SVG export: fix lost semi-transparent line shapes

The line shape itself didn't really have a height, rather it had a
stroke. For some reason, the SVG mask then decides that nothing has to
be painted there, so unless the line is entirely opaque, the line shape
gets lost on export.

Fix the problem by handling transparency similar to the PDF export,
which detects if the whole purpose of the transparency gradient is to
pass around a transparency percentage. We don't need a mask in that
case, we can just use opacity as described at e.g.
<https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/opacity>.

Change-Id: I0355b9b09b6dd48bbacc5b7cc54fb71866304ef1
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/91932
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins

3 files changed, 57 insertions, 24 deletions

diff --git a/filter/qa/unit/data/semi-transparent-line.odg b/filter/qa/unit/data/semi-transparent-line.odg
new file mode 100644
index 000000000000..2d28a694cea5
--- /dev/null
+++ b/filter/qa/unit/data/semi-transparent-line.odg
diff --git a/filter/qa/unit/svg.cxx b/filter/qa/unit/svg.cxx
index d6bb68283b74..8e6524c688ba 100644
--- a/filter/qa/unit/svg.cxx
+++ b/filter/qa/unit/svg.cxx
@@ -20,9 +20,7 @@
 
 using namespace ::com::sun::star;
 
-#if !defined MACOSX
 char const DATA_DIRECTORY[] = "/filter/qa/unit/data/";
-#endif
 
 /// SVG filter tests.
 class SvgFilterTest : public test::BootstrapFixture, public unotest::MacrosTest, public XmlTestTools
@@ -34,10 +32,8 @@ public:
     void setUp() override;
     void tearDown() override;
     void registerNamespaces(xmlXPathContextPtr& pXmlXpathCtx) override;
-#if !defined MACOSX
     uno::Reference<lang::XComponent>& getComponent() { return mxComponent; }
     void load(const OUString& rURL);
-#endif
 };
 
 void SvgFilterTest::setUp()
@@ -55,13 +51,11 @@ void SvgFilterTest::tearDown()
     test::BootstrapFixture::tearDown();
 }
 
-#if !defined MACOSX
 void SvgFilterTest::load(const OUString& rFileName)
 {
     OUString aURL = m_directories.getURLFromSrc(DATA_DIRECTORY) + rFileName;
     mxComponent = loadFromDesktop(aURL);
 }
-#endif
 
 void SvgFilterTest::registerNamespaces(xmlXPathContextPtr& pXmlXpathCtx)
 {
@@ -100,6 +94,34 @@ CPPUNIT_TEST_FIXTURE(SvgFilterTest, testPreserveJpg)
 #endif
 }
 
+CPPUNIT_TEST_FIXTURE(SvgFilterTest, testSemiTransparentLine)
+{
+    // Load a document with a semi-transparent line shape.
+    load("semi-transparent-line.odg");
+
+    // Export it to SVG.
+    uno::Reference<frame::XStorable> xStorable(getComponent(), uno::UNO_QUERY_THROW);
+    SvMemoryStream aStream;
+    uno::Reference<io::XOutputStream> xOut = new utl::OOutputStreamWrapper(aStream);
+    utl::MediaDescriptor aMediaDescriptor;
+    aMediaDescriptor["FilterName"] <<= OUString("draw_svg_Export");
+    aMediaDescriptor["OutputStream"] <<= xOut;
+    xStorable->storeToURL("private:stream", aMediaDescriptor.getAsConstPropertyValueList());
+    aStream.Seek(STREAM_SEEK_TO_BEGIN);
+
+    // Get the style of the group around the actual <path> element.
+    xmlDocPtr pXmlDoc = parseXmlStream(&aStream);
+    OUString aStyle = getXPath(
+        pXmlDoc, "//svg:g[@class='com.sun.star.drawing.LineShape']/svg:g/svg:g", "style");
+    OUString aPrefix("opacity: ");
+    // Without the accompanying fix in place, this test would have failed, as the style was
+    // "mask:url(#mask1)", not "opacity: <value>".
+    CPPUNIT_ASSERT(aStyle.startsWith(aPrefix));
+    int nPercent = std::round(aStyle.copy(aPrefix.getLength()).toDouble() * 100);
+    // Make sure that the line is still 30% opaque, rather than completely invisible.
+    CPPUNIT_ASSERT_EQUAL(30, nPercent);
+}
+
 CPPUNIT_PLUGIN_IMPLEMENT();
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */
diff --git a/filter/source/svg/svgwriter.cxx b/filter/source/svg/svgwriter.cxx
index 8d491d122d0b..c276fd903048 100644
--- a/filter/source/svg/svgwriter.cxx
+++ b/filter/source/svg/svgwriter.cxx
@@ -2382,32 +2382,43 @@ void SVGActionWriter::ImplWriteMask( GDIMetaFile& rMtf,
     if( nMoveX || nMoveY )
         rMtf.Move( nMoveX, nMoveY );
 
