GPv3: Draw Tool: Random Rotation

This was previously named `UV` in the UI.
Now renamed to the more appropriate `Rotation` since
it controls the random rotation of the texture on
each stroke point.

Otherwise implements the same functionality as GPv2.

Pull Request: https://projects.blender.org/blender/blender/pulls/123741

scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -2805,7 +2805,7 @@ class VIEW3D_PT_tools_grease_pencil_v3_brush_random(View3DPanel, Panel):
             col.template_curve_mapping(gp_settings, "curve_random_strength", brush=True, use_negative_slope=True)
 
         row = col.row(align=True)
-        row.prop(gp_settings, "uv_random", text="UV", slider=True)
+        row.prop(gp_settings, "uv_random", text="Rotation", slider=True)
         row.prop(gp_settings, "use_stroke_random_uv", text="", icon='GP_SELECT_STROKES')
         row.prop(gp_settings, "use_random_press_uv", text="", icon='STYLUS_PRESSURE')
         if gp_settings.use_random_press_uv and self.is_popover is False:

source/blender/editors/sculpt_paint/grease_pencil_paint.cc

@@ -253,6 +253,7 @@ class PaintOperation : public GreasePencilStrokeOperation {
 
   float stroke_random_radius_factor_;
   float stroke_random_opacity_factor_;
+  float stroke_random_rotation_factor_;
 
   friend struct PaintOperationExecutor;
 
@@ -368,6 +369,29 @@ struct PaintOperationExecutor {
     return math::interpolate(opacity, opacity * random_factor, settings_->draw_random_strength);
   }
 
+  float randomize_rotation(PaintOperation &self, const float pressure)
+  {
+    if (!use_settings_random_ || !(settings_->uv_random > 0.0f)) {
+      return 0.0f;
+    }
+    /* Random value in range [-1 .. 1]. */
+    float random_factor = [&]() {
+      if ((settings_->flag2 & GP_BRUSH_USE_UV_AT_STROKE) == 0) {
+        return self.rng_.get_float();
+      }
+      else {
+        return self.stroke_random_rotation_factor_;
+      }
+    }();
+
+    if ((settings_->flag2 & GP_BRUSH_USE_UV_RAND_PRESS) != 0) {
+      random_factor *= BKE_curvemapping_evaluateF(settings_->curve_rand_uv, 0, pressure);
+    }
+
+    const float random_rotation = (random_factor * 2.0f - 1.0f) * float(M_PI);
+    return math::interpolate(0.0f, random_rotation, settings_->uv_random);
+  }
+
   void process_start_sample(PaintOperation &self,
                             const bContext &C,
                             const InputSample &start_sample,
@@ -393,6 +417,8 @@ struct PaintOperationExecutor {
         start_sample.pressure, brush_, settings_);
     start_opacity = randomize_opacity(self, 0.0f, start_opacity, start_sample.pressure);
 
+    const float start_rotation = randomize_rotation(self, start_sample.pressure);
+
     Scene *scene = CTX_data_scene(&C);
     const bool on_back = (scene->toolsettings->gpencil_flags & GP_TOOL_FLAG_PAINT_ONBACK) != 0;
 
@@ -441,6 +467,14 @@ struct PaintOperationExecutor {
     materials.finish();
     softness.finish();
 
+    if (settings_->uv_random > 0.0f || attributes.contains("rotation")) {
+      bke::SpanAttributeWriter<float> rotations = attributes.lookup_or_add_for_write_span<float>(
+          "rotation", bke::AttrDomain::Point);
+      rotations.span[last_active_point] = start_rotation;
+      point_attributes_to_skip.add("rotation");
+      rotations.finish();
+    }
+
     /* Only set the attribute if the type is not the default or if it already exists. */
     if (settings_->caps_type != GP_STROKE_CAP_TYPE_ROUND || attributes.contains("start_cap")) {
       bke::SpanAttributeWriter<int8_t> start_caps =
@@ -728,6 +762,18 @@ struct PaintOperationExecutor {
       linear_interpolation<float>(prev_opacity, opacity, new_opacities, is_first_sample);
     }
 
+    /* Randomize rotations. */
+    if (use_settings_random_ && (settings_->uv_random > 0.0f || attributes.contains("rotation"))) {
+      bke::SpanAttributeWriter<float> rotations = attributes.lookup_or_add_for_write_span<float>(
+          "rotation", bke::AttrDomain::Point);
+      const MutableSpan<float> new_rotations = rotations.span.slice(new_points);
+      for (const int i : IndexRange(new_points_num)) {
+        new_rotations[i] = randomize_rotation(self, extension_sample.pressure);
+      }
+      point_attributes_to_skip.add("rotation");
+      rotations.finish();
+    }
+
     self.accum_distance_ += distance_px;
 
     /* Update screen space buffers with new points. */
@@ -839,6 +885,7 @@ void PaintOperation::on_stroke_begin(const bContext &C, const InputSample &start
   if ((settings->flag & GP_BRUSH_GROUP_RANDOM) != 0) {
     stroke_random_radius_factor_ = rng_.get_float();
     stroke_random_opacity_factor_ = rng_.get_float();
+    stroke_random_rotation_factor_ = rng_.get_float();
   }
 
   Material *material = BKE_grease_pencil_object_material_ensure_from_active_input_brush(