http://bugs.winehq.org/show_bug.cgi?id=11846
--- Comment #23 from John Klehm xixsimplicityxix@gmail.com 2008-04-16 03:15:20 --- I've looked into this sai business a bit further. SAI is different in how it captures the tablet messages. I believe that openCanvas and Photoshop grab packets by repeatedly using WTPacketsGet, the "active" way to get tablet info.
Based on my wintab32 logs SAI uses the "passive" way to get tablet info in that it expects to find tablet messages (WT_PACKET) coming through Windows Messaging channels. I think this is where our wintab breaks down as when I run +msg logs I don't see any WT_PACKETS coming through.
Of course this is just my theory and I can't confirm it for sure yet but in case I don't get back to this in a couple weeks it could maybe help someone out.
See the wintab spec[1] page 5, section 13 "Polling" for the two different ways of monitoring tablet info.
[1] http://www.wacomeng.com/devsupport/downloads/pc/wt1pt1.zip