Fix T84183: Dark area in the bevel custom profile widget

If there was a control point at an extreme position when drawing a curve
profile (in the bottom corner), the fill's trianglulation could fail, giving
a misleading view of the curve. This is because the extra points added to
create a closed shape were exactly on the border of the view.

This commit adds a small margin to those points, so the triangulation
doesn't fail because the line overlaps itself.

Another possible solution is to use a different algorithm to fill
the polygon, such as scanfill, which is used by curve objects.
This seemed simpler, and seems to work fairly robustly.

Differential Revision: https://developer.blender.org/D9989

1 files changed, 15 insertions, 13 deletions

diff --git a/source/blender/editors/interface/interface_draw.c b/source/blender/editors/interface/interface_draw.c
index d9571dc98bd..5bb6b0f21e7 100644
--- a/source/blender/editors/interface/interface_draw.c
+++ b/source/blender/editors/interface/interface_draw.c
@@ -2245,33 +2245,36 @@ void ui_draw_but_CURVEPROFILE(ARegion *region,
     table_coords[i][0] = pts[i].x;
     table_coords[i][1] = pts[i].y;
   }
+  /* Using some extra margin (-1.0f) for the coordinates used to complete the polygon
+   * avoids the profile line crossing itself in some common situations, which can lead to
+   * incorrect triangulation. See T841183. */
   if (add_left_tri && add_bottom_tri) {
     /* Add left side, bottom left corner, and bottom side points. */
-    table_coords[tot_points - 3][0] = profile->view_rect.xmin;
+    table_coords[tot_points - 3][0] = profile->view_rect.xmin - 1.0f;
     table_coords[tot_points - 3][1] = 1.0f;
-    table_coords[tot_points - 2][0] = profile->view_rect.xmin;
-    table_coords[tot_points - 2][1] = profile->view_rect.ymin;
+    table_coords[tot_points - 2][0] = profile->view_rect.xmin - 1.0f;
+    table_coords[tot_points - 2][1] = profile->view_rect.ymin - 1.0f;
     table_coords[tot_points - 1][0] = 1.0f;
-    table_coords[tot_points - 1][1] = profile->view_rect.ymin;
+    table_coords[tot_points - 1][1] = profile->view_rect.ymin - 1.0f;
   }
   else if (add_left_tri) {
     /* Add the left side and bottom left corner points. */
-    table_coords[tot_points - 2][0] = profile->view_rect.xmin;
+    table_coords[tot_points - 2][0] = profile->view_rect.xmin - 1.0f;
     table_coords[tot_points - 2][1] = 1.0f;
-    table_coords[tot_points - 1][0] = profile->view_rect.xmin;
-    table_coords[tot_points - 1][1] = 0.0f;
+    table_coords[tot_points - 1][0] = profile->view_rect.xmin - 1.0f;
+    table_coords[tot_points - 1][1] = -1.0f;
   }
   else if (add_bottom_tri) {
     /* Add the bottom side and bottom left corner points. */
-    table_coords[tot_points - 2][0] = 0.0f;
-    table_coords[tot_points - 2][1] = profile->view_rect.ymin;
+    table_coords[tot_points - 2][0] = -1.0f;
+    table_coords[tot_points - 2][1] = profile->view_rect.ymin - 1.0f;
     table_coords[tot_points - 1][0] = 1.0f;
-    table_coords[tot_points - 1][1] = profile->view_rect.ymin;
+    table_coords[tot_points - 1][1] = profile->view_rect.ymin - 1.0f;
   }
   else {
     /* Just add the bottom corner point. Side points would be redundant anyway. */
-    table_coords[tot_points - 1][0] = 0.0f;
-    table_coords[tot_points - 1][1] = 0.0f;
+    table_coords[tot_points - 1][0] = -1.0f;
+    table_coords[tot_points - 1][1] = -1.0f;
   }
 
   /* Calculate the table point indices of the triangles for the profile's fill. */
@@ -2427,7 +2430,6 @@ void ui_draw_but_CURVEPROFILE(ARegion *region,
 
   immUniformColor3ubv((const uchar *)wcol->outline);
   imm_draw_box_wire_2d(pos, rect->xmin, rect->ymin, rect->xmax, rect->ymax);
-
   immUnbindProgram();
 }