source/blender/blenkernel/intern/brush.cc

@@ -2116,7 +2116,7 @@ float BKE_brush_sample_tex_3d(const Scene *scene,
     if (mtex->brush_map_mode == MTEX_MAP_MODE_VIEW) {
       /* keep coordinates relative to mouse */
 
-      rotation += ups->brush_rotation;
+      rotation -= ups->brush_rotation;
 
       x = point_2d[0] - ups->tex_mouse[0];
       y = point_2d[1] - ups->tex_mouse[1];
@@ -2134,7 +2134,7 @@ float BKE_brush_sample_tex_3d(const Scene *scene,
       y = point_2d[1];
     }
     else if (mtex->brush_map_mode == MTEX_MAP_MODE_RANDOM) {
-      rotation += ups->brush_rotation;
+      rotation -= ups->brush_rotation;
       /* these contain a random coordinate */
       x = point_2d[0] - ups->tex_mouse[0];
       y = point_2d[1] - ups->tex_mouse[1];
@@ -2229,7 +2229,7 @@ float BKE_brush_sample_masktex(
     if (mtex->brush_map_mode == MTEX_MAP_MODE_VIEW) {
       /* keep coordinates relative to mouse */
 
-      rotation += ups->brush_rotation_sec;
+      rotation -= ups->brush_rotation_sec;
 
       x = point_2d[0] - ups->mask_tex_mouse[0];
       y = point_2d[1] - ups->mask_tex_mouse[1];
@@ -2247,7 +2247,7 @@ float BKE_brush_sample_masktex(
       y = point_2d[1];
     }
     else if (mtex->brush_map_mode == MTEX_MAP_MODE_RANDOM) {
-      rotation += ups->brush_rotation_sec;
+      rotation -= ups->brush_rotation_sec;
       /* these contain a random coordinate */
       x = point_2d[0] - ups->mask_tex_mouse[0];
       y = point_2d[1] - ups->mask_tex_mouse[1];

source/blender/blenkernel/intern/paint.cc

@@ -1352,11 +1352,11 @@ bool paint_calculate_rake_rotation(UnifiedPaintSettings *ups,
     }
 
     float dpos[2];
-    sub_v2_v2v2(dpos, ups->last_rake, mouse_pos);
+    sub_v2_v2v2(dpos, mouse_pos, ups->last_rake);
 
     /* Limit how often we update the angle to prevent jitter. */
     if (len_squared_v2(dpos) >= r * r) {
-      rotation = atan2f(dpos[0], dpos[1]);
+      rotation = atan2f(dpos[1], dpos[0]) + float(0.5f * M_PI);
 
       copy_v2_v2(ups->last_rake, mouse_pos);
 

source/blender/editors/sculpt_paint/paint_cursor.cc

@@ -584,7 +584,7 @@ static bool paint_draw_tex_overlay(UnifiedPaintSettings *ups,
 
       /* Brush rotation. */
       GPU_matrix_translate_2fv(center);
-      GPU_matrix_rotate_2d(-RAD2DEGF(primary ? ups->brush_rotation : ups->brush_rotation_sec));
+      GPU_matrix_rotate_2d(RAD2DEGF(primary ? ups->brush_rotation : ups->brush_rotation_sec));
       GPU_matrix_translate_2f(-center[0], -center[1]);
 
       /* Scale based on tablet pressure. */

source/blender/editors/sculpt_paint/paint_stroke.cc

@@ -46,7 +46,7 @@
 #include "paint_intern.hh"
 #include "sculpt_intern.hh"
 
-//#define DEBUG_TIME
+// #define DEBUG_TIME
 
 #ifdef DEBUG_TIME
 #  include "PIL_time_utildefines.h"
@@ -399,7 +399,7 @@ static bool paint_brush_update(bContext *C,
 
     ups->anchored_size = ups->pixel_radius = sqrtf(dx * dx + dy * dy);
 
-    ups->brush_rotation = ups->brush_rotation_sec = atan2f(dx, dy) + float(M_PI);
+    ups->brush_rotation = ups->brush_rotation_sec = atan2f(dy, dx) + float(0.5f * M_PI);
 
     if (brush->flag & BRUSH_EDGE_TO_EDGE) {
       halfway[0] = dx * 0.5f + stroke->initial_mouse[0];
@@ -1372,7 +1372,7 @@ static bool paint_stroke_curve_end(bContext *C, wmOperator *op, PaintStroke *str
 
       for (j = 0; j < PAINT_CURVE_NUM_SEGMENTS; j++) {
         if (do_rake) {
-          float rotation = atan2f(tangents[2 * j], tangents[2 * j + 1]);
+          float rotation = atan2f(tangents[2 * j + 1], tangents[2 * j]) + float(0.5f * M_PI);
           paint_update_brush_rake_rotation(ups, br, rotation);
         }
 

source/blender/editors/sculpt_paint/sculpt.cc

@@ -2708,20 +2708,22 @@ static void update_sculpt_normal(Sculpt *sd, Object *ob, Span<PBVHNode *> nodes)
   }
 }
 
