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Graphics
How to determine whether a clicked point is inside an irregular polygon outlining an ImageMap? by wgcs
Posted: 26 Sep 02

It's easy to tell if someone clicked on a convex shape (rectangle, circle) particularly because it's "Click" event get's fired!
However, if you use an ImageMaplike outline that surrounds a concave / irregular polygon outlining a feature on a graphic image, it gets a whole lot more complicated to determine whether that click was inside or outside the polygon.
Here's code that works (with a test sample) (this was converted by me into VFP from the noted code that was written in C++):
******************************************************************************** * The following routines are used to determine whether a point that is clicked * is inside an imagemap polygon. The original source was borrowed: *!* // Copyright 2001, softSurfer (www.softsurfer.com) *!* // This code may be freely used and modified for any purpose *!* // providing that this copyright notice is included with it. *!* // SoftSurfer makes no warranty for this code, and cannot be held *!* // liable for any real or imagined damage resulting from its use. *!* // Users of this code must verify correctness for their application. * *// a Point is defined by its coordinates {int x, y;} *//===================================================================
*!* // cn_PnPoly(): crossing number test for a point in a polygon *!* // Input: P = a point, *!* // V[] = vertex points of a polygon V[n+1] with V[n]=V[0] *!* // Return: 0 = outside, 1 = inside *!* // This code is patterned after [Franklin, 2000] FUNCTION InPoly( P_X, P_Y, V ) EXTERNAL ARRAY V LOCAL cn, lnI, lnALen, vt lnALen = ALEN( V, 1 ) && Number of Vertices cn = 0 && // the crossing number counter * // loop through all edges of the polygon for lnI = 1 TO lnALen1 && (int i=0; i<n; i++) { // edge from V[i] to V[i+1] ** if (((V[i,2] <= P_y) && (V[i+1].y > P.y)) // an upward crossing **  ((V[i].y > P.y) && (V[i+1].y <= P.y))) { // a downward crossing if (((V[lnI,2] <= P_y) AND (V[lnI+1,2] > P_y)) ; OR ((V[lnI,2] > P_y) AND (V[lnI+1,2] <= P_y))) ** // compute the actual edgeray intersect xcoordinate **float vt = (float)(P.y  V[i].y) / (V[i+1].y  V[i].y) vt = (P_y  V[lnI,2]) / (V[lnI+1,2]  V[lnI,2]) **if (P.x < V[i].x + vt * (V[i+1].x  V[i].x)) // P.x < intersect if (P_x < V[lnI,1] + vt * (V[lnI+1,1]  V[lnI,1])) && // P.x < intersect cn = cn + 1 && ++cn; // a valid crossing of y=P.y right of P.x ENDIF ENDIF ENDFOR RETURN cn=1 && // 0 if even (out), and 1 if odd (in)
PROCEDURE TestPoly * This routine tests the above routines that calculate whether * a point is within a polygon. * Vertices: * (these vertices define a convex, irregular polygon * that looks somewhat like: * /\ * / \ * \/\/ * and the test case runs horizontally * through it's "legs" )
DIMENSION aPoly[6,2] aPoly[1,1]=66 aPoly[1,2]=121 aPoly[2,1]=129 aPoly[2,2]=52 aPoly[3,1]=237 aPoly[3,2]=105 aPoly[4,1]=205 aPoly[4,2]=227 aPoly[5,1]=148 aPoly[5,2]=136 aPoly[6,1]=98 aPoly[6,2]=223
SET PROCEDURE TO AhRooming ?InPoly( 20, 140, @aPoly) && Out ?InPoly(104, 130, @aPoly) && in ?InPoly(147, 174, @aPoly) && Out ?InPoly(194, 130, @aPoly) && In ?InPoly(267, 140, @aPoly) && Out
RETURN

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