pgxgrid

Find all parts of the vertical lines of a grid inside a polygon. The grid is defined by the vectors x (length nx and y (length ny). The function detects all parts of the vertical grid lines <x[i],y[0]>..<x[i],y[ny-1]> (i=0..nx-1), which are inside the polygon p.

Usage

pgxgrid(P, IP, Xvector, Yvector)

P

a closed point-list or a polygon-stream defining one or more polygons.

IP

The index of the polygon of P to be tested; 0 ≤ IP < pgget(P)

X

The x-scale vector of the grid.

Y

The y-scale vector of the grid.

Result

The result r is a (N+1)x3 matrix, with N is the number of vertical grid lines intersecting the polygon.

Content of r[0,*]:

r[0,0]	ixmin	the X index of the first vertical line intersecting PIP
r[0,1]	ixmax	the X index of the last vertical line intersecting PIP
r[0,2]	iymin	the index of the minimum Y value of an intersection line
r[0,3]	iymax	the index of the maximum Y value of an intersection line
r[0,4]	ngrid	the accumlated number of grid points inside PIP
r[0,5]	aclen	the absolute length of the centerline
r[0,6]	rclen	the relative length of the centerline

Content of r[i,*], i=1..N (one row for each intersection line):

r[i,0]	ix	the X index of the intersection line i
r[i,1]	iy	the Y index of the first point <X[ix],Y[iy]> i inside PIP
r[i,2]	ny	the number of Y points inside PIP
r[i,3]	ymin	the y value of the point where the intersection line i enters the polygon PIP
r[i,4]	ymax	the y value of the point where the intersection line i leaves the polygon PIP
r[i,5]	acofs	the absolute length of the centerline from the begin to intersection line i
r[i,6]	rclen	the relative length of the centerline from the begin to intersection line i

Notes:

• The centerline is the line defined by the centerpoints <X[i],(ymin_i+ymax_i)/2> of all intersection lines. For the computation of aclen and acofs_i the x- and y-values of the points are used, while rclen and rcofs_i are computed from the x- and y-indices.
• The function fails if a vertical grid line intersects the polygon more than 2 times.

See also

pginit, pgget, pgitest, pgiline, pgsplit, pgtrans, pghull

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