R help - Oct 2013 - Problem with coordinates when trying to draw lines into a raster (image) file

Hi All,

   I am struggling with something, and could use some help

   I have a scenario where I need to draw lines onto a base image using R ???
   briefly, the image has what amounts to an outline ???map??? of locations, and
   the lines will correspond to ???routes??? between two locations.

   The locations are known in terms of image pixel coordinates ??? let???s call
   them (px1, py1) and (px2, py2), but when I try and plot a line into the
   image using these coordinates, the visual positions are incorrect ??? the
   start and end points of the line are offset from the desired position, and
   the amount of offset changes as I resize the window. I've tried such
things
   as normalising them into the [0,1] range used by the viewport but this does
   not correct the problem.

   So,  I  figured  that I must have made some mistake with my scaling of
   coordinates from image to viewport, but I cannot find where or what. I???ve
   fiddled around a bit (well, a lot!) but cannot get the desired result. So,
   it is time to ask for help, hence this message???.

   Any  suggestions  gratefully  received???   I???ve done a fair amount of R
   programming, but have not used these extended graphics capabilities much at
   all, so I really am getting frustrated....

   Regards and thanks in advance,
   Rick

   ----------------------------------------------------------------------------
   --------------------------------------------------

   The code segment in question is:

   # load packages

   library(jpeg)

   library(grid)


   # read the image file

   baseimg <- readJPEG("loc_map.jpg", native=FALSE)

   xsize <- ncol(baseimg)                   # Get image size ??? this one is
1344
   px wide

   ysize <- nrow(baseimg)                 # and 1008 px high


   # create a viewport

   xrange <- c(0, xsize)                        # set up the viewport range
to
   match the image size

   yrange <- c(0, ysize)

   vp  <-  viewport(x=0.5,  y=0.5,  width=0.9, height=0.9, xscale=xrange,
   yscale=yrange)

   pushViewport(vp)

   grid.rect(gp=gpar(lty="dashed"))        # draw a dashed line around
it.


   # display the base image

   grid.raster(baseimg)


   # First location ??? image pixel coordinates (748, 177). Normalise these to
   [0,1] to

   # match the viewpoint coordinate scheme. Note that we need to invert the

   # y coordinate as R coords run from bottom up, but image ones are top down

   px1 <- (748/xsize)                                            # 748/1344 ~
0.556, so in range [0,1]

   py1 <- (1.0 - (177/ysize))                               # 1-(177/1008) ~ 
0.824, so also in range [0,1]


   # position of the St Johns Hill enterance (image coords

   # [769, 892]) normalised to the viewport

     x2 <- (769/xsize)

     y2 <- (1.0 - (892/ysize))


     # draw a line from pixel (px1,py1) to pixel (px2,py2) in blue

     xx <- c(px1, px2)

     yy <- c(py1, py2)

     grid.lines(xx, yy, gp=gpar(col="blue"))

Rick,

This uses a different approach than what you propose, but it gets the job
done and perhaps you will find it helpful.

Jean

library(jpeg)
# download image from internet for use in example
# http://www.tootsie.com/wallpaper/wp_concord2_1344x1008.jpg
img <- readJPEG("c:/temp/wp_concord2_1344x1008.jpg")
# define location of two yellow gumballs, could use locator() to get these
coordinates
px.x <- c(172, 576)
px.y <- c(433, 657)
# set up plot
par(xaxs="i", yaxs="i")
eqscplot(x, y, type="n", xlim=c(0, 1334), ylim=c(0, 1008))
# add image
rasterImage(img, 0, 0, 1334, 1008)
# add line connecting two yellow gumballs
lines(px.x, px.y, lwd=5, col="yellow")



