GLBasic forum

Codesnippets => Math => Topic started by: doimus on 2011-Oct-09

Title: Point inside polygon
Post by: doimus on 2011-Oct-09
Here's some code that finds whether a particular point is inside a polygon.
Actual routine is not mine, I've found the C code online (http://www.visibone.com/inpoly/inpoly.c) and translated it into GLB.

It's great for implementing various regions, zones, collision or whatever else into your programs.
Code is pure math, it has nothing to do with graphical polygons (no acceleration) but it supports all kinds of mad polygonal shapes.


Code: (glbasic) [Select]
SYSTEMPOINTER TRUE

//create polygon
LOCAL poly[]
DIM poly[RND(3)+3][2]

FOR i = 0 TO BOUNDS(poly[], 0) - 1
poly[i][0] = RND(640)
poly[i][1] = RND(480)
NEXT


// main loop
WHILE TRUE

LOCAL mx,my,mba,mbb
MOUSESTATE mx, my, mba, mbb

drawPoly(poly[])

IF inPoly(poly[], mx, my)
PRINT "Inside!",0,0
ENDIF

SHOWSCREEN

WEND



FUNCTION inPoly: poly[], tx, ty

LOCAL newx, newy, oldx, oldy
LOCAL x1, y1, x2, y2
LOCAL inside

LOCAL npoints = BOUNDS(poly[], 0)

IF npoints < 3 THEN RETURN 0

oldx = poly[npoints - 1][0]
oldy = poly[npoints - 1][1]

FOR i = 0 TO npoints - 1

newx = poly[i][0]
newy = poly[i][1]

IF newx > oldx

x1 = oldx
x2 = newx
y1 = oldy
y2 = newy

ELSE

x1 = newx
x2 = oldx
y1 = newy
y2 = oldy

ENDIF


IF ((newx < tx) = (tx <= oldx) AND (ty-y1)*(x2-x1) < (y2-y1)*(tx-x1))

inside = NOT inside

ENDIF

oldx = newx
oldy = newy

NEXT

RETURN inside

ENDFUNCTION



FUNCTION drawPoly: poly[]

LOCAL npoints = BOUNDS(poly[], 0)

FOR i = 0 TO npoints-2
DRAWLINE poly[i][0],poly[i][1], poly[i+1][0],poly[i+1][1], RGB(200,200,200)
NEXT

DRAWLINE poly[-1][0],poly[-1][1], poly[0][0],poly[0][1], RGB(200,200,200)

ENDFUNCTION
Title: Re: Point inside polygon
Post by: AMateus on 2011-Oct-09
Nice :)

Thank you for sharing.
Title: Re: Point inside polygon
Post by: matchy on 2011-Oct-10
Cool. Looks faster than the code I'd converted from http://www.vb-helper.com/howto_find_angles.html

Code: (glbasic) [Select]

TYPE _vertex
pts[] // polygon
x
y
z
width
height
off_x
off_y
ENDTYPE

FUNCTION PointInPolygon: ply AS _vertex, point AS _vertex
LOCAL c, corners, total_angle
LOCAL ver AS _vertex

corners = BOUNDS(ply.pts[], 0) - 1
total_angle = GetAngleProduct(ply.pts[corners][0], ply.pts[corners][1], point.x, point.y, ply.pts[0][0], ply.pts[0][1])
FOR c = 0 TO corners - 1
ver.x = ply.pts[c][0] + ply.x
ver.y = ply.pts[c][1] + ply.y
ver.off_x = point.x
ver.off_y = point.y
ver.width = ply.pts[c + 1][0] + ply.x
ver.height = ply.pts[c + 1][1] + ply.y
    total_angle = total_angle + GetAngleProduct(ver.x, ver.y, ver.off_x, ver.off_y, ver.width, ver.height)
NEXT
RETURN ABS(INTEGER(total_angle))
ENDFUNCTION

FUNCTION GetAngleProduct: Ax,Ay,Bx,By,Cx,Cy
LOCAL dot_product, cross_product

dot_product = DotProduct(Ax, Ay, Bx, By, Cx, Cy)
cross_product = CrossProductLength(Ax, Ay, Bx, By, Cx, Cy)
RETURN ATan2(cross_product, dot_product)
ENDFUNCTION

FUNCTION ATan2: opp,adj
LOCAL angle
LOCAL iPI = 3.14159265358979323846

IF ABS(adj) < 0.0001
    angle = iPI / 2
ELSE
    angle = ABS(ATAN(opp, adj))
ENDIF
IF adj < 0
    angle  = iPI - angle
ENDIF
IF opp < 0
    angle = -angle
ENDIF
RETURN angle
ENDFUNCTION

FUNCTION CrossProductLength: Ax,Ay,Bx,By,Cx,Cy
LOCAL BAx=Ax-Bx
LOCAL BAy=Ay-By
LOCAL BCx=Cx-Bx
LOCAL BCy=Cy-By
RETURN (BAx*BCy) - (BAy*BCx)
ENDFUNCTION

FUNCTION DotProduct: Ax,Ay,Bx,By,Cx,Cy
LOCAL BAx= Ax - Bx
LOCAL BAy= Ay - By
LOCAL BCx= Cx - Bx
LOCAL BCy= Cy - By
RETURN (BAx*BCx) + (BAy*BCy)
ENDFUNCTION
Title: Re: Point inside polygon
Post by: Wampus on 2012-Apr-19
Cheers me dears. I was just about to write a routine to do this myself. Now I won't have to. :)
Title: Re: Point inside polygon
Post by: xbee on 2013-Nov-11
Perfect. Just what I was looking for :)
Many thanks.
Title: Re: Point inside polygon
Post by: mentalthink on 2013-Nov-12
Thanks -domius very interesting the code, perhaps for void make a for over  a huge mesh can works fine?¿...
Thanks I think I use your code in my simple 3D Editor...  :booze: