Question

I want to plot a wireframe in R using the lattice package. However, I get the following error message “error using packet 1 negative length vectors are not allowed”. The data looks like the following:

> result_mean
  experiment alpha beta   packet
1          0   1.0    1 3.000000
2          0   1.1    1 2.571429

The command to create the data is the following

  png(file=paste("foobar.png"),width=1280, height=1280);
    plot <- wireframe(result_mean$packet ~ result_mean$alpha * result_mean$beta,       
      data=result_mean, scales = list(arrows=FALSE, cex= .45, col = "black", font = 3),
      drape = TRUE, colorkey = TRUE, main = "Foo",
      col.regions = terrain.colors(100),
      screen = list(z = -60, x = -60),
      xlab="alpha", ylab="beta", zlab="mean \npackets");
  print(plot);
  dev.off();

I’m wondering what this error message means and if there is a good way to debug this?

Thanks in advance!

Answer 1

Debugging lattice graphics is a bit difficult because (a) the code is complex and multi-layered and (b) the errors get trapped in a way that makes them hard to intercept. However, you can at least get some way in diagnosing the problem.

First create a minimal example. I suspected that your problem was that your data fall on a single line, so I created data that looked like that:

d <- data.frame(x=c(1,1.1),
                y=c(1,1),
                z=c(2,3))

library(lattice)
wireframe(z~y*x,data=d)

Now confirm that fully three-dimensional data (data that define a plane) work just fine:

d2 <- data.frame(expand.grid(x=c(1,1.1),
                             y=c(1,1.1)),
                 z=1:4)
wireframe(z~y*x,data=d2) 

So the question is really — did you intend to draw a wireframe of two points lying on a line? If so, what did you want to have appear in the plot? You could hack things a little bit to set the y values to differ by a tiny bit — I tried it, though, and got no wireframe appearing (but no error either).

edit: I did a bit more tracing, with various debug() incantations (and searching the source code of the lattice package and R itself for “negative length”) to deduce the following: within a function called lattice:::panel.3dwire, there is a call to a C function wireframePanelCalculations, which you can see at https://r-forge.r-project.org/scm/viewvc.php/pkg/src/threeDplot.c?view=markup&root=lattice

Within this function:

nh = (nx-1) * (ny-1) * ng; /* number of quadrilaterals */
sHeights = PROTECT(allocVector(REALSXP, nh));

In this case nx is zero, so this code is asking R to allocate a negative-length vector, which is where the error comes from.

In this case, though, I think the diagnosis is more useful than the explicit debugging.