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C Library Functions  - inpolygon (3)

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NAME

inpolygon(3f) - [M_math:geometry] determine whether or not an integer point is in an integer polygon

CONTENTS

Synopsis
Description
Options
Result
Example

SYNOPSIS

logical function inpolygon(xin, yin, xconv, yconv, nconv)

     integer,intent(in)  xin, yin
     integer,intent(in)  nconv
     integer,intent(in)  xconv(nconv), yconv(nconv)

DESCRIPTION

Given a closed polygon find if a point lies inside the polygon. Intended for integer values, like pixel images.

OPTIONS

xin	the X coordinate of the point to be checked
yin	the Y coordinate of the point to be checked
xconv	contains the X coords of the polygon
yconv	contains the Y coords of the polygon
nconv	the number of points in the polygon

RESULT

INPOLYGON returns .true if the point lies inside the polygon, otherwise it returns .false.

EXAMPLE

Sample program

Draw a polygon and an envelope of the polygon and then calculate random points in the region and determine if they fall inside the polygon, within the accuracy of integer values.

   program demo_inpolygon
   use M_draw
   use M_math,     only : envelope        ! Find vertices (in clockwise order) of a polygon enclosing the points
   use M_math,     only : inpolygon       ! find if a point is inside a polygonal path
   use M_math,     only : polyarea        ! compute the area bounded by a closed polygonal curve
   implicit none
   integer,parameter :: n=6
   !  3--------------4
   !   \           /
   !     \       /
   !       \   /
   !         X 2,5
   !       /  \
   !     /      \
   !   /          \
   !  1--------------6
   integer,parameter    :: x(n)=[-5, 0,-5, 5, 0, 5]
   integer,parameter    :: y(n)=[-5, 0, 5, 5, 0,-5]
   real              :: xy(2,n)
   integer           :: vertex(n)
   integer           :: nvert
   integer           :: i
   integer           :: idum
   xy(1,:)=x
   xy(2,:)=y
   call vinit(’ ’)
   call page(-10.0,10.0,-10.0,10.0)
   call color(D_BLACK) ! set current color to black
   call clear()        ! clear to current color
   call polyfill(.true.)
   call color(D_BLUE)  ! we want to draw polygon in this color
   call poly2(n,xy)    ! draw filled polygon using points given
   idum=getkey()       ! pause for some input
   call color(D_CYAN)
   call polyhatch(.true.)
   call envelope(real(x), real(y), n, vertex, nvert)   ! calculate envelope

   call poly2(nvert,xy(:,vertex(1:nvert))) ! draw hatched envelope
   idum=getkey()       ! pause for some input
   call polyhatch(.false.)
   call linewidth(50)
   call color(D_WHITE)
   call poly2(n,xy)    ! draw line along original points
   idum=getkey()       ! pause for some input
   call random_seed()
   do i=1,70
      call pickrandom()
   enddo
   idum=getkey()       ! pause for some input
   call vexit()        ! wrap up and exit graphics mode
   write(*,*)’polyarea=’,polyarea(real(x),real(y))
   write(*,*)’polyarea=’,polyarea( xy(1,vertex(1:nvert)), xy(2,vertex(1:nvert)))
   contains
   subroutine pickrandom()
   ! randomly pick a point in the plot area and color it according to whether it is inside
   ! the original polygon
   real :: pointx, pointy
   integer :: l, m
      call random_number(pointx)
      call random_number(pointy)
      pointx=int(pointx*20.0-10.0)
      pointy=int(pointy*20.0-10.0)
      !call locpt(pointx,pointy,x,y,n,l,m)
      if(inpolygon(int(pointx),int(pointy),x,y,n))then
         call color(D_GREEN)
      else
         call color(D_RED)
      endif
      call circle(pointx,pointy,0.2)
   end subroutine pickrandom
   end program demo_inpolygon

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Nemo Release 3.1

inpolygon (3)

February 23, 2025

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