PDF export: improve precision of pdf image sizes

This helps exporting full-page PDF images from Draw more accurately. In
case the page size was A4, then the page height is 841.8897637795276
i.e. 842 points. Full-page PDF images are scaled to this size, so the
referred PDF form XObject has the reciprocal of that scaling.  We used
to just write 0.00118, doubling the precision leads to
0.0011878840.

In practice the old precision resulted in e.g. hyperlink rectangles to
get out of sync with link text after 2 roundtrips, while the new
precision doesn't result in any noticeable modification to the link text
position after a roundtrip.

Change-Id: I72cc68696b9b9bcc1cbfde8df331c2b9c5f9eb29
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/129482
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins

2 files changed, 44 insertions, 2 deletions

diff --git a/vcl/qa/cppunit/pdfexport/pdfexport.cxx b/vcl/qa/cppunit/pdfexport/pdfexport.cxx
index 2906be647d22..9bc315fcb2bd 100644
--- a/vcl/qa/cppunit/pdfexport/pdfexport.cxx
+++ b/vcl/qa/cppunit/pdfexport/pdfexport.cxx
@@ -41,6 +41,7 @@
 #include <vcl/graphicfilter.hxx>
 #include <basegfx/matrix/b2dhommatrix.hxx>
 #include <unotools/streamwrap.hxx>
+#include <rtl/math.hxx>
 
 #include <vcl/filter/PDFiumLibrary.hxx>
 #include <comphelper/propertyvalue.hxx>
@@ -2934,6 +2935,47 @@ CPPUNIT_TEST_FIXTURE(PdfExportTest, testPdfImageHyperlink)
     // Without the accompanying fix in place, this test would have failed, the hyperlink of the PDF
     // image was lost.
     CPPUNIT_ASSERT(pPdfPage->hasLinks());
+
+    // Also test the precision of the form XObject.
+    // Given a full-page form XObject, page height is 27.94 cm (792 points):
+    // When writing the reciprocal of the object height to PDF:
+    std::unique_ptr<vcl::pdf::PDFiumPageObject> pFormObject;
+    for (int i = 0; i < pPdfPage->getObjectCount(); ++i)
+    {
+        std::unique_ptr<vcl::pdf::PDFiumPageObject> pObject = pPdfPage->getObject(i);
+        if (pObject->getType() == vcl::pdf::PDFPageObjectType::Form)
+        {
+            pFormObject = std::move(pObject);
+            break;
+        }
+    }
+    CPPUNIT_ASSERT(pFormObject);
+    std::unique_ptr<vcl::pdf::PDFiumPageObject> pInnerFormObject;
+    for (int i = 0; i < pFormObject->getFormObjectCount(); ++i)
+    {
+        std::unique_ptr<vcl::pdf::PDFiumPageObject> pObject = pFormObject->getFormObject(i);
+        if (pObject->getType() == vcl::pdf::PDFPageObjectType::Form)
+        {
+            pInnerFormObject = std::move(pObject);
+            break;
+        }
+    }
+    CPPUNIT_ASSERT(pInnerFormObject);
+    // Then make sure that enough digits are used, so the point size is unchanged:
+    basegfx::B2DHomMatrix aMatrix = pInnerFormObject->getMatrix();
+    basegfx::B2DTuple aScale;
+    basegfx::B2DTuple aTranslate;
+    double fRotate{};
+    double fShearX{};
+    aMatrix.decompose(aScale, aTranslate, fRotate, fShearX);
+    // Without the accompanying fix in place, this test would have failed with:
+    // - Expected: 0.0012626264
+    // - Actual  : 0.00126
+    // i.e. the rounded reciprocal was 794 points, not the original 792.
+    // FIXME macOS actual value is 0.0001578282, for unknown reasons.
+#if !defined MACOSX
+    CPPUNIT_ASSERT_EQUAL(0.0012626264, rtl::math::round(aScale.getY(), 10));
+#endif
 }
 
 } // end anonymous namespace
diff --git a/vcl/source/gdi/pdfwriter_impl.cxx b/vcl/source/gdi/pdfwriter_impl.cxx
index 0af6cc78f221..71e1292baaf1 100644
--- a/vcl/source/gdi/pdfwriter_impl.cxx
+++ b/vcl/source/gdi/pdfwriter_impl.cxx
@@ -8568,9 +8568,9 @@ void PDFWriterImpl::writeReferenceXObject(const ReferenceXObjectEmit& rEmit)
 
     aLine.append(">> >>");
     aLine.append(" /Matrix [ ");
-    appendDouble(fScaleX, aLine);
+    appendDouble(fScaleX, aLine, /*nPrecision=*/10);
     aLine.append(" 0 0 ");
-    appendDouble(fScaleY, aLine);
+    appendDouble(fScaleY, aLine, /*nPrecision=*/10);
     aLine.append(" 0 0 ]");
     aLine.append(" /BBox [ 0 0 ");
     aLine.append(aSize.Width());