What information does dwExtraInfo hold for the PMOUSEHOOKSTRUCT? - c++

Question

LRESULT CALLBACK LowLevelMouseProc(_In_ int nCode, _In_ WPARAM wParam, _In_ LPARAM lParam)
{
BOOL fpassmove = FALSE;
if (nCode == HC_ACTION)
{
switch (wParam)
{
case WM_MOUSEMOVE: // how do i catch a dx instead of a cursor points?
PMOUSEHOOKSTRUCT me = (PMOUSEHOOKSTRUCT)lParam;
printf("x:%d\ny:%d\nextrainfo:%04X\n", me->pt.x,me->pt.y, me->dwExtraInfo );
break;
}
}
return(fpassmove ? 1 : CallNextHookEx(NULL, nCode, wParam, lParam));
}
int main()
{
// Install the low-level keyboard & mouse hooks
HHOOK hhkLowLevelMouse = SetWindowsHookEx(WH_MOUSE_LL, LowLevelMouseProc, 0, 0);
// Keep this app running until we're told to stop
MSG msg;
while (!GetMessage(&msg, NULL, NULL, NULL)) { //this while loop keeps the hook
TranslateMessage(&msg);
DispatchMessage(&msg);
}
UnhookWindowsHookEx(hhkLowLevelMouse);
return(0);
}
I am looking at all mouse moment from a global hook, and I'm trying to find the dx and dy of the mouse event, And I was hoping to find it in "dwExtraInfo". However, I have no idea how to make sense of the data inside "dwExtraInfo".
the windows documentation is not helpful in telling me what the data in dwExtraInfo means
dwExtraInfo
Type: ULONG_PTR
Additional information associated with the message.

Answer 1

Could not find any documentation on dwExtraInfo directly, but found something related: https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getmessageextrainfo:
Return value
Type: LPARAM
The return value specifies the extra information. The meaning of the
extra information is device specific.
One example of such device specific extra information is Pen gestures: https://learn.microsoft.com/en-us/windows/win32/tablet/system-events-and-mouse-messages
Distinguishing Pen Input from Mouse and Touch
When your application receives a mouse message (such as
WM_LBUTTONDOWN), it may call the GetMessageExtraInfo function to
evaluate whether the message originated from a pen or a mouse device.
The value returned from GetMessageExtraInfo needs to be mask-checked
against 0xFFFFFF00, and then compared with 0xFF515700. The following
definitions may make this clearer:
#define MI_WP_SIGNATURE 0xFF515700
#define SIGNATURE_MASK 0xFFFFFF00
#define IsPenEvent(dw) (((dw) & SIGNATURE_MASK) == MI_WP_SIGNATURE
If the comparison is true, then this mouse message was generated by a
Tablet PC pen or touch screen. In all other cases, you can assume that
this message was generated by a mouse device.
The lower 8 bits returned from GetMessageExtraInfo are variable. Of
those bits, 7 (the lower 7, masked by 0x7F) are used to represent the
cursor ID, zero for the mouse or a variable value for the pen ID.
Additionally, in Windows Vista, the eighth bit, masked by 0x80, is
used to differentiate touch input from pen input (0 = pen, 1 = touch).