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# triangle/plane intersection points

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I am making a BSP system. As you know, triangles have to be split along the plane of another triangle. I know how to tell if a triangle is spanning a plane, but how can I tell where it intersects? Sleeping might be a good idea now. Proceeding on a brutal rampage is the obvious choice.

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Intersect the edges of the triangle. Each edge can be expressed as

P = P0 + t * (P1 - P0)

where P0 and P1 are two vertices. Plug the components in the plane equation and solve for t.

Cédric

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what is t?

Proceeding on a brutal rampage is the obvious choice.

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It''s a parametric equation. t is a scalar value. Breaking the equaiton into components yield:

x = x0 + t * (x1 - x0)
y = y0 + t * (y1 - y0)
z = z0 + t * (z1 - z0)

These are all known quantities except t. Use them in the plane equation to find the intersection point of the line and the plane.

Note that if t is not comprised between 0 and 1, then the point is not part of the line segment.

Cédric

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Sorry, but I really don''t understand what you are saying. For clarity, lets follow this diagram:
 |      |  /|P1        |      | / |          |    I1|/  | |     /|   |y|    / |   | | P0/__|___|P2 |      |I2 |      | |___________________           x

P1, P2, and P3 make up the triangle. The vertical line is the plane, and I1 and I2 are the intersection points. How can I find the coordinates of I1 and I2?
Are you suggesting starting at P1 or P2, making a parametric that moves it to P0, and using the plane equation to find the value of t? Please give me an example. Thank you.

Proceeding on a brutal rampage is the obvious choice.

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Ok. I''ll try to be clearer

The idea is to test each and every edge (line) of the triangle to see if it collides with the plane. There are three lines:
P0-P1
P1-P2
P2-P0

For example, with P0-P1, any point on the line can be represented as a linear interpolation between P0-P1. Hence,

P = P0 + t * (P1 - P0)

If 0 < t < 1, then P is somewhere between P1 and P0. This is a vectorial equation, so there are three components. We can substitute them in the plane equation:

Ax + By + Cz + D = 0
A(P0.x + t * (P1.x - P0.x)) + B(P0.y + t * ... = 0

Isolate t. Once you have it, you know that, if t>1 or t<0, the edge doesn''t intersect the plane. Else, you can find the intersection point by replacing t in the original equation.

Hope this helps,

Cédric

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Okay. Thanx for all your help.

Proceeding on a brutal rampage is the obvious choice.

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