Pcbnew: fix some issues with very small arcs (size a few internal units)

these very small arcs do not allow to calculate a reliable center position,
and therefore a reliable radius.
So seeing them as a very small segment fix plotting/drawing issues.
Fixes #15639
https://gitlab.com/kicad/code/kicad/-/issues/15639

3d-viewer/3d_canvas/create_3Dgraphic_brd_items.cpp

@@ -286,6 +286,19 @@ void BOARD_ADAPTER::createTrack( const PCB_TRACK* aTrack, CONTAINER_2D_BASE* aDs
     {
         const PCB_ARC* arc = static_cast<const PCB_ARC*>( aTrack );
 
+        if( arc->IsDegenerated() )
+        {
+            // Draw this very small arc like a track segment (a PCB_TRACE_T)
+            PCB_TRACK track( arc->GetParent() );
+            track.SetStart( arc->GetStart() );
+            track.SetEnd( arc->GetEnd() );
+            track.SetWidth( arc->GetWidth() );
+            track.SetLayer( arc->GetLayer() );
+
+            createTrack( &track, aDstContainer );
+            return;
+        }
+
         VECTOR2D  center( arc->GetCenter() );
         EDA_ANGLE arc_angle = arc->GetAngle();
         double    radius = arc->GetRadius();

pcbnew/pcb_track.cpp

@@ -1171,6 +1171,16 @@ EDA_ANGLE PCB_ARC::GetArcAngleEnd() const
     return angleEnd.Normalize();
 }
 
+bool PCB_ARC::IsDegenerated( int aThreshold ) const
+{
+    // Too small arcs cannot be really handled: arc center (and arc radius)
+    // cannot be safely computed if the distance between mid and end points
+    // is too small (a few internal units)
+
+    return ( GetMid() - GetStart() ).EuclideanNorm() < aThreshold
+            || ( GetMid() - GetEnd() ).EuclideanNorm() < aThreshold;
+}
+
 
 bool PCB_TRACK::cmp_tracks::operator() ( const PCB_TRACK* a, const PCB_TRACK* b ) const
 {

pcbnew/pcb_track.h

@@ -352,6 +352,14 @@ public:
 
     EDA_ITEM* Clone() const override;
 
+    /**
+     * @return true if the arc is too small to allow a safe calculation
+     * of center position and arc angles i.e if distance between m_Mid and each arc end
+     * is only a few internal units.
+     * @param aThreshold is the minimal dist in internal units. Default id 5 IU
+     */
+    bool IsDegenerated( int aThreshold = 5 ) const;
+
 protected:
     virtual void swapData( BOARD_ITEM* aImage ) override;
 

pcbnew/plot_board_layers.cpp

@@ -645,8 +645,19 @@ void PlotStandardLayer( BOARD* aBoard, PLOTTER* aPlotter, LSET aLayerMask,
         {
             const PCB_ARC* arc = static_cast<const PCB_ARC*>( track );
 
-            aPlotter->ThickArc( arc->GetCenter(), arc->GetArcAngleStart(), arc->GetAngle(),
-                                arc->GetRadius(), width, plotMode, &gbr_metadata );
+            // Too small arcs cannot be really handled: arc center (and arc radius)
+            // cannot be safely computed
+            if( !arc->IsDegenerated( 10 /* in IU */ ) )
+            {
+                aPlotter->ThickArc( arc->GetCenter(), arc->GetArcAngleStart(), arc->GetAngle(),
+                                    arc->GetRadius(), width, plotMode, &gbr_metadata );
+            }
+            else
+            {
+                // Approximate this very small arc by a segment.
+                aPlotter->ThickSegment( track->GetStart(), track->GetEnd(), width, plotMode,
+                                        &gbr_metadata );
+            }
         }
         else
         {