tdf#164247 sw a11y check: Adhere to WCAG contrast threshold for large text

Quoting WCAG 2.2 "Success Criterion 1.4.3 Contrast (Minimum)" [1]:

> The visual presentation of text and images of text has a contrast
> ratio of at least 4.5:1, except for the following:
>
> Large Text
>
>     Large-scale text and images of large-scale text have a contrast
>     ratio of at least 3:1;

Regarding large text, the corresponding "Understanding SC 1.4.3" [2]
clarifies:

> Text that is larger and has wider character strokes is easier to read at
> lower contrast. The contrast requirement for larger text is therefore
> lower. This allows authors to use a wider range of color choices for
> large text, which is helpful for design of pages, particularly titles.
> 18 point text or 14 point bold text is judged to be large enough to
> require a lower contrast ratio.

Therefore, lower the text contrast requirement in
Writer's accessibility check from 4.5 to 3.0 for
text that is considered large according to that
specification.

Add a unit test with 2 test documents:

* one that adheres to the requirements and would have triggered false
  positives without this change in place
  (sw/qa/core/accessibilitycheck/data/ContrastTestOK.odt)
* one that doesn't adhere to the requirements and still fails the
  check, as it should
  (sw/qa/core/accessibilitycheck/data/ContrastTestFail.odt)

[1] https://www.w3.org/TR/WCAG22/#contrast-minimum
[2] https://www.w3.org/WAI/WCAG22/Understanding/contrast-minimum.html

Change-Id: I398f72aa5bdcd77c42834632575d6465a5ecd586
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/178127
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins
Reviewed-by: Balazs Varga <balazs.varga.extern@allotropia.de>

sw/qa/core/accessibilitycheck/AccessibilityCheckTest.cxx

@@ -318,6 +318,37 @@ CPPUNIT_TEST_FIXTURE(AccessibilityCheckTest, testStylesWithHeader)
     CPPUNIT_ASSERT_EQUAL(sfx::AccessibilityIssueID::DIRECT_FORMATTING, aIssues[4]->m_eIssueID);
 }
 
+// Text contrast tests
+// see https://www.w3.org/WAI/WCAG22/Understanding/contrast-minimum.html
+CPPUNIT_TEST_FIXTURE(AccessibilityCheckTest, testCheckTextContrast)
+{
+    // first test doc has these issues:
+    // * normal text with contrast < 4.5
+    // * large text with contrast < 3.0
+    // * bold text with font size 13 (i.e. not considered large) with contrast < 4.5
+    createSwDoc("ContrastTestFail.odt");
+    SwDoc* pDoc = getSwDoc();
+    sw::AccessibilityCheck aCheck(pDoc);
+    aCheck.check();
+    auto& aIssues = aCheck.getIssueCollection().getIssues();
+    CPPUNIT_ASSERT_EQUAL(size_t(3), aIssues.size());
+    CPPUNIT_ASSERT_EQUAL(sfx::AccessibilityIssueID::TEXT_CONTRAST, aIssues[0]->m_eIssueID);
+    CPPUNIT_ASSERT_EQUAL(sfx::AccessibilityIssueID::TEXT_CONTRAST, aIssues[1]->m_eIssueID);
+    CPPUNIT_ASSERT_EQUAL(sfx::AccessibilityIssueID::TEXT_CONTRAST, aIssues[2]->m_eIssueID);
+
+    // second test doc has large text with contrast between 3.0 and 4.5,
+    // which is sufficient for large text
+    // both of these are considered large text according to the spec:
+    // * non-bold text font with size 18
+    // * bold text with font size 14
+    createSwDoc("ContrastTestOK.odt");
+    SwDoc* pDocOK = getSwDoc();
+    sw::AccessibilityCheck aCheckOK(pDocOK);
+    aCheckOK.check();
+    auto& aIssuesOK = aCheckOK.getIssueCollection().getIssues();
+    CPPUNIT_ASSERT_EQUAL(size_t(0), aIssuesOK.size());
+}
+
 namespace
 {
 std::vector<std::shared_ptr<sfx::AccessibilityIssue>>

sw/source/core/access/AccessibilityCheck.cxx

@@ -21,6 +21,7 @@
 #include <svx/svdpage.hxx>
 #include <sortedobjs.hxx>
 #include <swtable.hxx>
+#include <com/sun/star/awt/FontWeight.hpp>
 #include <com/sun/star/frame/XModel.hpp>
 #include <com/sun/star/text/XTextContent.hpp>
 #include <com/sun/star/document/XDocumentPropertiesSupplier.hpp>
@@ -716,6 +717,32 @@ double calculateContrastRatio(Color const& rColor1, Color const& rColor2)
     return (aMinMax.second + 0.05) / (aMinMax.first + 0.05);
 }
 
+// Determine required minimum contrast ratio for text with the given properties
+// according to https://www.w3.org/WAI/WCAG22/Understanding/contrast-minimum.html
+// * 3.0 for large text (font size >= 18 or bold and font size >= 14)
+// * 4.5 otherwise
+double minimumContrastRatio(const uno::Reference<beans::XPropertySet>& xProperties)
+{
+    double fMinimumContrastRatio = 4.5;
+    double fFontSize = 0;
+    if (xProperties->getPropertyValue(u"CharHeight"_ustr) >>= fFontSize)
+    {
+        if (fFontSize >= 18)
+            fMinimumContrastRatio = 3.0;
+        else if (fFontSize >= 14)
+        {
+            double fCharWeight = 0;
+            if (xProperties->getPropertyValue(u"CharWeight"_ustr) >>= fCharWeight)
+            {
+                if (fCharWeight == css::awt::FontWeight::BOLD
+                    || fCharWeight == css::awt::FontWeight::ULTRABOLD)
+                    fMinimumContrastRatio = 3.0;
+            }
+        }
+    }
+    return fMinimumContrastRatio;
+}
+
 class TextContrastCheck : public NodeCheck
 {
 private:
@@ -836,7 +863,7 @@ private:
             aBackgroundColor = COL_WHITE;
 
         double fContrastRatio = calculateContrastRatio(aForegroundColor, aBackgroundColor);
-        if (fContrastRatio < 4.5)
+        if (fContrastRatio < minimumContrastRatio(xProperties))
         {
             auto pIssue = lclAddIssue(m_rIssueCollection, SwResId(STR_TEXT_CONTRAST),
                                       sfx::AccessibilityIssueID::TEXT_CONTRAST,