I've recently bought myself a Windows Tablet PC with a touchscreen thinking my Allegro project would just work with it. Turns out I was wrong 
Since I invested a lot of money in the tablet, I was determined to get Allegro working with it. The good news is that I managed to finally get everything working (stylus/mouse/touch.) The bad news is I'm not sure the best way to implement this directly into Allegro.
Some specs:
Toshiba Portege M750 with touchscreen
Windows Vista Business
Allegro 4.2.2
First I searched these forums and found no solutions. A search for "tablet" brings up a bunch of posts on Wacom tablets. And searching for "touch screen" brings up a few posts which describe my problem, but their solutions seem to work for Wacom tablets, not touch screens or styluses (styli?) I tried commenting out the handlers for DIMOFS_X and DIMOFS_Y in wmmouse.c, function mouse_dinput_handle_event. No luck, just broke everything.
The behavior I see is the cursor is always at one corner of the screen. But I found if I point the stylus at the center and move just a little, then it will bounce around dramatically, but still be somewhere on the screen.
I checked out what data is being returned from Direct Input. It looks like data being passed to mouse_dinput_handle_event are in range (-2560, -1600) - (2560, 1600) Absolute coordinates. But if you use the mouse then relative coordinates are passed. Touch screen is also absolute, but I did not check to see the range. Since the behavior is similar to the stylus input, my guess is that it's the same.
PROBLEM 1: How does someone tell if the data being passed is relative or absolute?
PROBLEM 2: How do I know what device is sending me the data?
PROBLEM 3: How do I know the range of that absolute device?
Since I could not answer these questions, I decided I needed a different approach. I tried to edit wwnd.c and handle WM_MOUSEMOVE, WM_LBUTTONDOWN, and WM_LBUTTONDOWN messages in directx_wnd_proc.
I found that I wasn't getting WM_MOUSEMOVE messages in full screen mode.
I managed to find a work around for this. GetCursorPos will work, but only if you don't initialize Allegro's mouse routines and you account for the window location in windowed mode.
So I export wnd_x and wnd_y in allegro.def, and include aintwin.h in my project. Then I can get the mouse position with this code...
Now I need to handle mouse clicks. I am getting button down/up messages so I'll use those. I add a variable to winalleg.h
And define it in wwnd.c
And add this to the switch in wwnd.c, function directx_wnd_proc.
Then export windows_mouse_b1 in allegro.def. It sort of works. Mouse is fine, you have to press really hard on the stylus to acknowledge a click, and the touchscreen does not move the cursor before you get the click message. 
Dig through WinUser.h, find messages WM_TABLET_FIRST and WM_TABLET_LAST with nothing in between them. I start capturing the messages in this range. I only find one, when I click with the stylus or touch the screen, it's undocumented message 716 (0x2CC) I figure out this is a message saying click and set the cursor at the same time. I add this code to directx_wnd_proc
The mouse up message is received as normal and everything works! I can switch between mouse and stylus and touch screen and the input is always interpreted correctly whether in full screen or windowed. Since message #716 is not documented, I'm afraid it will change in Windows 7. I haven't figured out right clicking yet either. I have not tested it in Allegro 4.4 but I checked the code and didn't see anything that would indicate a fix for this. I haven't tested or checked Allegro 5.
So that's what I've figured out. Maybe that will help with tablet/touch screen support in the future. It's at least a workaround for people who want tablet support now. Although really we should be using the tablet SDK to write a windows mouse driver. I'm just glad I finally got this working.
-Mark
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