Re: [ferret_users] Creating and using an irregular mask, as in the recent question regarding filling data over China

[mick spillane <mick.spillane@xxxxxxxx>]

Hi Ansley,

   Thanks for catching my omission of the shell script get_vertices in 
my original email.  Since some of my applications use polydef.jnl in 
situations where I don't need or want to close the polygon, I chose 
instead to make a variants polydef_close.jnl and get_vertices_close that 
do the closing. Either choice would work with InsidePolygon.jnl . Also 
the lengthy email may have masked the role of x0,y0.

   So another demo and graphic are attached to clarify how the scheme 
works; it selects cells in the underlying grid whose centers fall within 
the user-prescribed polygon.  The variables x0,y0 identify the x and 
y-axes of the grid.

! shade a coarsely gridded field and mark cell centers
shade/i=1:10/j=1:8 i+j
let x0=x[x=1:10] ; let y0=y[y=1:8]
plot/o/vs/nolab/sym=17/i=1:10/j=1:8/set x0+0*y0,y0+0*x0 ; ppl pen,1,7 ; 
ppl plot/over

! select polygon vertices (closed) and plot boundary
go polydef_close
poly/o/nolab/line=2 vx,vy

! alternately a vertex file for the closed polygon might be otherwise 
available, in which case
! file/form=free/var=vx,vy/g=gz vx,vy
! where "gz" is a 1-D z-axis grid for the vertex coords, could 
substitute for "go polydef..."

! identify and highlight the cells whose centers fall inside the polygon
go InsidePolygon
shade/o/nolab/pal=red/pat=dark_vertical inside

NOTE in the demo plot that cells are treated as inside or outside which 
has an implication for area averages or integrals that might result from 
using the "inside" variable as a mask.  If the cells adjacent to the 
boundary are too many, it might be a good idea to regrid the data to be 
masked to a finer grid to reduce the raggedness of the polygon edges.
   Another thing to note is that the polygon vertex file "vertices.xy" 
is read in along the z-axis.  If one were using the scheme for a 
vertical section, a variant employing another axis might be appropriate.

Mick

> > ------------------------------------------------------------------------

Attachment: get_vertices_close
Description: video/flv

! polydef_close : use mouse click to define polygon vertices;
!                 the shell script get_vertices_close will 
!                 close the polygon
can mode verify
let done=0 ; sp rm -f vertices.xy

say "****************************************************"
say "*                                                  *"
say "* Add polygon vertices by mouse clicks.  Terminate *"
say "*     by clicking to the left of the plot area.    *"
say "*                                                  *"
say "****************************************************"

! add new vertices to the file vertices.xy
repeat/range=1:1000 go add_vertex

! then read in the resulting file 
sp get_vertices_close ; go get_vertices

set mode verify