-static void calc_local_y(ViewContext *vc, const float center[3], float y[3])
+static void calc_local_from_screen(ViewContext *vc,
+                                   const float center[3],
+                                   const float screen_dir[2],
+                                   float r_local_dir[3])
 {
   Object *ob = vc->obact;
   float loc[3];
-  const float xy_delta[2] = {0.0f, 1.0f};
 
-  mul_v3_m4v3(loc, ob->world_to_object, center);
+  mul_v3_m4v3(loc, ob->object_to_world, center);
   const float zfac = ED_view3d_calc_zfac(vc->rv3d, loc);
 
-  ED_view3d_win_to_delta(vc->region, xy_delta, zfac, y);
-  normalize_v3(y);
+  ED_view3d_win_to_delta(vc->region, screen_dir, zfac, r_local_dir);
+  normalize_v3(r_local_dir);
 
-  add_v3_v3(y, ob->loc);
-  mul_m4_v3(ob->world_to_object, y);
+  add_v3_v3(r_local_dir, ob->loc);
+  mul_m4_v3(ob->world_to_object, r_local_dir);
 }
 
 static void calc_brush_local_mat(const float rotation,
@@ -2734,7 +2736,6 @@ static void calc_brush_local_mat(const float rotation,
   float mat[4][4];
   float scale[4][4];
   float angle, v[3];
-  float up[3];
 
   /* Ensure `ob->world_to_object` is up to date. */
   invert_m4_m4(ob->world_to_object, ob->object_to_world);
@@ -2745,17 +2746,33 @@ static void calc_brush_local_mat(const float rotation,
   mat[2][3] = 0.0f;
   mat[3][3] = 1.0f;
 
-  /* Get view's up vector in object-space. */
-  calc_local_y(cache->vc, cache->location, up);
+  /* Read rotation (user angle, rake, etc.) to find the view's movement direction (negative X of
+   * the brush). */
+  angle = rotation + cache->special_rotation;
+  /* By convention, motion direction points down the brush's Y axis, the angle represents the X
+   * axis, normal is a 90 deg ccw rotation of the motion direction. */
+  float motion_normal_screen[2];
+  motion_normal_screen[0] = cosf(angle);
+  motion_normal_screen[1] = sinf(angle);
+  /* Convert view's brush transverse direction to object-space,
+   * i.e. the normal of the plane described by the motion */
+  float motion_normal_local[3];
+  calc_local_from_screen(cache->vc, cache->location, motion_normal_screen, motion_normal_local);
 
-  /* Calculate the X axis of the local matrix. */
-  cross_v3_v3v3(v, up, cache->sculpt_normal);
-  /* Apply rotation (user angle, rake, etc.) to X axis. */
-  angle = rotation - cache->special_rotation;
-  rotate_v3_v3v3fl(mat[0], v, cache->sculpt_normal, angle);
+  /* Calculate the movement direction for the local matrix.
+   * Note that there is a deliberate prioritization here: Our calculations are
+   * designed such that the _motion vector_ gets projected into the tangent space;
+   * in most cases this will be more intuitive than projecting the transverse
+   * direction (which is orthogonal to the motion direction and therefore less
+   * apparent to the user).
+   * The Y-axis of the brush-local frame has to lie in the intersection of the tangent plane
+   * and the motion plane. */
+
+  cross_v3_v3v3(v, cache->sculpt_normal, motion_normal_local);
+  normalize_v3_v3(mat[1], v);
 
   /* Get other axes. */
-  cross_v3_v3v3(mat[1], cache->sculpt_normal, mat[0]);
+  cross_v3_v3v3(mat[0], mat[1], cache->sculpt_normal);
   copy_v3_v3(mat[2], cache->sculpt_normal);
 
   /* Set location. */

source/blender/makesdna/DNA_scene_types.h

@@ -1351,6 +1351,9 @@ typedef struct UnifiedPaintSettings {
   float average_stroke_accum[3];
   int average_stroke_counter;
 
+  /* How much brush should be rotated in the view plane, 0 means x points right, y points up.
+   * The convention is that the brush's _negative_ Y axis points in the tangent direction (of the
+   * mouse curve, Bezier curve, etc.) */
   float brush_rotation;
   float brush_rotation_sec;