0.0,
modelMatrix,
projMatrix,
viewport,
&x_obj1,&y_obj1,&z_obj1);

gluUnProject(screenPoint.x,view_height-screenPoint.y,1.0,
modelMatrix,
projMatrix,
viewport,
&x_obj2,&y_obj2,&z_obj2);

x_AB = (x_obj2 - x_obj1);
y_AB = (y_obj2 - y_obj1);
z_AB = (z_obj2 - z_obj1);

factor_t = - (y_obj2 / (y_AB));

space_x = x_obj2 + (factor_t * x_AB);
space_y = y_obj2 + (factor_t * y_AB);
space_z = z_obj2 + (factor_t * z_AB);

What I do is to retrieve 2 points, with 2 calls to gluUnProject() the firts with screen_z=0.0f, and the second with screen_z=1.0f. Thus, I have prepare the parametric equation from a line with to 3D points. And then calculate the intersectioin point.

Basicaly, as I know that my plane is at height y = 0, the normal vector is (0,1,0).

Plane equation: Ax + By + Cz + D = 0; As the distance to the origin is 0, then D = 0; Therefore my plane equation is this:

My plane equation: By = 0;

The parametric equation of a line is like this:

x = x1 + ta;
y = y1 + tb; -> The important one to substitute at plane eq.
z = z1 + tc;

In the code it corresponds with this:

space_x = x_obj2 + (factor_t * x_AB);
space_y = y_obj2 + (factor_t * y_AB);
space_z = z_obj2 + (factor_t * z_AB);

Easy to calculate factor_t.

The results are, incredibly precise. Just without painting any kind of invisble plane



Hi.
My mind is similar to yours, but I know where you are.