On Mon, Oct 21, 2013 at 8:57 AM, Rick Turner
<rickturner@btconnect.com>wrote:
>
>    Hi All,
>
>    I am struggling with something, and could use some help
>
>    I have a scenario where I need to draw lines onto a base image using R
> –
>    briefly, the image has what amounts to an outline ‘map’ of
> locations, and
>    the lines will correspond to ‘routes’ between two locations.
>
>    The locations are known in terms of image pixel coordinates – let’s
> call
>    them (px1, py1) and (px2, py2), but when I try and plot a line into the
>    image using these coordinates, the visual positions are incorrect –
> the
>    start and end points of the line are offset from the desired position,
> and
>    the amount of offset changes as I resize the window. I've tried such
> things
>    as normalising them into the [0,1] range used by the viewport but this
> does
>    not correct the problem.
>
>    So,  I  figured  that I must have made some mistake with my scaling of
>    coordinates from image to viewport, but I cannot find where or what.
> I’ve
>    fiddled around a bit (well, a lot!) but cannot get the desired result.
> So,
>    it is time to ask for help, hence this message….
>
>    Any  suggestions  gratefully  received…   I’ve done a fair amount
> of R
>    programming, but have not used these extended graphics capabilities
> much at
>    all, so I really am getting frustrated....
>
>    Regards and thanks in advance,
>    Rick
>
>
> 
----------------------------------------------------------------------------
>    --------------------------------------------------
>
>    The code segment in question is:
>
>    # load packages
>
>    library(jpeg)
>
>    library(grid)
>
>
>    # read the image file
>
>    baseimg <- readJPEG("loc_map.jpg", native=FALSE)
>
>    xsize <- ncol(baseimg)                   # Get image size – this one
> is 1344
>    px wide
>
>    ysize <- nrow(baseimg)                 # and 1008 px high
>
>
>    # create a viewport
>
>    xrange <- c(0, xsize)                        # set up the viewport
> range to
>    match the image size
>
>    yrange <- c(0, ysize)
>
>    vp  <-  viewport(x=0.5,  y=0.5,  width=0.9, height=0.9,
xscale=xrange,
>    yscale=yrange)
>
>    pushViewport(vp)
>
>    grid.rect(gp=gpar(lty="dashed"))        # draw a dashed line
around it.
>
>
>    # display the base image
>
>    grid.raster(baseimg)
>
>
>    # First location – image pixel coordinates (748, 177). Normalise
> these to
>    [0,1] to
>
>    # match the viewpoint coordinate scheme. Note that we need to invert the
>
>    # y coordinate as R coords run from bottom up, but image ones are top
> down
>
>    px1 <- (748/xsize)                                            #
> 748/1344 ~>    0.556, so in range [0,1]
>
>    py1 <- (1.0 - (177/ysize))                               #
1-(177/1008)
> ~>    0.824, so also in range [0,1]
>
>
>    # position of the St Johns Hill enterance (image coords
>
>    # [769, 892]) normalised to the viewport
>
>      x2 <- (769/xsize)
>
>      y2 <- (1.0 - (892/ysize))
>
>
>      # draw a line from pixel (px1,py1) to pixel (px2,py2) in blue
>
>      xx <- c(px1, px2)
>
>      yy <- c(py1, py2)
>
>      grid.lines(xx, yy, gp=gpar(col="blue"))
> ______________________________________________
> 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.
>
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Rick,

I see a couple errors in my code ... you will need to attach the MASS
package for the eqscplot() function and that function's first arguments
should be any coordinates, I use (1, 1)

library(jpeg)
library(MASS)
# download image from internet for use in example
# http://www.tootsie.com/wallpaper/wp_concord2_1344x1008.jpg
img <- readJPEG("c:/temp/wp_concord2_1344x1008.jpg")
# define location of two yellow gumballs, could use locator() to get these
coordinates
px.x <- c(172, 576)
px.y <- c(433, 657)
# set up plot
par(xaxs="i", yaxs="i")
eqscplot(1, 1, type="n", xlim=c(0, 1334), ylim=c(0, 1008))
# add image
rasterImage(img, 0, 0, 1334, 1008)
# add line connecting two yellow gumballs
lines(px.x, px.y, lwd=5, col="yellow")

Jean



On Mon, Oct 28, 2013 at 4:12 AM, Rick Turner
<rickturner@btconnect.com>wrote:
> I forgot to mention that I get an error from your code - the
'lines()'
> command fails with the error 'object px.x not found'.  I am not
sure why
> (yet) but need to dig into it... sigh.
>
> Rick
>
>
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