Thick dashed lines are not dashed with GTK3 (Issue #26449)

[taler21]

taler21 created an issue (wxWidgets/wxWidgets#26449)

With a wxPen width of 10 or more, wxDC::DrawLine() draws dashed lines not dashed, but rather solid.

This issue can be seen in the drawing sample, even more with thicker line widths, for example by using the following patch:

Patch for drawing sample

diff -r -u -p a/samples/drawing/drawing.cpp b/samples/drawing/drawing.cpp
--- a/samples/drawing/drawing.cpp	2026-05-12 10:22:24.793184400 +0200
+++ b/samples/drawing/drawing.cpp	2026-05-12 08:33:40.209233300 +0200
@@ -780,23 +780,24 @@ void MyCanvas::DrawTestPoly(wxDC& dc)
 
 void MyCanvas::DrawTestLines( int x, int y, int width, wxDC &dc )
 {
+    const int vStep = 5 * (2 + (width / 6));
     dc.SetPen( wxPen( *wxBLACK, width ) );
     dc.SetBrush( *wxWHITE_BRUSH );
     dc.DrawText(wxString::Format("Testing lines of width %d", width), dc.FromDIP(x + 10), dc.FromDIP(y - 10));
-    dc.DrawRectangle(dc.FromDIP(x + 10), dc.FromDIP(y + 10), dc.FromDIP(100), dc.FromDIP(190));
+    dc.DrawRectangle(dc.FromDIP(x + 10), dc.FromDIP(y + 1*vStep), dc.FromDIP(200), dc.FromDIP(6*vStep));
 
-    dc.DrawText("Solid/dot/short dash/long dash/dot dash", dc.FromDIP(x + 150), dc.FromDIP(y + 10));
+    dc.DrawText("Solid/dot/short dash/long dash/dot dash", dc.FromDIP(x + 250), dc.FromDIP(y + 10));
     dc.SetPen( wxPen( *wxBLACK, width ) );
-    dc.DrawLine(dc.FromDIP(x + 20), dc.FromDIP(y + 20), dc.FromDIP(100), dc.FromDIP(y + 20));
+    dc.DrawLine(dc.FromDIP(x + 20), dc.FromDIP(y + 2*vStep), dc.FromDIP(200), dc.FromDIP(y + 2*vStep));
     dc.SetPen( wxPen( *wxBLACK, width, wxPENSTYLE_DOT) );
-    dc.DrawLine(dc.FromDIP(x + 20), dc.FromDIP(y + 30), dc.FromDIP(100), dc.FromDIP(y + 30));
+    dc.DrawLine(dc.FromDIP(x + 20), dc.FromDIP(y + 3*vStep), dc.FromDIP(200), dc.FromDIP(y + 3*vStep));
     dc.SetPen( wxPen( *wxBLACK, width, wxPENSTYLE_SHORT_DASH) );
-    dc.DrawLine(dc.FromDIP(x + 20), dc.FromDIP(y + 40), dc.FromDIP(100), dc.FromDIP(y + 40));
+    dc.DrawLine(dc.FromDIP(x + 20), dc.FromDIP(y + 4*vStep), dc.FromDIP(200), dc.FromDIP(y + 4*vStep));
     dc.SetPen( wxPen( *wxBLACK, width, wxPENSTYLE_LONG_DASH) );
-    dc.DrawLine(dc.FromDIP(x + 20), dc.FromDIP(y + 50), dc.FromDIP(100), dc.FromDIP(y + 50));
+    dc.DrawLine(dc.FromDIP(x + 20), dc.FromDIP(y + 5*vStep), dc.FromDIP(200), dc.FromDIP(y + 5*vStep));
     dc.SetPen( wxPen( *wxBLACK, width, wxPENSTYLE_DOT_DASH) );
-    dc.DrawLine(dc.FromDIP(x + 20), dc.FromDIP(y + 60), dc.FromDIP(100), dc.FromDIP(y + 60));
-
+    dc.DrawLine(dc.FromDIP(x + 20), dc.FromDIP(y + 6*vStep), dc.FromDIP(200), dc.FromDIP(y + 6*vStep));
+/*
     dc.DrawText("Hatches", dc.FromDIP(x + 150), dc.FromDIP(y + 70));
     dc.SetPen( wxPen( *wxBLACK, width, wxPENSTYLE_BDIAGONAL_HATCH) );
     dc.DrawLine(dc.FromDIP(x + 20), dc.FromDIP(y + 70), dc.FromDIP(100), dc.FromDIP(y + 70));
@@ -855,6 +856,7 @@ void MyCanvas::DrawTestLines( int x, int
     dc.SetPen(penWithCap);
     dc.DrawText("Projecting cap", dc.FromDIP(x + 270), dc.FromDIP(y + 100));
     dc.DrawLine(dc.FromDIP(x + 200), dc.FromDIP(y + 110), dc.FromDIP(x + 250), dc.FromDIP(y + 110));
+*/
 }
 
 void MyCanvas::DrawCrossHair(int x, int y, int width, int heigth, wxDC &dc)
@@ -2232,10 +2234,10 @@ void MyCanvas::Draw(wxDC& pdc)
             break;
 
         case File_ShowLines:
-            DrawTestLines( 0, 100, 0, dc );
-            DrawTestLines( 0, 320, 1, dc );
-            DrawTestLines( 0, 540, 2, dc );
-            DrawTestLines( 0, 760, 6, dc );
+            DrawTestLines( 0, 100, 2, dc );
+            DrawTestLines( 0, 190, 5, dc );
+            DrawTestLines( 0, 280, 10, dc );
+            DrawTestLines( 0, 410, 15, dc );
             DrawCrossHair( 0, 0, 400, 90, dc);
             break;
 

In comparison with MSW and GTK2:

GTK3	MSW	GTK2
dashed-lines-GTK3.png (view on web)	dashed-lines-MSW.png (view on web)	dashed-lines-GTK2.png (view on web)

Platform and version information

• wxWidgets version you use: 3.2.8 and master (4ed05f7)
• wxWidgets port you use: wxGTK
• OS and its version: Linux Mint 22
  • GTK version: 3.24.41
  • Which GDK backend is used: X11
  • Desktop environment : Cinnamon
  • Current theme: Mint-Y

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[VZ]

vadz left a comment (wxWidgets/wxWidgets#26449)

We probably need to scale the values hardcoded in wxCairoPenData ctor by the pen width. Including 2.0 used for dotted lines which are not dotted at all with thicker pens.

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