Geometry/halfspace_triangle_int_3d.f90 at master · lguez/Geometry · GitHub

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subroutine halfspace_triangle_int_3d ( dist1, dist2, dist3, t, int_num, pint )

!*****************************************************************************80
!
!! HALFSPACE_TRIANGLE_INT_3D: intersection ( halfspace, triangle ) in 3D.
!
!  Discussion:
!
!    The triangle is specified by listing its three vertices.
!
!    The halfspace is not described in the input data.  Rather, the
!    distances from the triangle vertices to the halfspace are given.
!
!    The intersection may be described by the number of vertices of the
!    triangle that are included in the halfspace, and by the location of
!    points between vertices that separate a side of the triangle into
!    an included part and an unincluded part.
!
!    0 vertices, 0 separators    (no intersection)
!    1 vertex, 0 separators      (point intersection)
!    2 vertices, 0 separators    (line intersection)
!    3 vertices, 0 separators    (triangle intersection)
!
!    1 vertex, 2 separators,     (intersection is a triangle)
!    2 vertices, 2 separators,   (intersection is a quadrilateral).
!
!  Licensing:
!
!    This code is distributed under the GNU LGPL license.
!
!  Modified:
!
!    06 May 2005
!
!  Author:
!
!    John Burkardt
!
!  Parameters:
!
!    Input, double precision DIST1, DIST2, DIST3, the distances from each of
!    the three vertices of the triangle to the halfspace.  The distance is
!    zero if a vertex lies within the halfspace, or on the plane that
!    defines the boundary of the halfspace.  Otherwise, it is the
!    distance from that vertex to the bounding plane.
!
!    Input, double precision T(3,3), the vertices of the triangle.
!
!    Output, integer INT_NUM, the number of intersection points
!    returned, which will always be between 0 and 4.
!
!    Output, double precision PINT(3,4), the coordinates of the INT_NUM
!    intersection points.  The points will lie in sequence on the triangle.
!    Some points will be vertices, and some may be separators.
!
  implicit none

  integer, parameter :: dim_num = 2

  double precision dist1
  double precision dist2
  double precision dist3
  integer int_num
  double precision pint(dim_num,4)
  double precision t(dim_num,3)
!
!  Walk around the triangle, looking for vertices that are included,
!  and points of separation.
!
  int_num = 0

  if ( dist1 <= 0.0D+00 ) then

    int_num = int_num + 1
    pint(1:dim_num,int_num) = t(1:dim_num,1)

  end if

  if ( dist1 * dist2 < 0.0D+00 ) then

    int_num = int_num + 1
    pint(1:dim_num,int_num) = &
      ( dist1 * t(1:dim_num,2) - dist2 * t(1:dim_num,1) ) &
      / ( dist1 - dist2 )

  end if

  if ( dist2 <= 0.0D+00 ) then

    int_num = int_num + 1
    pint(1:dim_num,int_num) = t(1:dim_num,2)

  end if

  if ( dist2 * dist3 < 0.0D+00 ) then

    int_num = int_num + 1

    pint(1:dim_num,int_num) = &
      ( dist2 * t(1:dim_num,3) - dist3 * t(1:dim_num,2) ) &
      / ( dist2 - dist3 )

  end if

  if ( dist3 <= 0.0D+00 ) then

    int_num = int_num + 1
    pint(1:dim_num,int_num) = t(1:dim_num,3)

  end if

  if ( dist3 * dist1 < 0.0D+00 ) then

    int_num = int_num + 1
    pint(1:dim_num,int_num) = &
      ( dist3 * t(1:dim_num,1) - dist1 * t(1:dim_num,3) ) &
      / ( dist3 - dist1 )

  end if

  return
end