3 points are known, and the distance from point to surface is obtained.

// Obtain the plane equation at three known points
BOOL CGe: PanelEquationFromThreePt (CPoint3dArray ptArr,
Double & a, double & B, double & c, double & d)
{// From http://blog.csdn.net/hoya5121
CPoint3d p1, p2, p3;

If (ptArr. GetSize () <3)
{
Return FALSE;
}
P1 = ptArr [0];
P2 = ptArr [1];
P3 = ptArr [2];

A = (p2.y-p1.y) * (p3.z-p1.z)-(p2.z-p1.z) * (p3.y-p1.y ));
B = (p2.z-p1.z) * (p3.x-p1.x)-(p2.x-p1.x) * (p3.z-p1.z ));
C = (p2.x-p1.x) * (p3.y-p1.y)-(p2.y-p1.y) * (p3.x-p1.x ));
D = (0-(a * p1.x + B * p1.y + c * p1.z ));

Return TRUE;
}

// The distance from the point to the surface, where the coordinate is P (x, y, z), and the plane equation is ax + by + cz + d = 0.
Double CGe: DistPt2Panel (double x, double y, double z,
Double a, double B, double c, double d)
{
Return fabs (a * x + B * y + c * z + d)/sqrt (a * a + B * B + c * c );
}