Mate connector: a roll mark that survives a grazing view, and a quieter warning

Two open findings from the first rig judgement of the connector glyph.

F4 — the quadrant collapses to a blob at grazing angles, which is exactly when
the roll is hardest to read. Adds a radial tick along +X extending past the disc
rim. As the disc flattens to a line the sector loses all its area, but a radial
spoke keeps its length and its direction along the one axis that still projects.

The alternative on the issue was to billboard the quadrant while the disc stayed
in-plane. Rejected, and not on taste: at true grazing the view direction lies IN
the connector's plane, so every in-plane direction projects onto the same screen
line and the roll is geometrically unrecoverable. Billboarding would not recover
it — it would face the camera and read as a definite orientation that is not the
frame's. Degrading to a direction that can still be trusted beats drawing a
confident lie. The tick is additive, so unlike billboarding it cannot make the
non-grazing case worse; it still wants judging on the rig at a true grazing view
before F4 is called closed.

F5 — roll-undefined was a loud red: the strongest colour in the viewport spent on
the least important connector, pulling the eye off the mate being made. It marks
"this one could not be derived", not an error. Muted amber says look-here without
shouting.

No tick is drawn when the roll is undefined — a tick there would assert a
direction that does not exist, which is the silent guess the hatched quadrant
exists to avoid.

Reviewed and compiled (libslic3r_gui, RC=0); not exercised. snaporca-wgsc.

src/slic3r/GUI/DesignSketchTool.cpp

@@ -3365,7 +3365,10 @@ void DesignSketchTool::render_mate_connectors()
     const ColorRGBA gold (0.93f, 0.66f, 0.09f, 1.0f);
     const ColorRGBA blue (0.18f, 0.44f, 0.93f, 1.0f);
     const ColorRGBA grey (0.42f, 0.46f, 0.52f, 1.0f);
-    const ColorRGBA warn (0.76f, 0.26f, 0.05f, 1.0f);
+    // F5: roll-undefined was a loud red — the strongest colour in the viewport spent on the LEAST
+    // important connector, which pulled the eye away from the mate being made. It is a "this one
+    // could not be derived" mark, not an error: muted amber says look-here without shouting.
+    const ColorRGBA warn (0.72f, 0.55f, 0.22f, 1.0f);
 
     const SketchPlane saved = m_plane;
 
@@ -3414,11 +3417,26 @@ void DesignSketchTool::render_mate_connectors()
             }
             q.emplace_back(Vec2d(0, 0), Vec2d(qr, 0));
             q.emplace_back(Vec2d(0, 0), Vec2d(0, qr));
-            if (!g.roll_undefined)
+            if (!g.roll_undefined) {
                 for (int i = 1; i < 5; ++i) {          // fan lines read as "filled" at any angle
                     const double a = (M_PI_2 * i) / 5.0;
                     q.emplace_back(Vec2d(0, 0), Vec2d(qr * std::cos(a), qr * std::sin(a)));
                 }
+                // F4: the quadrant collapses to a blob at grazing angles — exactly when the roll
+                // is hardest to read. A radial tick along +X, extending PAST the disc rim, is what
+                // survives that: as the disc flattens to a line the sector loses all area, but a
+                // radial spoke keeps its length and its direction along the one axis that still
+                // projects.
+                //
+                // The alternative on the issue was billboarding the quadrant. Rejected, and not
+                // on taste: at true grazing the view direction lies IN the connector's plane, so
+                // every in-plane direction projects onto the same screen line and the roll is
+                // geometrically unrecoverable. A billboarded quadrant would not recover it — it
+                // would face the camera and read as a definite orientation that is not the frame's.
+                // Better to degrade to a direction you can still trust than to draw a confident
+                // lie. Judge the tick on the rig at a true grazing view before calling F4 closed.
+                q.emplace_back(Vec2d(R, 0), Vec2d(R * 1.28, 0));
+            }
             draw_strokes(m_mc_stroke_model, q, lw, g.roll_undefined ? warn : gold);
         }