Sculpt: Preview the active side of the line gestures

This adds a small gradient to the right side of the line to preview
which side of the mesh is going to be affected by the gesture operation.

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D9106

source/blender/editors/sculpt_paint/paint_mask.c

@@ -1419,7 +1419,7 @@ void PAINT_OT_mask_line_gesture(wmOperatorType *ot)
   ot->idname = "PAINT_OT_mask_line_gesture";
   ot->description = "Add mask to the right of a line as you move the brush";
 
-  ot->invoke = WM_gesture_straightline_invoke;
+  ot->invoke = WM_gesture_straightline_active_side_invoke;
   ot->modal = WM_gesture_straightline_oneshot_modal;
   ot->exec = paint_mask_gesture_line_exec;
 
@@ -1516,7 +1516,7 @@ void SCULPT_OT_project_line_gesture(wmOperatorType *ot)
   ot->idname = "SCULPT_OT_project_line_gesture";
   ot->description = "Project the geometry onto a plane defined by a line";
 
-  ot->invoke = WM_gesture_straightline_invoke;
+  ot->invoke = WM_gesture_straightline_active_side_invoke;
   ot->modal = WM_gesture_straightline_oneshot_modal;
   ot->exec = project_gesture_line_exec;
 

source/blender/windowmanager/WM_api.h

@@ -636,6 +636,9 @@ const int (*WM_gesture_lasso_path_to_array(struct bContext *C,
 int WM_gesture_straightline_invoke(struct bContext *C,
                                    struct wmOperator *op,
                                    const struct wmEvent *event);
+int WM_gesture_straightline_active_side_invoke(struct bContext *C,
+                                               struct wmOperator *op,
+                                               const struct wmEvent *event);
 int WM_gesture_straightline_modal(struct bContext *C,
                                   struct wmOperator *op,
                                   const struct wmEvent *event);

source/blender/windowmanager/WM_types.h

@@ -485,6 +485,8 @@ typedef struct wmGesture {
   /** optional, maximum amount of points stored. */
   int points_alloc;
   int modal_state;
+  /** optional, draw the active side of the straightline gesture. */
+  bool draw_active_side;
 
   /**
    * For modal operators which may be running idle, waiting for an event to activate the gesture.

source/blender/windowmanager/intern/wm_gesture.c

@@ -199,10 +199,61 @@ int wm_gesture_evaluate(wmGesture *gesture, const wmEvent *event)
 
 /* ******************* gesture draw ******************* */
 
+static void wm_gesture_draw_line_active_side(rcti *rect)
+{
+  GPUVertFormat *format = immVertexFormat();
+  uint shdr_pos = GPU_vertformat_attr_add(format, "pos", GPU_COMP_F32, 2, GPU_FETCH_FLOAT);
+  uint shdr_col = GPU_vertformat_attr_add(format, "color", GPU_COMP_F32, 4, GPU_FETCH_FLOAT);
+
+  GPU_blend(GPU_BLEND_ALPHA);
+  immBindBuiltinProgram(GPU_SHADER_2D_SMOOTH_COLOR);
+
+  const float gradient_length = 150.0f * U.pixelsize;
+  float line_dir[2];
+  float gradient_dir[2];
+  float gradient_point[2][2];
+
+  const float line_start[2] = {rect->xmin, rect->ymin};
+  const float line_end[2] = {rect->xmax, rect->ymax};
+  const float color_line_gradient_start[4] = {0.2f, 0.2f, 0.2f, 0.4f};
+  const float color_line_gradient_end[4] = {0.0f, 0.0f, 0.0f, 0.0f};
+
+  sub_v2_v2v2(line_dir, line_end, line_start);
+  normalize_v2(line_dir);
+  ortho_v2_v2(gradient_dir, line_dir);
+  mul_v2_fl(gradient_dir, -1.0f);
+  mul_v2_fl(gradient_dir, gradient_length);
+  add_v2_v2v2(gradient_point[0], line_start, gradient_dir);
+  add_v2_v2v2(gradient_point[1], line_end, gradient_dir);
+
+  immBegin(GPU_PRIM_TRIS, 6);
+  immAttr4f(shdr_col, UNPACK4(color_line_gradient_start));
+  immVertex2f(shdr_pos, line_start[0], line_start[1]);
+  immAttr4f(shdr_col, UNPACK4(color_line_gradient_start));
+  immVertex2f(shdr_pos, line_end[0], line_end[1]);
+  immAttr4f(shdr_col, UNPACK4(color_line_gradient_end));
+  immVertex2f(shdr_pos, gradient_point[1][0], gradient_point[1][1]);
+
+  immAttr4f(shdr_col, UNPACK4(color_line_gradient_start));
+  immVertex2f(shdr_pos, line_start[0], line_start[1]);
+  immAttr4f(shdr_col, UNPACK4(color_line_gradient_end));
+  immVertex2f(shdr_pos, gradient_point[1][0], gradient_point[1][1]);
+  immAttr4f(shdr_col, UNPACK4(color_line_gradient_end));
+  immVertex2f(shdr_pos, gradient_point[0][0], gradient_point[0][1]);
+  immEnd();
+
+  immUnbindProgram();
+  GPU_blend(GPU_BLEND_NONE);
+}
+
 static void wm_gesture_draw_line(wmGesture *gt)
 {
   rcti *rect = (rcti *)gt->customdata;
 
+  if (gt->draw_active_side) {
+    wm_gesture_draw_line_active_side(rect);
+  }
+
   uint shdr_pos = GPU_vertformat_attr_add(
       immVertexFormat(), "pos", GPU_COMP_F32, 2, GPU_FETCH_FLOAT);
 

source/blender/windowmanager/intern/wm_gesture_ops.c

@@ -849,6 +849,17 @@ int WM_gesture_straightline_invoke(bContext *C, wmOperator *op, const wmEvent *e
 
   return OPERATOR_RUNNING_MODAL;
 }
+/**
+ * This invoke callback starts the straightline gesture with a viewport preview to the right side
+ * of the line.
+ */
+int WM_gesture_straightline_active_side_invoke(bContext *C, wmOperator *op, const wmEvent *event)
+{
+  WM_gesture_straightline_invoke(C, op, event);
+  wmGesture *gesture = op->customdata;
+  gesture->draw_active_side = true;
+  return OPERATOR_RUNNING_MODAL;
+}
 
 /**
  * This modal callback calls exec once per mouse move event while the gesture is active with the