Tetra - Tetrahedra subroutines

tetra_volume - find volume of tetrahedra give 4 points
area3d - find area of a triangle in 3D defined by 3 points

Function Prototypes:
float tetra_volume(float x0[3], float x1[3], float x2[3], float x3[3]);
float area3d( float x0[3], float x1[3], float x2[3]);
tetra_volume:
Input:
x0[3]	x,y,z of 1st point
x1[3]	x,y,z of 2nd point
x2[3]	x,y,z of 3rd point
x3[3]	x,y,z of 4th point

Returns:  tetrahedra volume

area_3d:
Input:
x0[3]		x,y,z of 1st point
x1[3]		x,y,z of 2nd point
x2[3]		x,y,z of 3rd point

Returns:
Notes:
tetra_volume:
The formula:
                | x0[0]   x0[1]   x0[2]   1 |
                | x1[0]   x1[1]   x1[2]   1 |
           v =  | x2[0]   x2[1]   x2[2]   1 | / 6;
                | x3[0]   x3[1]   x3[2]   1 |

area3d:

References:
Author: CWP: Zhaobo Meng, October 1997