OPENGL_GAL::DrawArcSegment(): use a better number of segm to approximate the arc.

Previously, the count of segments used a magic number optimized for Pcbnew.
This is not good, and does not work well on Gerbview.
The count uses now a max error acceptable is approximation (5 microns in Gerbview and Pcbnew).
Fixes #9101
https://gitlab.com/kicad/code/kicad/issues/9101

common/gal/cairo/cairo_gal.cpp

@@ -342,8 +342,10 @@ void CAIRO_GAL_BASE::DrawArc( const VECTOR2D& aCenterPoint, double aRadius, doub
 
 
 void CAIRO_GAL_BASE::DrawArcSegment( const VECTOR2D& aCenterPoint, double aRadius,
-                                     double aStartAngle, double aEndAngle, double aWidth )
+                                     double aStartAngle, double aEndAngle, double aWidth,
+                                     double aMaxError )
 {
+    // Note: aMaxError is not used because Cairo can draw true arcs
     if( m_isFillEnabled )
     {
         m_lineWidth = aWidth;

common/gal/opengl/opengl_gal.cpp

@@ -48,6 +48,7 @@
 #include <wx/frame.h>
 
 #include <macros.h>
+#include <geometry/geometry_utils.h>
 
 #ifdef KICAD_GAL_PROFILE
 #include <profile.h>
@@ -850,8 +851,9 @@ void OPENGL_GAL::DrawArc( const VECTOR2D& aCenterPoint, double aRadius, double a
 }
 
 
-void OPENGL_GAL::DrawArcSegment( const VECTOR2D& aCenterPoint, double aRadius, double aStartAngle,
+void OPENGL_GAL::DrawArcSegment( const VECTOR2D& aCenterPoint, double aRadius,
-                                 double aEndAngle, double aWidth )
+                                 double aStartAngle, double aEndAngle,
+                                 double aWidth, double aMaxError )
 {
     if( aRadius <= 0 )
     {
@@ -865,7 +867,10 @@ void OPENGL_GAL::DrawArcSegment( const VECTOR2D& aCenterPoint, double aRadius, d
     // Swap the angles, if start angle is greater than end angle
     SWAP( aStartAngle, >, aEndAngle );
 
-    double alphaIncrement = calcAngleStep( aRadius );
+    // Calculate the seg count to approximate the arc with aMaxError or less
+    int segCount360 = GetArcToSegmentCount( aRadius, aMaxError, 360.0 );
+    segCount360 = std::max( CIRCLE_POINTS, segCount360 );
+    double alphaIncrement = 2.0 * M_PI / segCount360;
 
     // Refinement: Use a segment count multiple of 2, because we have a control point
     // on the middle of the arc, and the look is better if it is on a segment junction

gerbview/gerbview_painter.cpp

@@ -351,7 +351,7 @@ void GERBVIEW_PAINTER::draw( /*const*/ GERBER_DRAW_ITEM* aItem, int aLayer )
             endAngle =  startAngle + 2*M_PI;
         }
 
-        m_gal->DrawArcSegment( center, radius, startAngle, endAngle, width );
+        m_gal->DrawArcSegment( center, radius, startAngle, endAngle, width, ARC_HIGH_DEF );
 
 #if 0   // Arc Debugging only
         m_gal->SetIsFill( false );

include/gal/cairo/cairo_gal.h

@@ -82,8 +82,10 @@ public:
                           double aStartAngle, double aEndAngle ) override;
 
     /// @copydoc GAL::DrawArcSegment()
+    /// Note: aMaxError is not used in Cairo, because Cairo can draw true arcs
     void DrawArcSegment( const VECTOR2D& aCenterPoint, double aRadius,
-                                 double aStartAngle, double aEndAngle, double aWidth ) override;
+                         double aStartAngle, double aEndAngle, double aWidth,
+                         double aMaxError ) override;
 
