Edge detection between the finiteelement mesh and a bounding box

Question

i have an finite element mesh ,i constucuted a bounding box which passes through the mesh ,so now i want to detect the edge of the rectangle which intersects the mesh and construct a new mesh which resides in the rectangular box.how to do this.any kind of help is appreciated.
P /------------------\ Q
A |---- |/---|-----|----|-----|--\--| B
|----/|----|-----|----|-----|---\-|
|---/-|----|-----|----|-----|----\|
|--/--|----|-----|----|-----|---- |\
D |-/---|----|-----|----|-----|---- | \ C
S /-------------------------------------\ R
suppose ABCD is FEMesh, PQRS is a bounding box,consider the edge PS when it intersect with the elements in the mesh i will get the pentagon element when i want to form the new mesh which contains the elements which are inside the box.the graphical represenation is not clear if you draw on a paper you can easily understand my problem.
now how to create a new mesh from the elements which reside inside the boxin which the left hand side of PS will be discarded and RHS of QR edge is discarded,so now mesh will have some zigzag shape i think.so please help me out how to solve this problem.

Answer 1

i think the figure is not clear , so i want to
simplify the problem by considering only one element
of the mesh and only one edge of the box.Okay.
suppose ABCD
is a 3D quadrilateral element and PQ is the edge
passing through the element.i will pass a function
with coordinates of ABCD and P (startpoint of
edge)coordinate and its normal vector,if there is an
intersection the function should return 2 elements
according to intersection
now we have element ABCD and an edge with P and its
normal vector,now we have intersection between element
and edge from p at X and Y, now the algorithm should
return AXYD as one element and XBCY as second
element.please help me out in solving this task.
Thanks in advance.I'll assume you mean 2D, planar, linear quad. I think the general 3D problem would entail surfaces and be much more difficult. If you use higher order shape functions the 2D problem isn't trivial, because then you might have more than two intersections.
It COULD return two points of intersection, but you'll have to be careful about special cases:
(1) The edge could intersect just one of the four corners,
(2) The edge could coincide with an edge of the element,
(3) The edge could miss the element entirely and not intersect.
You'll have to decide what to do in the degenerate cases. The sensible thing would be to do nothing and leave the element alone.
You'll also be concerned about mesh quality when you're done. Near degenerate elements won't perform well when you analyze them. Will you try to correct these after the fact?
What if you end up with triangular elements? An edge might slice off a triangle and leave you with a five-noded element. What will you do with those? Create two triangles to get back to a quad? It's not so easy. You really should look into quadtree meshing. There are more general ways to attack a problem like this.
What kind of physics are you ultimately trying to capture? What are you really doing here?
You've described the algorithm fairly well, even if you haven't thought it all the way through.
What's the problem? You'll have to write the code. No one will do that for you. Write some and come back with specific questions if you have difficulties. This is a Java forum, after all.