# PLOT THE IMAGE OF A DIGIT.  The arguments are the vector of pixel values
# for the image (0-255), and the threshold to apply in displaying the pixels
# as a binary array (default 128).

plot.digit = function (pixels, threshold=128)
{
  x = rep(1:28,times=28)
  y = rep(28:1,each=28)
  t = pixels>=threshold

  plot(x,y,pch=c(1,19)[1+t],xlab="",ylab="")
}