tdf#134128 Use Gdiplus::DashCapRound for round dash or dot.

If Skia and OpenGL are disabled, rendering in edit mode shows no round
dashes and dots on Windows. This becomes especially visible, when
importing OOXML documents. In that case it looks as if dots are lost.
The patch uses now Gdiplus::DashCapRound instead of the previously
used Gdiplus::DashCapFlat, and it adds a similar tweak as in OOXML
import and increases the dash length by the cap size for rendering.

Change-Id: I98a258809ef253a2cacb7c5c94f2b26b89ee2488
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/107181
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

1 files changed, 31 insertions, 4 deletions

diff --git a/vcl/win/gdi/gdiimpl.cxx b/vcl/win/gdi/gdiimpl.cxx
index de022280c4b2..b3b70320dd05 100644
--- a/vcl/win/gdi/gdiimpl.cxx
+++ b/vcl/win/gdi/gdiimpl.cxx
@@ -2343,12 +2343,39 @@ bool WinSalGraphicsImpl::drawPolyLine(
         std::vector<Gdiplus::REAL> aDashArray(pStroke->size());
         const double fFactor(fLineWidth == 0 ? 1.0 : 1.0 / fLineWidth);
 
-        for(size_t a(0); a < pStroke->size(); a++)
+        // tdf#134128. ODF adds caps to the dashes and dots, but GDI makes caps from the
+        // dash or dot themselve. We tweak aDashArray to look the same in GDI (e.g. Impress edit mode)
+        // and other renderes (e.g. Impress slide show), while keeping the total length of the
+        // pattern.
+        // Patterns are always a sequence dash space dash space ...
+        if (eLineCap != css::drawing::LineCap_BUTT)
         {
-            aDashArray[a] = Gdiplus::REAL((*pStroke)[a] * fFactor);
+            size_t nSize = pStroke->size();
+            // We want to treat dash and space in pairs. There should be no odd size. If so, we ignore
+            // last item.
+            nSize /= 2;
+            for(size_t a(0); a < nSize; a++)
+            {
+                double fDashLengthRel = (*pStroke)[2 * a] * fFactor;
+                double fSpaceLengthRel = (*pStroke)[2 * a + 1] * fFactor;
+                // GDI allows only positive lengths for space, Skia negative lengths too. Thus the
+                // appearance is different, in case space is too small.
+                double fCorrect = fSpaceLengthRel - 1.0 <= 0 ? fSpaceLengthRel - 0.01 : 1.0;
+                aDashArray[2 * a] = Gdiplus::REAL(fDashLengthRel + fCorrect);
+                aDashArray[2 * a + 1] = Gdiplus::REAL(fSpaceLengthRel - fCorrect);
+            }
         }
-
-        aPen.SetDashCap(Gdiplus::DashCapFlat);
+        else
+        {
+            for(size_t a(0); a < pStroke->size(); a++)
+            {
+                aDashArray[a] = Gdiplus::REAL((*pStroke)[a] * fFactor);
+            }
+        }
+        if (eLineCap == css::drawing::LineCap_ROUND)
+            aPen.SetDashCap(Gdiplus::DashCapRound);
+        else
+            aPen.SetDashCap(Gdiplus::DashCapFlat); // "square" doesn't exist in Gdiplus
         aPen.SetDashOffset(Gdiplus::REAL(0.0));
         aPen.SetDashPattern(aDashArray.data(), aDashArray.size());
     }