I think that's because of your definition for "touches". If I read it correctly, David's code assumes that a "cell" is a square 1 unit wide and then followed the normal geometric definition of "touches", ie. the line intersects with at least one point on the cell's boundary.
You seem to have a different definition: cell centers perhaps ? or cell lower-left corner points ? Whether or not any particular definition is valid depends on your requirements. In other words - it's all up to you :) Michael On 17 September 2010 21:22, Alaios <ala...@yahoo.com> wrote: > I would like to thank you again for your help. > But it seems that the floor function (ceiling, round too) create more dots in > the matrix that line really "touches". > > unique( floor( cbind( seq(2,62, by=0.1), linefn(seq(2,62, by=0.1)) ) ) ) > > You can see that in the picture below > > http://yfrog.com/5blineswj > > So, how to select the only the cells that the line "touches"? > > I would like to thank you in advance for your help > best Regards > Alex > > > > > ________________________________ > From: David Winsemius <dwinsem...@comcast.net> > > Cc: Rhelp list <r-help@r-project.org> > Sent: Wed, September 15, 2010 1:55:10 PM > Subject: Re: [R] Interpolate? a line > > > On Sep 15, 2010, at 7:24 AM, David Winsemius wrote: > >> Replacing context: >> >>>> Hello everyone. >>>> I have created a 100*100 matrix in R. >>>> Let's now say that I have a line that starts from (2,3) point and ends to > the >>>> (62,34) point. In other words this line starts at cell (2,3) and ends at > cell >>>> (62,34). >>>> >>>> Is it possible to get by some R function all the matrix's cells that this >>line >>>> transverses? >>>> >>>> I would like to thank you for your feedback. >>>> >>>> Best Regards >>>> Alex >> >> On Sep 15, 2010, at 6:52 AM, Michael Bedward wrote: >> >>> Hello Alex, >>> >>> Here is one way to do it. It works but it's not pretty :) >> >> If you want an alternative, consider that produces the Y cell indices (since >>the x cell indices are already 2:62): >> >> > linefn <- function(x) 3+((34-3)/(62-2)) *(x-2) >> > findInterval(linefn(2:62), 3:34) >> [1] 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 >> 13 >>13 14 >> [28] 14 15 15 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26 >> 27 >>27 28 >> [55] 28 29 29 30 30 31 32 >> # that seems "off" by two >> > linefn(62) >> [1] 34 >> > linefn(2) >> [1] 3 # but that checks out and I realized those were just indices for the >> 3:34 >>findInterval vector >> >> > (3:34)[findInterval(linefn(2:62), 3:34)] >> [1] 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 13 13 14 14 >> 15 >>15 16 >> [28] 16 17 17 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 28 28 >> 29 >>29 30 >> [55] 30 31 31 32 32 33 34 >> >> ( no rounding and I think the logic is clearer.) > > But I also realized it didn't enumerate all the the cells were crossed either, > only indicating which cell was associated with an integer value of x. Also > would > have even more serious problems if the slope were greater than unity. To > enumerate the cell indices that were crossed, try: > > unique( floor( cbind( seq(2,62, by=0.1), linefn(seq(2,62, by=0.1)) ) ) ) > [,1] [,2] > [1,] 2 3 > [2,] 3 3 > [3,] 4 4 > [4,] 5 4 > [5,] 5 5 > [6,] 6 5 > [7,] 7 5 > [8,] 7 6 > snipping interior results > [83,] 58 32 > [84,] 59 32 > [85,] 60 32 > [86,] 60 33 > [87,] 61 33 > [88,] 62 34 > > That could probably be passed to rect() to illustrate (and check logic): > > rect(cellidxs[,1], cellidxs[,2], cellidxs[,1]+1, cellidxs[,2]+1, col="red") > > #redraw line : > lines(2:62, 3+(34-3)/(62-2)*(0:60)) > > >> >> --David. >> >>> >>> interp <- approx(c(2, 62), c(3, 34), method="linear", xout=2:62) >>> m <- matrix(c(interp$x, round(interp$y)), ncol=2) >>> tie <- m[,2] == c(-Inf, m[-nrow(m),2]) >>> m <- m[ !tie, ] >>> >>> You might want to examine the result like this... >>> >>> plot(m) # plots points >>> lines(c(2,26), c(3, 34)) # overlay line for comparison >> you can add a grid with >> abline(v=2:62, h=3:34) >>> >>> Michael >>> >>> ______________________________________________ >>> R-help@r-project.org mailing list >>> https://stat.ethz.ch/mailman/listinfo/r-help >>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html >>> and provide commented, minimal, self-contained, reproducible code. >> >> David Winsemius, MD >> West Hartford, CT >> >> ______________________________________________ >> R-help@r-project.org mailing list >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. > > David Winsemius, MD > West Hartford, CT > > > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
