Using <window>.screenToWorld within vizcave powerwall

[VizMars]

How can one use the <window>.screenToWorld command within a vizcave powerwall?
Specifically, how can one get the length of the line passing through the given screen coordinates (in pixel units) with the begin point of the line being the users head position (center of the eyes)?

Unfortunately, the following code seems to be incorrect:

line = viz.MainWindow.screenToWorld([pixel_x,pixel_y], eye = viz.BOTH_EYE, mode = viz.WINDOW_PIXELS)
print line.length