UI: Consistent Pen Tilt Values

Pen tilt values come into Blender through ghost platforms and device
drivers with inconsistent and ambiguous directions. This PR codifies
that we consider tilt-x positive values to be rightward, negative
values as leftward. Similarly tilt-y positive is away from the user,
negative is toward the user. Comments are updated to make this clear.
This fixes Windows Ink (currently always returning left and away). This
also reverses the Wayland tily-y values to match. We might still have
to consider a user preference to swap directions if we find differences
within a platform.

Pull Request: https://projects.blender.org/blender/blender/pulls/137501

intern/ghost/GHOST_Types.h

@@ -165,8 +165,8 @@ typedef enum {
 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 0.0 (upright) to 1.0 (tilted fully against the tablet surface) */
-  float Ytilt; /* as above */
+  float Xtilt;              /* range -1.0 (left) to +1.0 (right) */
+  float Ytilt;              /* range -1.0 (toward user) to +1.0 (away from user) */
 } GHOST_TabletData;
 
 static const GHOST_TabletData GHOST_TABLET_DATA_NONE = {

intern/ghost/intern/GHOST_SystemCocoa.mm

@@ -1481,9 +1481,10 @@ GHOST_TSuccess GHOST_SystemCocoa::handleTabletEvent(void *eventPtr, short eventT
       }
 
       ct.Pressure = event.pressure;
+      /* Range: -1 (left) to 1 (right). */
       ct.Xtilt = event.tilt.x;
-      /* On macOS, the y tilt behavior is inverted; an increase in the tilt
-       * value corresponds to tilting the device away from the user. */
+      /* 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. */
       ct.Ytilt = -event.tilt.y;
       break;
 

intern/ghost/intern/GHOST_SystemWayland.cc

@@ -4791,10 +4791,10 @@ 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..1.0`. */
+  /* Map degrees to `-1.0 (left/back)..1.0 (right/forward)`. */
   const float tilt_unit[2] = {
-      float(wl_fixed_to_double(tilt_x) / 90.0),
-      float(wl_fixed_to_double(tilt_y) / 90.0),
+      float(wl_fixed_to_double(tilt_x) / 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

@@ -6,12 +6,14 @@
  * \ingroup GHOST
  */
 
-#include "GHOST_WindowWin32.hh"
+#include <algorithm>
+
 #include "GHOST_ContextD3D.hh"
 #include "GHOST_ContextNone.hh"
 #include "GHOST_DropTargetWin32.hh"
 #include "GHOST_SystemWin32.hh"
 #include "GHOST_WindowManager.hh"
+#include "GHOST_WindowWin32.hh"
 #include "utf_winfunc.hh"
 #include "utfconv.hh"
 
@@ -1022,11 +1024,19 @@ GHOST_TSuccess GHOST_WindowWin32::getPointerInfo(
     }
 
     if (pointerPenInfo[i].penMask & PEN_MASK_TILT_X) {
-      outPointerInfo[i].tabletData.Xtilt = fmin(fabs(pointerPenInfo[i].tiltX / 90.0f), 1.0f);
+      /* Input value is a range of -90 to +90, with a positive value
+       * indicating a tilt to the right. Convert to what Blender
+       * expects: -1.0f (left) to +1.0f (right). */
+      outPointerInfo[i].tabletData.Xtilt = std::clamp(
+          pointerPenInfo[i].tiltX / 90.0f, -1.0f, 1.0f);
     }
 
     if (pointerPenInfo[i].penMask & PEN_MASK_TILT_Y) {
-      outPointerInfo[i].tabletData.Ytilt = fmin(fabs(pointerPenInfo[i].tiltY / 90.0f), 1.0f);
+      /* 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). */
+      outPointerInfo[i].tabletData.Ytilt = std::clamp(
+          pointerPenInfo[i].tiltY / -90.0f, -1.0f, 1.0f);
     }
   }
 

source/blender/makesrna/intern/rna_brush.cc

@@ -3937,12 +3937,12 @@ static void rna_def_operator_stroke_element(BlenderRNA *brna)
   prop = RNA_def_property(srna, "x_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 X", "");
+  RNA_def_property_ui_text(prop, "Tilt X", "Pen tilt from left (-1.0) to right (+1.0)");
 
   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", "");
+  RNA_def_property_ui_text(prop, "Tilt Y", "Pen tilt from forward (-1.0) to back (+1.0)");
 
   /* used in uv painting */
   prop = RNA_def_property(srna, "time", PROP_FLOAT, PROP_UNSIGNED);

source/blender/windowmanager/WM_types.hh

@@ -700,9 +700,9 @@ struct wmTabletData {
   int active;
   /** Range 0.0 (not touching) to 1.0 (full pressure). */
   float pressure;
-  /** Range 0.0 (upright) to 1.0 (tilted fully against the tablet surface). */
+  /** range -1.0 (left) to +1.0 (right). */
   float x_tilt;
-  /** As above. */
+  /** range -1.0 (toward user) to +1.0 (away from user). */
   float y_tilt;
   /** Interpret mouse motion as absolute as typical for tablets. */
   char is_motion_absolute;