[Plib-users] ssgIsect question

[McEvoy, N. <nic...@ds...>]

What's up with ssgIsect (ie. how does it work) ???  YES, I have read the
PLIB documentation.

I want to find the distance between my player's center and the plane I hit.
I use a sphere radius 10 at 0,0,0 as per the code below.

When my player is at (0,0,9.68326) and the ground (or plane) is on the X-Y
plane with normal (0,0,1) I use ssgIsect to return info about the ground
below me that I wish to 'rest' on, ssgHit has plane
(0.00,0.00,1.00,9.68326).

Why is plane[3] = 9.68326 ?  According to the equation of a plane D would be
zero in my case cause my plane is X-Y (ie. all point on plane have z=0)
(ref: http://astronomy.swin.edu.au/pbourke/geometry/planeeq/)

Ax = By + Cz + D = 0

A = y1 (z2 - z3) + y2 (z3 - z1) + y3 (z1 - z2) 
B = z1 (x2 - x3) + z2 (x3 - x1) + z3 (x1 - x2) 
C = x1 (y2 - y3) + x2 (y3 - y1) + x3 (y1 - y2) 
- D = x1 (y2 z3 - y3 z2) + x2 (y3 z1 - y1 z3) + x3 (y1 z2 - y2 z1)

Say O is my player (& sphere with radius 10 for isect test).

+Z   +Y
 |  /
 | /
 O/
 .------+X

Now I hope I don't confuse the issue (by bringing ODE into the equation) ...
but ...

The problem is that when I pass the plane to a physics engine (ODE) it works
out the depth of penetration by:
#define dDOT(a,b)   ((a)[0]*(b)[0] + (a)[1]*(b)[1] + (a)[2]*(b)[2])
dReal k = dDOT(pos,plane);
dReal depth = -plane[3] - k + sphere->radius;

ie. depth = -9.68326 -9.68326 + 10 = -9.36652

The incorrect depth result causes my sphere to 'bounce' due to the large
depth in penetration.

Note I get same depth calculation result with PLIB:
float dist = sgDistToPlaneVec3(h->plane, player_pos);
float depth = sphere.getRadius() - dist;

Is it something to do with the invmat[3][2] or setting the sphere at 0,0,0
???

Do I need to raise the sphere or invmat[3][2] ?  I'm quite not sure how PLIB
works backwards to get the ssgHit.  Believe me I have traced the code !!!

Any ideas ???

--------------------------------------------------
sgMat4 invmat;
sgMakeIdentMat4(invmat);
sgSphere sphere;

invmat[3][0] = -player_pos[SG_X];
invmat[3][1] = -player_pos[SG_Y];
invmat[3][2] = -player_pos[SG_Z];
{
...
	sphere.setCenter(0.0, 0.0, 0.0);
	sphere.setRadius(10.0);

	ssgHit *results ;
	int num_hits = ssgIsect(&mWorld, &sphere, invmat, &results);

	for (int i = 0; i < num_hits; i++)
	{
		ssgHit *h = &results[i];
		printf("pos <%.1f:%.1f:%.1f> plane <%.1f:%.1f:%.1f:%.1f>\n",
			player_pos[0], player_pos[1], player_pos[2],
			h->plane[0], h->plane[1], h->plane[2], h->plane[3]);
	}
...
}

--------------------------------------------------
pos <0.00:0.00:9.68326> plane <0.00:0.00:1.00:9.68326>
...etc
--------------------------------------------------
Nick
http://members.ozemail.com.au/~ndmcevoy/