I am trying to use z-buffer algorithm to find the visible surface detection for my college computer graphics project.I have used following co-ordinate system that the positive x-axis to the right, the positive z-axis upward, and the positive y-axis forward on the screen. My maximum depth is 200 units.

Problem is that when I calculate the depth of all surface of cube I found the depth of back and front accurate but depth of other side is more than my maximum depth.

My code for calculating co-efficient of plane is

public Coefficient_Of_Plane Coefficient_Value(_3Dpoint pt1, _3Dpoint pt2, _3Dpoint pt3)
{
Coefficient_Of_Plane pt;
pt = new Coefficient_Of_Plane();
pt.A = (pt2.z - pt3.z) * (pt1.y - pt2.y) - (pt1.z - pt2.z) * (pt2.y - pt3.y);
pt.B = (pt2.x - pt3.x) * (pt1.z - pt2.z) - (pt1.x - pt2.x) * (pt2.z - pt3.z);
pt.C = (pt2.y - pt3.y) * (pt1.x - pt2.x) - (pt1.y - pt2.y) * (pt2.x - pt3.x);
pt.D = - pt1.x * (pt2.y * pt3.z - pt2.z * pt3.y) + pt1.y * (pt2.x * pt3.z - pt2.z * pt3.x) - pt1.z * (pt2.x * pt3.y - pt2.y * pt3.x);
return pt;
}

and finding the depth value is

public double DepthValue(Coefficient_Of_Plane surface,int x,int y)
{
double z = 0;
if (surface.B != 0)
z = (-surface.A * x - surface.C * y - surface.D) / surface.B;
return z;
}

I need a help to solve this problem and I am also ready to post other part of code if required and further description of problem if my question is incomplete.

z = (-Ax - By - D) / C

Regards

I have solve my error . While finding depth the points x and y must be kept fixed

I have taken origin (x,y)=(0,0) as a fixed point.