how to design a tablet driver?

Daniel O'Connor doconnor at
Thu Mar 6 00:40:12 PST 2008

On Thu, 6 Mar 2008, Chuck Robey wrote:
> My current project is making a usb driver for my "Superpen" WP8060
> Tablet. It's a cheaper version of the better known Wacom Tablet,
> which i couldn't afford for my use in Gimp.  I figure, I better ask
> myu questions here, because they don't really bear so much on usb as
> they bear upon the basic architecture of a mouselike drvice that must
> be absolute, not relative, in approach.

I would say that you should report the physical size of the tablet, and 
then report movement in terms of real physical units. You could do all 
this in fixed point 32 bit ints and then have an enum which says what 
units it's reporting in (tenths of a millimetre, or whatever).

After that it's up to the client to do the right thing. Passing screen 
size to the display drive seems like a gross hack liable to much 

All this said I suspect that this problem has already been solved by 
other tablet drivers so I'd look there. Not to mention that there 
should already be a protocol for tablets that goes between the kernel 
and X (or from an X driver that opens ugen and speaks to the rest of X)

> OK, that means that I have to know the graphic point size (is there a
> Well, if it does, then it also means that I need to find the
> absoluite point size of whatever screen you're playing with, no
> matter if I'm talking about an X11 screen or a screen from a vty.  I
> mean, vty's use mice now, isn't it possible for use a tablet on a
> vtyt also?  OR, extra possibility, maybe if we're in a non-X11
> environment, fall back to a relative use, and only go to absolute
> when you go to X11.  If I have too much trouble getting and keeping
> an idea of the screen size oout of X11, I could do that.

Personally, I'd forget about vty's unless you want to do mouse emulation 
stuff there.. I imagine it would be a pretty major rearchitecting of 
sysmouse, moused etc etc.. Too hard to for too little reward :)

> I'm suggesting this because I'm very, very confused as to how I would
> figure out what the screen size is for a vty.  If I gotta do a
> absolute gig on a bty, then I gotta find some way to ask the size of
> the cry, and get me driver in the list of those to be notified if the
> size of a vty changes.

As above - basically "don't bother".

> Please, if you have any comment, feel free to interrupt all over me,
> sir! The sooner I get some of this, the sooner that really cheap
> (~$60) 8"X6" tablet is ours, especially for Gimp.

Wow, pretty nice :)
I might see if I can grab one..

Daniel O'Connor software and network engineer
for Genesis Software -
"The nice thing about standards is that there
are so many of them to choose from."
  -- Andrew Tanenbaum
GPG Fingerprint - 5596 B766 97C0 0E94 4347 295E E593 DC20 7B3F CE8C
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