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# how to count the point value in the 3d triangle

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if I have known the triangle''s three vertexs value in 3d axis, how can I get the point''z axis value that I have known the point is inside the triangle surface and knew the point''s x and y value?

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If I understand you correctly, what you need to do is calculate the equation of the plane upon which the triangle lies, then plug the x and y values of the unknown point into the equation and solve for z.

Given 3 points (p1,p2,p3) which form a triangle, you can calculate the normal vector of the plane as the cross-product of two of the triangle's sides, normalized to unit length:
Vector v1=p3-p1;Vector v2=p2-p1;Vector normal=v1.CrossProduct(v2);normal.normalize();

Now, the equation of a plane is expressed as:
Ax + By + Cz + D = 0ornormal.x * x + normal.y * y + normal.z * z + D = 0

where A,B and C are the components of the normal vector, and D is a constant. Take one of the points of the triangle (say, p1) and plug it into the equation, along with the normal components, and solve for D:
D = -( (normal.x * p1.x) + (normal.y * p1.y) + (normal.z * p1.z) )

Now, for p4 you know the x and y values and wish to calculate p4.z. To do so, plug p4.x and p4.y into the plane equation, and solve for p4.z:
       -( (normal.x * p4.x) + (normal.y * p4.y) + D )p4.z = ----------------------------------------------                          normal.z

Remember to check for the special case where normal.z==0 , in which case the value of p4.z will also always be 0. If you try to solve this case using the equation, you will of course get a divide-by-zero exception.

Now, if you need to determine whether or not this calculated point (p4) is within the triangle, you can do it in this fashion:

First, calculate a few dot products based on the edge of the triangle:
c00 = v1.dotproduct(v1);c01 = v1.dotproduct(v2);c11 = v2.dotproduct(v2);

Now, calculate some vectors and dot products based on the intersection point:
Vector Q = p4 - p1;q0 = v1.dotproduct(Q);q1 = v2.dotproduct(Q);delta = (c00 * c11) - (c01 * c01);s0=(c11*q0 - c01*q1) / delta;s1=(c00*q1 - c01*q0) / delta;

If s0>=0 and s1>=0 and s0+s1<=1.0, the point lies within the triangle.

Golem
Blender--The Gimp--Python--Lua--SDL

[edited by - VertexNormal on April 12, 2004 8:30:51 PM]

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Triangle vertices = 'p1', 'p2', 'p3'
point of interest = 'p'
direction 'v' = (0, 0, -1) (down the z axis)
resulting point on triangle plane = 'q'

// triangle planeVector v1=p3-p1; // edge 3Vector v2=p2-p1; // edge 1Vector n = v1.CrossProduct(v2); // normal of planefloat  d = normal.Dot(p1); // distance of plane from origin// intersection of ray (p, v) with triangle planet = (d - p.Dot(n)) / v.Dot(n);// intersection point on planeq = p + v * t;

you don't need to normalise the plane normal, but you can if you want.

edit : it's exactly what vertex normal said. damn IE.

[edited by - oliii on April 13, 2004 4:25:01 AM]

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Thank you for reply , VertexNormal

but I don''t know why use v1 vector and v1 vector to calculate the dot product, what is the result value?

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quote:
Original post by juwen_chao
Thank you for reply , VertexNormal

but I don't know why use v1 vector and v1 vector to calculate the dot product, what is the result value?

The second part of my earlier post (where I make use of v1.dotproduct(v1) ) is an algorithm to test a point to see if it is within the boundary of a triangle. The algorithm depends on the parametric form of a plane equation, which is expressed as:
P(s,t) = p1 + s(p2-p1) + t(p3-p1)orP(s,t) = p1 + s*v1 + t*v2

Any point on the plane can be represented as an (s,t) coordinate pair (in my earlier post, I used s0 and s1 rather than s and t, but they are the same values) which, when plugged into the parametric equation, yields the Cartesian (x,y,z) point. The set of points for which s>=0 && t>=0 && s+t<=1 are all points within the triangle or on the triangle edges. So if you can find the (s,t) pair for a given point and test it, if the (s,t) pair meets the criteria, then the corresponding point lies within the triangle.

The (s,t) coordinates for your original triangle points--p1, p2 and p3--are:
p1: s=0, t=0
p2: s=1, t=0
p3: s=0, t=1

The derivation of the equations to calculate s and t (of which, the v1.dotproduct(v1) computation is a part) is a little complicated, but you can find an explanation of it here (which details an algorithim for testing for linesegment/triangle intersection; the relevant section for here comes after the diagram).

Golem
Blender--The Gimp--Python--Lua--SDL

[edited by - VertexNormal on April 13, 2004 1:10:37 PM]

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if calculate the normal vector of one triangle ,
the result is two of normal vector value x y z are zero,
x = 0, y = 0, z = 1, for example.

then use the equation of a plane:
Ax + By + Cz + D = 0;
to calculate one point whether into the triangle will wrong, any x y value will ok,

what can i do?

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quote:
Original post by juwen_chao
if calculate the normal vector of one triangle ,
the result is two of normal vector value x y z are zero,
x = 0, y = 0, z = 1, for example.

then use the equation of a plane:
Ax + By + Cz + D = 0;
to calculate one point whether into the triangle will wrong, any x y value will ok,

what can i do?

I don''t really understand what you are asking here. What''s the problem?

With a normal vector of (0,0,1), the plane defined is all points (x,y,-D). In such a case, any point (x,y) plugged into the equation will have the same z value when solving for z, since the equation reduces down to x+D=0:
Ax + By + Cz + D = 00x + 0y + 1z + D = 0z + D = 0z=-D

So no matter what the value of x and y, as long as z==-D the equation is satisfied and the point lies on the plane.

Golem
Blender--The Gimp--Python--Lua--SDL

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