Given a polygon defined by the vertex coordinate vectors x & y, we've
seen that we can compute the indices of pixels roughly within that
polygon using polyfillv(). You can run the code attached to set-up a
framework for visualizing this. It shows a 10x10 pixel grid with an
overlain polygon by default, with pixels returned from polyfillv()
shaded.
You'll notice that polyfillv() considers only integer pixels, basically
truncating any fractional part of the input polygon vertices (you can
see this by plotting fix([x,x[0]]), etc.). For polygons on a fractional
grid, this error can be significant.
The problem posed consists of the following:
Expand on the idea of the polyfillv algorithm to calculate and return
those pixels for which *any* part of the pixel is contained within the
polygon, along with the fraction so enclosed.
For instance, the default polygon shown (invoked simply as
"poly_bounds"), would have a fraction about .5 for pixel 34, 1 for
pixels 33 & 43, and other values on the interval [0,1] for the others.
Return only those pixels with non-zero fractions, and retain polygon
vertices in fractional pixels (i.e. don't truncate like polyfillv()
does).
JD
pro poly_bounds,x,y,N=n
if n_elements(n) eq 0 then n=10
if n_elements(x) eq 0 then begin
x=[1.2,3,5.3,3.2] & y=[1.3,6.4,4.3,2.2]
endif
window,XSIZE=500,YSIZE=500
;; Set up the plot region, etc.
plot,[0],[0],XRANGE=[0.,n],YRANGE=[0.,n], XMINOR=-1,YMINOR=-1, $
XTICKS=n,YTICKS=n,POSITION=[.05,.05,.95,.95],TICKLEN=0,/NODA TA
p=polyfillv(x,y,n,n)
for i=0,n_elements(p)-1 do begin
xp=p[i] mod n
yp=p[i]/n
polyfill,[xp,xp,xp+1,xp+1],[yp,yp+1,yp+1,yp],COLOR=!D.N_COLO RS/2
endfor
oplot,[x,x[0]],[y,y[0]]
for i=0,n-1 do begin
plots,i,!Y.CRANGE
plots,!X.CRANGE,i
for j=0,n-1 do begin
plots,i+.5,j+.5,PSYM=3
xyouts,i+.1,j+.1,strtrim(i+j*n,2)
endfor
endfor
end
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