-    OUString aMaskId = "mask" + OUString::number( mnCurMaskId++ );
-
+    OUString aStyle;
+    if (rGradient.GetStartColor() == rGradient.GetEndColor())
     {
-        SvXMLElementExport aElemDefs( mrExport, XML_NAMESPACE_NONE, aXMLElemDefs, true, true );
+        // Special case: constant alpha value.
+        const Color& rColor = rGradient.GetStartColor();
+        const double fOpacity = 1.0 - static_cast<double>(rColor.GetLuminance()) / 255;
+        aStyle = "opacity: " + OUString::number(fOpacity);
+    }
+    else
+    {
+        OUString aMaskId = "mask" + OUString::number(mnCurMaskId++);
 
-        mrExport.AddAttribute( XML_NAMESPACE_NONE, aXMLAttrId, aMaskId );
         {
-            SvXMLElementExport aElemMask( mrExport, XML_NAMESPACE_NONE, "mask", true, true );
+            SvXMLElementExport aElemDefs(mrExport, XML_NAMESPACE_NONE, aXMLElemDefs, true, true);
 
-            const tools::PolyPolygon aPolyPolygon( tools::PolyPolygon( tools::Rectangle( rDestPt, rDestSize ) ) );
-            Gradient aGradient( rGradient );
+            mrExport.AddAttribute(XML_NAMESPACE_NONE, aXMLAttrId, aMaskId);
+            {
+                SvXMLElementExport aElemMask(mrExport, XML_NAMESPACE_NONE, "mask", true, true);
 
-            // swap gradient stops to adopt SVG mask
-            Color aTmpColor( aGradient.GetStartColor() );
-            sal_uInt16 nTmpIntensity( aGradient.GetStartIntensity() );
-            aGradient.SetStartColor( aGradient.GetEndColor() );
-            aGradient.SetStartIntensity( aGradient.GetEndIntensity() ) ;
-            aGradient.SetEndColor( aTmpColor );
-            aGradient.SetEndIntensity( nTmpIntensity );
+                const tools::PolyPolygon aPolyPolygon(tools::PolyPolygon(tools::Rectangle(rDestPt, rDestSize)));
+                Gradient aGradient(rGradient);
 
-            ImplWriteGradientEx( aPolyPolygon, aGradient, nWriteFlags );
+                // swap gradient stops to adopt SVG mask
+                Color aTmpColor(aGradient.GetStartColor());
+                sal_uInt16 nTmpIntensity(aGradient.GetStartIntensity());
+                aGradient.SetStartColor(aGradient.GetEndColor());
+                aGradient.SetStartIntensity(aGradient.GetEndIntensity());
+                aGradient.SetEndColor(aTmpColor);
+                aGradient.SetEndIntensity(nTmpIntensity);
+
+                ImplWriteGradientEx(aPolyPolygon, aGradient, nWriteFlags);
+            }
         }
-    }
 
-    OUString aMaskStyle = "mask:url(#" + aMaskId + ")";
-    mrExport.AddAttribute( XML_NAMESPACE_NONE, aXMLAttrStyle, aMaskStyle );
+        aStyle = "mask:url(#" + aMaskId + ")";
+    }
+    mrExport.AddAttribute(XML_NAMESPACE_NONE, aXMLAttrStyle, aStyle);
 
     {
         SvXMLElementExport aElemG( mrExport, XML_NAMESPACE_NONE, aXMLElemG, true, true );