UI: Reverse Pen Y Tilt Expected Direction

It is more convenient for math reasons, for Blender to treat a pen tilt
toward the user as a positive angle. This PR just reverses all the
comments about the expected values. And also reverses the values from
WinTab, Windows Ink, and Wayland to match.

Pull Request: https://projects.blender.org/blender/blender/pulls/137636

intern/ghost/GHOST_Types.h

@@ -166,7 +166,7 @@ typedef struct GHOST_TabletData {
   GHOST_TTabletMode Active; /* 0=None, 1=Stylus, 2=Eraser */
   float Pressure;           /* range 0.0 (not touching) to 1.0 (full pressure) */
   float Xtilt;              /* range -1.0 (left) to +1.0 (right) */
-  float Ytilt;              /* range -1.0 (toward user) to +1.0 (away from user) */
+  float Ytilt;              /* range -1.0 (away from user) to +1.0 (toward user) */
 } GHOST_TabletData;
 
 static const GHOST_TabletData GHOST_TABLET_DATA_NONE = {

intern/ghost/intern/GHOST_SystemCocoa.mm

@@ -1484,7 +1484,8 @@ GHOST_TSuccess GHOST_SystemCocoa::handleTabletEvent(void *eventPtr, short eventT
       /* Range: -1 (left) to 1 (right). */
       ct.Xtilt = event.tilt.x;
       /* On macOS, the y tilt behavior is inverted from what we expect: negative
-       * meaning a tilt toward the user, positive meaning away from the user. */
+       * meaning a tilt toward the user, positive meaning away from the user.
+       * Convert to what Blender expects: -1.0 (away from user) to +1.0 (toward user). */
       ct.Ytilt = -event.tilt.y;
       break;
 

intern/ghost/intern/GHOST_SystemWayland.cc

@@ -4791,10 +4791,11 @@ static void tablet_tool_handle_tilt(void *data,
                                     const wl_fixed_t tilt_x,
                                     const wl_fixed_t tilt_y)
 {
-  /* Map degrees to `-1.0 (left/back)..1.0 (right/forward)`. */
+  /* Map X tilt to `-1.0 (left)..1.0 (right)`.
+   * Map Y tilt to `-1.0 (away from user)..1.0 (toward user)`. */
   const float tilt_unit[2] = {
       float(wl_fixed_to_double(tilt_x) / 90.0f),
-      float(wl_fixed_to_double(tilt_y) / -90.0f),
+      float(wl_fixed_to_double(tilt_y) / 90.0f),
   };
   CLOG_INFO(LOG, 2, "tilt (x=%.4f, y=%.4f)", UNPACK2(tilt_unit));
   GWL_TabletTool *tablet_tool = static_cast<GWL_TabletTool *>(data);

intern/ghost/intern/GHOST_WindowWin32.cc

@@ -1013,9 +1013,9 @@ GHOST_TSuccess GHOST_WindowWin32::getPointerInfo(
     if (pointerPenInfo[i].penMask & PEN_MASK_TILT_Y) {
       /* Input value is a range of -90 to +90, with a positive value
        * indicating a tilt toward the user. Convert to what Blender
-       * expects: -1.0f (toward user) to +1.0f (away from user). */
+       * expects: -1.0f (away from user) to +1.0f (toward user). */
       outPointerInfo[i].tabletData.Ytilt = std::clamp(
-          pointerPenInfo[i].tiltY / -90.0f, -1.0f, 1.0f);
+          pointerPenInfo[i].tiltY / 90.0f, -1.0f, 1.0f);
     }
   }
 

intern/ghost/intern/GHOST_Wintab.cc

@@ -362,8 +362,12 @@ void GHOST_Wintab::getInput(std::vector<GHOST_WintabInfoWin32> &outWintabInfo)
       float vecLen = cos(altRad);
 
       /* From there calculate X and Y components based on azimuth. */
+
+      /* Blender expects: -1.0f (left) to +1.0f (right). */
       out.tabletData.Xtilt = sin(azmRad) * vecLen;
-      out.tabletData.Ytilt = float(sin(M_PI_2 - azmRad) * vecLen);
+
+      /* Blender expects: -1.0f (away from user) to +1.0f (toward user). */
+      out.tabletData.Ytilt = -float(sin(M_PI_2 - azmRad) * vecLen);
     }
 
     out.time = pkt.pkTime;

source/blender/makesrna/intern/rna_brush.cc

@@ -3942,7 +3942,7 @@ static void rna_def_operator_stroke_element(BlenderRNA *brna)
   prop = RNA_def_property(srna, "y_tilt", PROP_FLOAT, PROP_FACTOR);
   RNA_def_property_flag(prop, PROP_IDPROPERTY);
   RNA_def_property_range(prop, -1.0f, 1.0f);
-  RNA_def_property_ui_text(prop, "Tilt Y", "Pen tilt from forward (-1.0) to back (+1.0)");
+  RNA_def_property_ui_text(prop, "Tilt Y", "Pen tilt from backward (-1.0) to forward (+1.0)");
 
   /* used in uv painting */
   prop = RNA_def_property(srna, "time", PROP_FLOAT, PROP_UNSIGNED);

source/blender/windowmanager/WM_types.hh

@@ -702,7 +702,7 @@ struct wmTabletData {
   float pressure;
   /** range -1.0 (left) to +1.0 (right). */
   float x_tilt;
-  /** range -1.0 (toward user) to +1.0 (away from user). */
+  /** range -1.0 (away from user) to +1.0 (toward user). */
   float y_tilt;
   /** Interpret mouse motion as absolute as typical for tablets. */
   char is_motion_absolute;