Windows tablets don't report pen events when relative mouse mode is enabled

[Susko3]

• Originally reported in SDL3: external tablet driver bug with high precision mouse enabled ppy/osu#28223

SDL has special logic to convert raw absolute mouse (i.e. tablet) input to relative mouse motion. This tablet pen input never generates SDL_PenMotionEvents.

SDL/src/video/windows/SDL_windowsevents.c

Lines 650 to 658 in 31f9cb4

	} else {
	const int MAXIMUM_TABLET_RELATIVE_MOTION = 32;
	if (SDL_abs(relX) > MAXIMUM_TABLET_RELATIVE_MOTION ||
	SDL_abs(relY) > MAXIMUM_TABLET_RELATIVE_MOTION) {
	/* Ignore this motion, probably a pen lift and drop */
	} else {
	SDL_SendMouseMotion(timestamp, window, mouseID, true, (float)relX, (float)relY);
	}
	}

I would expect this pen to mouse logic to only fire when SDL_HINT_PEN_MOUSE_EVENTS is enabled. I would also expect to get the usual SDL_PenMotionEvent for this pen input.

This issue only affects tablet drivers that generate absolute mouse input. (Example: Wacom drivers with windows ink disabled, OpenTabletDriver, hawku Tablet Driver.) Tablet drivers that generate windows ink events work as expected via WM_POINTER & co. handling. When relative mouse mode is disabled, absolute tablet events are reported as absolute mouse input, which is not ideal, but is not really a problem as it's mapping absolute to absolute.

[Susko3]

This patch makes absolute pen input from OpenTablet Driver work when relative mouse mode is enabled. You shouldn't apply the patch as-is, as the previous behaviour of mapping absolute pen input to relative mouse motion is lost. The SDL_pen.c logic doesn't consider relative mouse mode when simulating mouse input.

diff --git a/src/video/windows/SDL_windowsevents.c b/src/video/windows/SDL_windowsevents.c
index af3407c40..8fddd0d55 100644
--- a/src/video/windows/SDL_windowsevents.c
+++ b/src/video/windows/SDL_windowsevents.c
@@ -579,6 +579,11 @@ static void WIN_HandleRawMouseInput(Uint64 timestamp, SDL_VideoData *data, HANDL
         return;
     }
 
+    SDL_Mouse *mouse = SDL_GetMouse();
+    if (!mouse) {
+        return;
+    }
+
     if (GetMouseMessageSource(rawmouse->ulExtraInformation) != SDL_MOUSE_EVENT_SOURCE_MOUSE) {
         return;
     }
@@ -648,13 +653,10 @@ static void WIN_HandleRawMouseInput(Uint64 timestamp, SDL_VideoData *data, HANDL
                     }
                 }
             } else {
-                const int MAXIMUM_TABLET_RELATIVE_MOTION = 32;
-                if (SDL_abs(relX) > MAXIMUM_TABLET_RELATIVE_MOTION ||
-                    SDL_abs(relY) > MAXIMUM_TABLET_RELATIVE_MOTION) {
-                    /* Ignore this motion, probably a pen lift and drop */
-                } else {
-                    SDL_SendMouseMotion(timestamp, window, mouseID, true, (float)relX, (float)relY);
+                if (!data->raw_input_fake_pen_id) {
+                    data->raw_input_fake_pen_id = SDL_AddPenDevice(timestamp, "raw mouse input", NULL, SDL_malloc(1));
                 }
+                SDL_SendPenMotion(timestamp, data->raw_input_fake_pen_id, window, (float)(x - window->x), (float)(y - window->y));
             }
 
             data->last_raw_mouse_position.x = x;
diff --git a/src/video/windows/SDL_windowsrawinput.c b/src/video/windows/SDL_windowsrawinput.c
index fa249914d..b9d4f979a 100644
--- a/src/video/windows/SDL_windowsrawinput.c
+++ b/src/video/windows/SDL_windowsrawinput.c
@@ -111,6 +111,11 @@ static DWORD WINAPI WIN_RawInputThread(LPVOID param)
         WIN_PollRawInput(_this, poll_start);
     }
 
