sw HTML export: fix writing transparent images

It is possible ImpGraphic::ImplSetPrepared() should handle this
directly, though.

Change-Id: I1a020a45eaded140493d7207f8e0e0dfe551cc88
Reviewed-on: https://gerrit.libreoffice.org/57623
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

sw/qa/extras/htmlexport/htmlexport.cxx

@@ -589,6 +589,17 @@ DECLARE_HTMLEXPORT_TEST(testList, "list.html")
     CPPUNIT_ASSERT(aStream.indexOf("<li>") != -1);
 }
 
+DECLARE_HTMLEXPORT_TEST(testTransparentImage, "transparent-image.odt")
+{
+    htmlDocPtr pDoc = parseHtml(maTempFile);
+    CPPUNIT_ASSERT(pDoc);
+
+    OUString aSource = getXPath(pDoc, "/html/body/p/img", "src");
+    OUString aMessage = "src attribute is: " + aSource;
+    // This was a jpeg, transparency was lost.
+    CPPUNIT_ASSERT_MESSAGE(aMessage.toUtf8().getStr(), aSource.endsWith(".gif"));
+}
+
 CPPUNIT_PLUGIN_IMPLEMENT();
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */

sw/source/filter/html/htmlflywriter.cxx

@@ -1840,7 +1840,13 @@ static Writer& OutHTML_FrameFormatGrfNode( Writer& rWrt, const SwFrameFormat& rF
                 aMimeType = "image/png";
             }
 
-            ErrCode nErr = XOutBitmap::WriteGraphic( pGrfNd->GetGrf(), aGraphicURL,
+            const Graphic& rGraphic = pGrfNd->GetGrf();
+
+            // So that Graphic::IsTransparent() can report true.
+            if (!rGraphic.isAvailable())
+                const_cast<Graphic&>(rGraphic).makeAvailable();
+
+            ErrCode nErr = XOutBitmap::WriteGraphic( rGraphic, aGraphicURL,
                     aFilterName, nFlags, &aMM100Size );
             if( nErr )
             {