[R] lattice: wireframe "eats up" points; how to make points on wireframe visible?

[Deepayan Sarkar]

On Fri, Mar 4, 2011 at 1:47 PM, Marius Hofert <m_hofert at web.de> wrote:
> Dear expeRts,
>
> I would like to add two points to a wireframe plot. The points have (x,y,z) coordinates
> where z is determined to be on the wireframe [same z-value]. Now something strange
> happens. One point is perfectly plotted, the other isn't shown at all. It only
> appears if I move it upwards in z-direction by adding a positive number. So somehow
> it disappears in the wireframe-surface *although* the plot symbol [the cross] has
> a positive length in each dimension [I also chose cex=5 to make it large enough so
> that it should (theoretically) be visible].
>
> My wireframe plot is a complicated function which I cannot post here. Below is a minimal
> example, however, it didn't show the same problem [the surface is too nice I guess].
> I therefore *artifically* create the problem in the example below so that you know
> what I mean. For one of the points, I subtract an epsilon [=0.25] in z-direction and
> suddenly the point completely disappears. The strange thing is that the point is
> not even "under" the surface [use the screen-argument to rotate the wireframe plot to check this],
> it's simply gone, eaten up by the surface.
>
> How can I make the two points visible?
> I also tried to use the alpha-argument to make the wireframe transparent, but I couldn't
> solve the problem.
>
> Cheers,
>
> Marius
>
> PS: One also faces this problem for example if one wants to make points visible that are on "opposite sides" of the wireframe.
>
> library(lattice)
>
> f <- function(x) 1/((1-x[1])*(1-x[2])+1)
>
> u <- seq(0, 1, length.out=20)
> grid <- expand.grid(x=u, y=u)
> x <- grid[,1]
> y <- grid[,2]
> z <- apply(grid, 1, f)
>
> pt.x <- c(0.2, 0.5)
> pt.y <- c(0.6, 0.8)
> eps <- 0.25
> pts <- rbind(c(pt.x, f(pt.x)-eps), c(pt.y, f(pt.y))) # points to add to the wireframe

The reason in this case is fairly obvious: you have

> pts
     [,1] [,2]      [,3]
[1,]  0.2  0.5 0.4642857
[2,]  0.6  0.8 0.9259259

So the z-value for Point 1 is 0.4642857, which is less than 0.5, the
minimum of the z-axis.panel.3dscatter() "clips" any points outside the
range of the bounding box, so this point is not plotted.

I can't say what the problem was in your original example without
looking at it, but I would guess it's caused by something similar.

Hope this is not too late to be useful.

-Deepayan



>
> wireframe(z~x*y, pts=pts, aspect=1, scales=list(col=1, arrows=FALSE),
>          panel.3d.wireframe = function(x,y,z,xlim,ylim,zlim,xlim.scaled,
>          ylim.scaled,zlim.scaled,pts,...){
>              panel.3dwire(x=x, y=y, z=z, xlim=xlim, ylim=ylim, zlim=zlim,
>                           xlim.scaled=xlim.scaled, ylim.scaled=ylim.scaled,
>                           zlim.scaled=zlim.scaled, ...)
>              panel.3dscatter(x=pts[,1], y=pts[,2], z=pts[,3],
>                              xlim=xlim, ylim=ylim, zlim=zlim,
>                              xlim.scaled=xlim.scaled, ylim.scaled=ylim.scaled,
>                              zlim.scaled=zlim.scaled, type="p", col=c(2,3),
>                              cex=1.8, .scale=TRUE, ...)
>          }, key=list(x=0.5, y=0.95, points=list(col=c(2,3)),
>             text=list(c("Point 1", "Point 2")),
>             cex=1, align=TRUE, transparent=TRUE))
>
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