A solution for one specific case
After posting, I figured out a kludge that is “good enough” for the specific project I’m setting up now.
This series of experiments involves subjects looking at stimuli on the screen and also smelling scratch and sniff labels. In order to reduce experimenter errors, I wanted to display the two-digit number printed on the paper that the odor labels are mounted on, in gray digits, all the way in the lower left and right corners of the display (both corners, because the experimenter hands the labels to right-handed subjects from the left and vice versa). SInce these “ticket codes” are presented only during the ITI, nothing else really has to appear on the screen with them.
What I did was to use some pieces of the netpbm package to generate a black 640x480 png file, with the codes superimposed in gray in the lower left and right corners. Superlab is then programmed to expand the image to fill the screen, so the codes are always in the corners.
If anyone is interested, here is a Korn shell function to do this:
Usage: cornerpng XX file.png
XX is a two-digit code
ppmmake black 640 480
| ppmlabel -x 0 -y 478 -size 10 -background black -color gray -text $1
| ppmlabel -x 620 -y 478 -size 10 -background black -color gray -text $1
| pnmtopng > $2 2>/dev/null