Hi

say I have two cube which I can move around the scene - how could I go about showing their resultant area when they intersect - either in a different colour or transparent - would I have to set up vectors, normals etc.. also say I have a small sphere which I want to light up only when it enters the area of the cube? how could you show that it is in side the cube?

cheers

DFrey

01-10-2001, 04:02 AM

For displaying the intersecting volume. You just use the planes of each side of one cube to clip the sides of the other cube, keeping the part behind the clipping plane, and adding new faces where the clipping planes intersect the other cube. Then you just draw the intersecting volume, then each cube, with depth testing appropriately set so that the intersecting parts of the cubes don't overwrite the just drawn intersected volume.

could you sample code? what about if i rotate one cube at 45 degrees then intersect them - also would this work with say cylinders?

thanx

DFrey

01-10-2001, 07:31 AM

I don't have any code available, but the idea is rather straight forward. There is a method that can be implemented for two simple convex polyhedra. In this method you ignore the fact that you have cubes or cylinders or whatever as what only matters are the planes of each face, of each polyhedron. What you do is first intersect each plane with every other plane. This will produce a set of lines. Then you intersect each line with every other line. This will give you a set of points. Then clip the set of points by all of the planes, keeping only the points that are on or behind all of the planes. This final set of points are the vertices of the intersecting volume. Then intersect this final set of points by each plane, to pick only the points that are on that plane. If there is more than one point in the plane, then these points define an edge if it is two points and so can be ignored as it will be part of a face as well, and a face if it is more than two points. To create the face, the vertices must be connected in either ccw or cw order (whichever you are using). After this process is completed, you will have the set of faces for the intersecting volume (if any such volume exists).

[This message has been edited by DFrey (edited 01-11-2001).]

Could you illustrate with the below:

I have three cubes of the same dimension drawn counter clockwise say 2 units apart if one is movedand made to intersect the second how could i show the resultant area

glPushMatrix();

//Top of cube

glPushMatrix();

glMaterialfv(GL_FRONT, GL_AMBIENT_AND_DIFFUSE, BlueSurface);

glBegin(GL_POLYGON);

glNormal3d( 0.0, 0.1776, 0.0);

glVertex3d( 0.363, 0.0575, -0.1225);//FRIGHT

glVertex3d( -0.363, 0.0575, -0.1225);//FLEFT

glVertex3d( -0.363, 0.0575, 0.1225);//NLEFT

glVertex3d( 0.363, 0.0575, 0.1225);//NRIGHT

glEnd();

glPopMatrix();

//Front face of cube

glPushMatrix();

glMaterialfv(GL_FRONT, GL_AMBIENT_AND_DIFFUSE, BlueSurface);

glBegin(GL_POLYGON);

glNormal3d( 0.0, 0.0, 0.0);

glVertex3d( 0.363, -0.0575, 0.1225);//top right corner

glVertex3d(-0.363, -0.0575, 0.1225); //top left corner

glVertex3d(-0.363, -0.0575,0.1225); //bottom left corner

glVertex3d( 0.363, -0.0575,0.1225); //bottom right corner

glEnd();

glPopMatrix();

//horizontal right face

glPushMatrix();

glMaterialfv(GL_FRONT, GL_AMBIENT_AND_DIFFUSE, BlueSurface);

glBegin(GL_POLYGON);

glNormal3d( 0.02818, 0.0, 0.0);

glVertex3d( 0.363, 0.0575, -0.1225);//FTOP

glVertex3d( 0.363, 0.0575, 0.1225);//NTOP

glVertex3d( 0.363, -0.0575, 0.1225);//NBOTTOM

glVertex3d( 0.363, -0.0575, -0.1225);//FBOTTOM

glEnd();

glPopMatrix();

//Back face of cube

glPushMatrix();

glMaterialfv(GL_FRONT, GL_AMBIENT_AND_DIFFUSE, BlueSurface);

glBegin(GL_POLYGON);

glNormal3d( 0.0, 0.0, -0.083375);

glVertex3d( -0.363, 0.0575, -0.1225);//top left

glVertex3d( 0.363, 0.0575, -0.1225);//top right

glVertex3d( 0.363, -0.0575, -0.1225);//bottom right

glVertex3d( -0.363, -0.0575, -0.1225);//bottom left

glEnd();

glPopMatrix();

//horizontal left face

glPushMatrix();

glMaterialfv(GL_FRONT, GL_AMBIENT_AND_DIFFUSE, BlueSurface);

glBegin(GL_POLYGON);

glNormal3d( -0.028175, 0.0, 0.0);

glVertex3d( -0.363, 0.0575, 0.1225);//NTOP

glVertex3d( -0.363, 0.0575, -0.1225);//FTOP

glVertex3d( -0.363, -0.0575, -0.1225);//FBOTTOM

glVertex3d( -0.363, -0.0575, 0.1225);//NBOTTOM

glEnd();

glPopMatrix();

//Bottom face of cube

glPushMatrix();

glMaterialfv(GL_FRONT, GL_AMBIENT_AND_DIFFUSE, BlueSurface);

glBegin(GL_POLYGON);

glNormal3d( 0.0, -0.1776, 0.0);

glVertex3d( 0.363, -0.0575, 0.1225);//NRIGHT

glVertex3d( -0.363, -0.0575, 0.1225);//NLEFT

glVertex3d( -0.363, -0.0575, -0.1225);//FLEFT

glVertex3d( 0.363, -0.0575, -0.1225);//FRIGHT

glEnd();

glPopMatrix();

glPopMatrix();

Michael Steinberg

01-11-2001, 03:31 AM

AS I think this belongs to CSG.

How does one make CSG stuff with UNconvex bodies. I found myself doing CSG with convex bodies, but the results tend to be UNconvex and I can't work any further with them. Is the trick to cut them down in convex parts again?

DFrey

01-11-2001, 04:44 AM

fox, I'm getting the feeling you don't want to do all the math involved. http://www.opengl.org/discussion_boards/ubb/smile.gif In that case you can also highlight the intersected volume by using the stencil buffer. Though this method is only simple if 2 objects are intersecting and both are simple convex polyhedra. It goes something like this:

1. Disable depth writes

2. Enable stencil testing

3. Clear stencil buffer to all 0

4. Set stencil function to GL_ALWAYS, 0, 0

5. Set stencil operation to GL_INCR, GL_KEEP, GL_KEEP

6. Disable color buffer writes

7. Enable backface culling

8. Draw each cube (no texture or lighting needed)

9. Enable color buffer writes

10. Set stencil function GL_LESS, 2, 255

11. Set stencil operation to GL_KEEP, GL_KEEP, GL_KEEP

12*. Set renderstate for each cube and draw them

13. Set stencil function to GL_EQUAL, 2, 255

14. Set renderstate for intersection

15. Draw one cube

16. Disable stencil test

* Enabling depth writes is ok now too, but is of course dependent upon the result you want.

[This message has been edited by DFrey (edited 01-11-2001).]

Thanx I'll try that out - yes I would want to avoid the maths otherwise it could get very messy but the other thing is i'm not quite sure how to go about it - would it involve vectors and matrices - also would it be similar to implementing collision detection?

cheers

[This message has been edited by fox (edited 01-11-2001).]

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