$si(dpi)$ - Return the DPI
$si(dwidth)$ - Return the real width of the device in pixels
$si(dheight)$ - Return the eeal height of the device in pixels
Yes. There is a way to obtain those values in some devices using sh() function, but it's not an stable way (unless Frank add something like "final variable") and it's not good if you're going to share it with a lot of devices (since some will work, some won't).