[R] jpeg() creating empty files with qplot() in a loop

[B. Bogart]

Hello all,

I'm stuck with a strange issue with writing jpegs of plots to a folder
in a loop.

This works:

for (step in 1:length(steps)) {
  jpeg(filename=paste("frame_",sprintf("%05d",step),".jpg",sep=""))
  plot(steps[[step]])
  dev.off()
}

But if I use qplot to generate the plot (which is my aim):

for (step in 1:length(steps)) {
 jpeg(filename=paste("frame_",sprintf("%05d",step),".jpg",sep=""))
 qplot(x, y, data=steps[[step]], geom="tile", fill=rgb(V1,V2,V3)) +
scale_fill_identity() + opts(aspect.ratio = .75)
 dev.off()
}

I end up with a directory of empty files named correctly.

The following does work:

step <- 10
jpeg(filename=paste("frame_",sprintf("%05d",step),".jpg",sep=""))
qplot(x, y, data=steps[[step]], geom="tile", fill=rgb(V1,V2,V3)) +
scale_fill_identity() + opts(aspect.ratio = .75)
dev.off()

So what could be the difference between the last example and it running
in a loop, that the former generates a proper jpeg file and the lattter
does not?

Does something more special than dev.off() need to be done with qplot
output when in a loop?

Thanks,
B. Bogart