add SDL_SetRelativeMouseTransform

include/SDL3/SDL_mouse.h

@@ -147,6 +147,38 @@ typedef enum SDL_MouseWheelDirection
  */
 typedef Uint32 SDL_MouseButtonFlags;
 
+/**
+ * A callback used to transform mouse motion delta from raw values.
+ *
+ * This is called during SDL's handling of platform mouse events to
+ * scale the values of the resulting motion delta. 
+ *
+ * \param userdata what was passed as `userdata` to SDL_SetRelativeMouseTransform().
+ * \param timestamp the associated time at which this mouse motion event was received.
+ * \param window the associated window to which this mouse motion event was addressed.
+ * \param mouseID the associated mouse from which this mouse motion event was emitted.
+ * \param x pointer to a variable that will be treated as the resulting x-axis motion.
+ * \param y pointer to a variable that will be treated as the resulting y-axis motion.
+ *
+ * \threadsafety This callback is called by SDL's internal mouse input processing
+ *               procedure, which may be a thread separate from the main event loop
+ *               that is run at realtime priority. Stalling this thread with too much
+ *               work in the callback can therefore potentially freeze the entire
+ *               system. Care should be taken with proper synchronization practices 
+ *               when adding other side effects beyond mutation of the x and y values.
+ *
+ * \since This datatype is available since SDL 3.2.6.
+ *
+ * \sa SDL_SetRelativeMouseTransform
+ */
+typedef void (SDLCALL *SDL_MouseMotionTransformCallback)(
+    void *userdata, 
+    Uint64 timestamp, 
+    SDL_Window *window, 
+    SDL_MouseID mouseID, 
+    float *x, float *y
+);
+
 #define SDL_BUTTON_LEFT     1
 #define SDL_BUTTON_MIDDLE   2
 #define SDL_BUTTON_RIGHT    3
@@ -380,6 +412,17 @@ extern SDL_DECLSPEC void SDLCALL SDL_WarpMouseInWindow(SDL_Window *window,
  */
 extern SDL_DECLSPEC bool SDLCALL SDL_WarpMouseGlobal(float x, float y);
 
+/**
+ * Set a user-defined function by which to transform relative mouse inputs.
+ * This overrides the relative system scale and relative speed scale hints.
+ *
+ * \param callback a callback used to transform relative mouse motion, or NULL for default behavior.
+ * \param userdata a pointer that is passed to `callback`.
+ *
+ * \since This function is available since SDL 3.2.6.
+ */
+extern SDL_DECLSPEC void SDLCALL SDL_SetRelativeMouseTransform(SDL_MouseMotionTransformCallback callback, void *userdata);
+
 /**
  * Set relative mouse mode for a window.
  *

src/events/SDL_mouse.c

@@ -705,6 +705,10 @@ static void SDL_PrivateSendMouseMotion(Uint64 timestamp, SDL_Window *window, SDL
 
     if (relative) {
         if (mouse->relative_mode) {
+            if (mouse->InputTransform) {
+                void *data = mouse->input_transform_data;
+                mouse->InputTransform(data, timestamp, window, mouseID, &x, &y);
+            } else {
                 if (mouse->enable_relative_system_scale) {
                     if (mouse->ApplySystemScale) {
                         mouse->ApplySystemScale(mouse->system_scale_data, timestamp, window, mouseID, &x, &y);
@@ -714,6 +718,7 @@ static void SDL_PrivateSendMouseMotion(Uint64 timestamp, SDL_Window *window, SDL
                     x *= mouse->relative_speed_scale;
                     y *= mouse->relative_speed_scale;
                 }
+            }
         } else {
             if (mouse->enable_normal_speed_scale) {
                 x *= mouse->normal_speed_scale;
@@ -1115,6 +1120,13 @@ void SDL_QuitMouse(void)
     SDL_mice = NULL;
 }
 
+void SDL_SetRelativeMouseTransform(SDL_MouseMotionTransformCallback transform, void *userdata)
+{
+    SDL_Mouse *mouse = SDL_GetMouse();
+    mouse->InputTransform = transform;
+    mouse->input_transform_data = userdata;
+}
+
 SDL_MouseButtonFlags SDL_GetMouseState(float *x, float *y)
 {
     SDL_Mouse *mouse = SDL_GetMouse();

src/events/SDL_mouse_c.h

@@ -87,10 +87,14 @@ typedef struct
     // Get absolute mouse coordinates. (x) and (y) are never NULL and set to zero before call.
     SDL_MouseButtonFlags (*GetGlobalMouseState)(float *x, float *y);
 
-    // Platform-specific system mouse transform
+    // Platform-specific system mouse transform applied in relative mode
-    void (*ApplySystemScale)(void *internal, Uint64 timestamp, SDL_Window *window, SDL_MouseID mouseID, float *x, float *y);
+    SDL_MouseMotionTransformCallback ApplySystemScale;
     void *system_scale_data;
 
+    // User-defined mouse input transform applied in relative mode
+    SDL_MouseMotionTransformCallback InputTransform;
+    void *input_transform_data;
+
     // Data common to all mice
     SDL_Window *focus;
     float x;

src/video/wayland/SDL_waylandevents.c

@@ -1012,6 +1012,7 @@ static void relative_pointer_handle_relative_motion(void *data,
     struct SDL_WaylandInput *input = data;
     SDL_VideoData *d = input->display;
     SDL_WindowData *window = input->pointer_focus;
+    SDL_Mouse *mouse = SDL_GetMouse();
 
     // Relative pointer event times are in microsecond granularity.
     const Uint64 timestamp = Wayland_GetEventTimestamp(SDL_US_TO_NS(((Uint64)time_hi << 32) | (Uint64)time_lo));
@@ -1019,7 +1020,7 @@ static void relative_pointer_handle_relative_motion(void *data,
     if (input->pointer_focus && d->relative_mouse_mode) {
         double dx;
         double dy;
-        if (!SDL_GetMouse()->enable_relative_system_scale) {
+        if (mouse->InputTransform || !mouse->enable_relative_system_scale) {
             dx = wl_fixed_to_double(dx_unaccel_w);
             dy = wl_fixed_to_double(dy_unaccel_w);
         } else {