+    if (_this->internal->raw_input_fake_pen_id) {
+        SDL_RemovePenDevice(0, _this->internal->raw_input_fake_pen_id);
+        _this->internal->raw_input_fake_pen_id = 0;
+    }
+
     devices[0].dwFlags |= RIDEV_REMOVE;
     devices[1].dwFlags |= RIDEV_REMOVE;
     RegisterRawInputDevices(devices, count, sizeof(devices[0]));
diff --git a/src/video/windows/SDL_windowsvideo.h b/src/video/windows/SDL_windowsvideo.h
index 53cfbadd7..cbf5639e1 100644
--- a/src/video/windows/SDL_windowsvideo.h
+++ b/src/video/windows/SDL_windowsvideo.h
@@ -440,6 +440,7 @@ struct SDL_VideoData
     bool raw_keyboard_enabled;
     bool pending_E1_key_sequence;
     Uint32 raw_input_enabled;
+    SDL_PenID raw_input_fake_pen_id;
 
     WIN_GameInputData *gameinput_context;
 

The simplest solution that works without adding any new flags would be to invoke the new code iff SDL_HINT_PEN_MOUSE_EVENTS is disabled. Applications that handle pen events will likely disable this flag.

diff --git a/src/video/windows/SDL_windowsevents.c b/src/video/windows/SDL_windowsevents.c
index af3407c40..36a9e4dc5 100644
--- a/src/video/windows/SDL_windowsevents.c
+++ b/src/video/windows/SDL_windowsevents.c
@@ -579,6 +579,11 @@ static void WIN_HandleRawMouseInput(Uint64 timestamp, SDL_VideoData *data, HANDL
         return;
     }
 
+    SDL_Mouse *mouse = SDL_GetMouse();
+    if (!mouse) {
+        return;
+    }
+
     if (GetMouseMessageSource(rawmouse->ulExtraInformation) != SDL_MOUSE_EVENT_SOURCE_MOUSE) {
         return;
     }
@@ -647,7 +652,7 @@ static void WIN_HandleRawMouseInput(Uint64 timestamp, SDL_VideoData *data, HANDL
                         }
                     }
                 }
-            } else {
+            } else if (mouse->pen_mouse_events) {
                 const int MAXIMUM_TABLET_RELATIVE_MOTION = 32;
                 if (SDL_abs(relX) > MAXIMUM_TABLET_RELATIVE_MOTION ||
                     SDL_abs(relY) > MAXIMUM_TABLET_RELATIVE_MOTION) {
@@ -655,6 +660,11 @@ static void WIN_HandleRawMouseInput(Uint64 timestamp, SDL_VideoData *data, HANDL
                 } else {
                     SDL_SendMouseMotion(timestamp, window, mouseID, true, (float)relX, (float)relY);
                 }
+            } else {
+                if (!data->raw_input_fake_pen_id) {
+                    data->raw_input_fake_pen_id = SDL_AddPenDevice(timestamp, "raw mouse input", NULL, SDL_malloc(1));
+                }
+                SDL_SendPenMotion(timestamp, data->raw_input_fake_pen_id, window, (float)(x - window->x), (float)(y - window->y));
             }
 
             data->last_raw_mouse_position.x = x;
diff --git a/src/video/windows/SDL_windowsrawinput.c b/src/video/windows/SDL_windowsrawinput.c
index fa249914d..b9d4f979a 100644
--- a/src/video/windows/SDL_windowsrawinput.c
+++ b/src/video/windows/SDL_windowsrawinput.c
@@ -111,6 +111,11 @@ static DWORD WINAPI WIN_RawInputThread(LPVOID param)
         WIN_PollRawInput(_this, poll_start);
     }
 
+    if (_this->internal->raw_input_fake_pen_id) {
+        SDL_RemovePenDevice(0, _this->internal->raw_input_fake_pen_id);
+        _this->internal->raw_input_fake_pen_id = 0;
+    }
+
     devices[0].dwFlags |= RIDEV_REMOVE;
     devices[1].dwFlags |= RIDEV_REMOVE;
     RegisterRawInputDevices(devices, count, sizeof(devices[0]));
diff --git a/src/video/windows/SDL_windowsvideo.h b/src/video/windows/SDL_windowsvideo.h
index 53cfbadd7..cbf5639e1 100644
--- a/src/video/windows/SDL_windowsvideo.h
+++ b/src/video/windows/SDL_windowsvideo.h
@@ -440,6 +440,7 @@ struct SDL_VideoData
     bool raw_keyboard_enabled;
     bool pending_E1_key_sequence;
     Uint32 raw_input_enabled;
+    SDL_PenID raw_input_fake_pen_id;
 
     WIN_GameInputData *gameinput_context;