Plotters (especially DXF) add more precision when plotting arcs (and others)

- in DXF coordinates were using 6 digits for coordinate mantissa: this is not
enough for coord in inches. Now use 16 digits
- Arc( VECTOR2I& aCentre, EDA_ANGLE& aStartAngle, EDA_ANGLE& aEndAngle, ...)
was using integers for coord. This creates significant errors for start point
and end points of the arc. Now the center is given in double, and its position
is calculated from angle end points (and radius) to do not generate a position error
for these end points (previously the error could be 20 ... 50 nm)
Fixes #15056
https://gitlab.com/kicad/code/kicad/-/issues/15056

common/plotters/plotter.cpp

@@ -145,13 +145,31 @@ double PLOTTER::GetDashGapLenIU( int aLineWidth ) const
     return userToDeviceSize( m_renderSettings->GetGapLength( aLineWidth ) );
 }
 
-
+#include <wx/log.h>
 void PLOTTER::Arc( const VECTOR2I& aCenter, const VECTOR2I& aStart, const VECTOR2I& aEnd,
                    FILL_T aFill, int aWidth, int aMaxError )
 {
-    EDA_ANGLE startAngle( aStart - aCenter );
-    EDA_ANGLE endAngle( aEnd - aCenter );
-    int       radius = ( aStart - aCenter ).EuclideanNorm();
+    // Recalculate aCenter using double to be sure we will use a exact value, from aStart and aEnd
+    // it must be on the line passing by the middle of segment {aStart, aEnd}
+    // To simplify calculations, use aStart as origin in intermediate calculations
+    VECTOR2D center = aCenter - aStart;
+    VECTOR2D end = aEnd - aStart;
+    EDA_ANGLE segAngle( end );
+
+    // Rotate end and center, to make segment {aStart, aEnd} horizontal
+    RotatePoint( end, segAngle );
+    RotatePoint( center, segAngle );
+
+    // center.x must be at end.x/2 coordinate
+    center.x = end.x / 2;
+
+    // Now calculate the right center position
+    RotatePoint( center, -segAngle );
+    center += aStart;
+
+    EDA_ANGLE startAngle( VECTOR2D( aStart ) - center );
+    EDA_ANGLE endAngle( VECTOR2D( aEnd ) - center );
+    double    radius = ( VECTOR2D( aStart ) - center ).EuclideanNorm();
 
     if( startAngle > endAngle )
     {
@@ -168,12 +186,12 @@ void PLOTTER::Arc( const VECTOR2I& aCenter, const VECTOR2I& aStart, const VECTOR
     startAngle = -startAngle;
     endAngle = -endAngle;
 
-    Arc( aCenter, startAngle, endAngle, radius, aFill, aWidth );
+    Arc( center, startAngle, endAngle, radius, aFill, aWidth );
 }
 
 
-void PLOTTER::Arc( const VECTOR2I& aCenter, const EDA_ANGLE& aStartAngle,
-                   const EDA_ANGLE& aEndAngle, int aRadius, FILL_T aFill, int aWidth )
+void PLOTTER::Arc( const VECTOR2D& aCenter, const EDA_ANGLE& aStartAngle,
+                   const EDA_ANGLE& aEndAngle, double aRadius, FILL_T aFill, int aWidth )
 {
     EDA_ANGLE       startAngle( aStartAngle );
     EDA_ANGLE       endAngle( aEndAngle );
@@ -570,8 +588,8 @@ void PLOTTER::ThickSegment( const VECTOR2I& start, const VECTOR2I& end, int widt
 }
 
 
-void PLOTTER::ThickArc( const VECTOR2I& centre, const EDA_ANGLE& aStartAngle,
-                        const EDA_ANGLE& aEndAngle, int aRadius, int aWidth,
+void PLOTTER::ThickArc( const VECTOR2D& centre, const EDA_ANGLE& aStartAngle,
+                        const EDA_ANGLE& aEndAngle, double aRadius, int aWidth,
                         OUTLINE_MODE aTraceMode, void* aData )
 {
     if( aTraceMode == FILLED )