A <- tess(xgrid=0:4,ygrid=0:4)
plot(A, col="blue", lwd=2, lty=2)
B <- A[c(1, 2, 5, 7, 9)]
plot(B, hatch=TRUE)
v <- as.im(function(x,y){factor(round(5 * (x^2 + y^2)))}, W=owin())
levels(v) <- letters[seq(length(levels(v)))]
E <- tess(image=v)
plot(E)
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