Braille cells across the graphic area.
Braille cells down the graphic area.
Cells on the separate braille text line. Zero on models without one.
Pins across the graphic area — cellColumns * 2.
Pins down the graphic area — cellRows * 4 on a DotPad. On a display
whose pin rows are not a whole number of cells, fewer: the last cell row
is then only partly on the display, and its other pins stay down.
Physical layout of a connected tactile display, as the device itself reports it.
Never hardcode these. Models differ — one DotPad has no text line at all — and a renderer that assumes one geometry silently draws garbage on another.
The graphic area travels as a DotPad lays it out, in cells of two pins by four, whichever display it is going to. A display laid out differently -- a Monarch, whose braille cells sit apart on its pins -- reads the pins back out of those cells, so the tactile service draws for either without knowing which it has.