BoolExtractPolygons

Section: C Library Functions (3)
Updated: IRIT Version 6.0
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NAME

BoolExtractPolygons()

SYNOPSIS

bool_lib/bool2low.c:889

IPObjectStruct *BoolExtractPolygons(IPObjectStruct *PObj, int AinB)

DESCRIPTION

This routine coordinates all the extraction of the polygons from the intersecting lists. Does it in the following steps:
1. Mark all polygons with no intersection at all as complete polygons. (this is because this polygon will be totally in or out, according to inter-polygon adjacencies propagation...) Also mark them as undefined (if in output or undefined) yet. Uses IPPolygonStruct Tags to save these bits.
2. do
    2.1. Convert the unordered segment list of each polygon to closed loops (if create a hole in polygon) or open (if crosses its boundary).
    2.2. Order the open loops along the perimeter of the polygon (note these loops cannt intersect. For example (5 vertices polygon):
                -----------------------------L3
               |  ---------------L1  -----L2 |          --------L4   --L5
               | |               |  |     |  |         |        |   |  |
         P0 ------ P1 ------- P2 ----- P3 -------- P4 ------ P5 -------- P0 Note L1, L2 are enclosed in L3 loop, and that the order is circular.
    2.3. "Break" the polygon at each open loop that has no enclosed loops in it. For example we can start at L1, L2, L4, L5 and then L3. "Break" means - replace the vertex chain between the two loop end points, with the loops itself. Depends upon the relation required we may need to output a new polygon form from the deleted chain and that loop. In addition we may form a new polygon from last loop and was was left from the original polygon For each formed polygon, for each complete edge of it (i.e. edge which was originally in the polygon) test the adjacent polygon if it is complete (as marked in 1.) and if in or undefined (marked undefined in 1.) is still undefined:
         2.3.1. set it to be in.
         2.3.2. push it on adjacency stack.
    2.4. For each closed loop - find in which polygon (from polygons created in 2.3.) it is enclosed, and decompose it.
3. While adjacencies stack not empty do:
    3.1. pop top polygon from stack and output it.
    3.2. For each of its edges (which obviousely must be complete edges) if adjacent polygon is complete and undefined:
         3.3.1. set it to be in.
         3.3.2. push it on adjacency stack.
    3.3  go back to 3.

The above algorithm defines in as in output, but dont be confused with the required inter-object AinB (or AoutB if FALSE), which used to determine which side of the trimming loop should be output. Note this routine may return non-convex polygons (but marked as so) even though the input for the booleans must be convex polygons only! In order to keep the given object unchanged, a whole new copy off the polygon list is made. The polygons of the list that are not in the output are freed: a global list of all polygons (pointers) is used to scan them in the end and free the unused ones (list PolysPtr).

PARAMETERS:

PObj: Object that need to be rebuilt according to the intersection curves that were found, both closed and open loops.

AinB: Type of inclusion/exclusion requested.

FUNCTION RETURN VALUE

IPObjectStruct *: The newly created clipped object.

ORIGIN

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Time: 07:27:11 GMT, October 17, 2022