     /// @copydoc GAL::DrawRectangle()
     void DrawRectangle( const VECTOR2D& aStartPoint, const VECTOR2D& aEndPoint ) override;

include/gal/graphics_abstraction_layer.h

@@ -151,9 +151,11 @@ public:
      * @param aStartAngle   is the start angle of the arc.
      * @param aEndAngle     is the end angle of the arc.
      * @param aWidth        is the thickness of the arc (pen size).
+     * @param aMaxError     is the max allowed error to create segments to approximate a circle.
+     *  It has meaning only for back ends that can't draw a true arc, and use segments to approximate.
      */
     virtual void DrawArcSegment( const VECTOR2D& aCenterPoint, double aRadius, double aStartAngle,
-                                 double aEndAngle, double aWidth ) {};
+                                 double aEndAngle, double aWidth, double aMaxError ) {};
 
     /**
      * Draw a rectangle.

include/gal/opengl/opengl_gal.h

@@ -128,7 +128,8 @@ public:
 
     /// @copydoc GAL::DrawArcSegment()
     void DrawArcSegment( const VECTOR2D& aCenterPoint, double aRadius,
-                                 double aStartAngle, double aEndAngle, double aWidth ) override;
+                         double aStartAngle, double aEndAngle, double aWidth,
+                         double aMaxError ) override;
 
     /// @copydoc GAL::DrawRectangle()
     void DrawRectangle( const VECTOR2D& aStartPoint, const VECTOR2D& aEndPoint ) override;

pcbnew/pcb_painter.cpp

@@ -683,7 +683,7 @@ void PCB_PAINTER::draw( const PCB_ARC* aArc, int aLayer )
         m_gal->SetIsFill( not outline_mode );
         m_gal->SetLineWidth( m_pcbSettings.m_outlineWidth );
 
-        m_gal->DrawArcSegment( center, radius, start_angle, start_angle + angle, width );
+        m_gal->DrawArcSegment( center, radius, start_angle, start_angle + angle, width, ARC_HIGH_DEF );
     }
 
     // Clearance lines
@@ -700,7 +700,7 @@ void PCB_PAINTER::draw( const PCB_ARC* aArc, int aLayer )
         m_gal->SetStrokeColor( color );
 
         m_gal->DrawArcSegment( center, radius, start_angle, start_angle + angle,
-                               width + clearance * 2 );
+                               width + clearance * 2, ARC_HIGH_DEF );
     }
 
 // Debug only: enable this code only to test the TransformArcToPolygon function
@@ -1081,7 +1081,7 @@ void PCB_PAINTER::draw( const PAD* aPad, int aLayer )
         std::unique_ptr<PAD>            dummyPad;
         std::shared_ptr<SHAPE_COMPOUND> shapes;
 
-        // Drawing components of compound shapes in outline mode produces a mess. 
+        // Drawing components of compound shapes in outline mode produces a mess.
         bool simpleShapes = !m_pcbSettings.m_sketchMode[LAYER_PADS_TH];
 
         if( simpleShapes )
@@ -1416,7 +1416,7 @@ void PCB_PAINTER::draw( const PCB_SHAPE* aShape, int aLayer )
             m_gal->DrawArcSegment( start, aShape->GetRadius(),
                     DECIDEG2RAD( aShape->GetArcAngleStart() ),
                     DECIDEG2RAD( aShape->GetArcAngleStart() + aShape->GetAngle() ), // Change this
-                    thickness );
+                    thickness, ARC_HIGH_DEF );
         }
         else
         {
@@ -1426,7 +1426,7 @@ void PCB_PAINTER::draw( const PCB_SHAPE* aShape, int aLayer )
             m_gal->DrawArcSegment( start, aShape->GetRadius(),
                     DECIDEG2RAD( aShape->GetArcAngleStart() ),
                     DECIDEG2RAD( aShape->GetArcAngleStart() + aShape->GetAngle() ), // Change this
-                    thickness );
+                    thickness, ARC_HIGH_DEF );
         }
         break;