Fix handling of transparency / Restore colour per face

The Materials module does a conversion from float to double when saving the transparency and again a conversion from double to float to double
when restoring it. This causes a considerable loss of accuracy so that the representation in percent leads to different numbers.

Using consistently some helper functions to do a proper conversion from float to long and back fixes the problem.

The new property ShapeAppearance is a PropertyMaterialList and always read after the DiffuseColor property when restoring a document.
Thus, the  method onChanged() doesn't override DiffuseColor when restoring a document. Additionally, the method finishRestoring() is re-implemented
to set the colours per face in case DiffuseColor has defined multiple colors.

src/Gui/ViewProviderGeometryObject.cpp

@@ -51,6 +51,19 @@
 
 using namespace Gui;
 
+// Helper functions to consistently convert between float and long
+namespace {
+float fromPercent(long value)
+{
+    return static_cast<float>(value) / 100.0F;
+}
+
+long toPercent(float value)
+{
+    return static_cast<long>(100.0 * value + 0.5);
+}
+}
+
 PROPERTY_SOURCE(Gui::ViewProviderGeometryObject, Gui::ViewProviderDragger)
 
 const App::PropertyIntegerConstraint::Constraints intPercent = {0, 100, 5};
@@ -137,9 +150,9 @@ void ViewProviderGeometryObject::onChanged(const App::Property* prop)
         setSelectable(Sel);
     }
     else if (prop == &Transparency) {
-        long value = (long)(100 * ShapeAppearance.getTransparency());
+        long value = toPercent(ShapeAppearance.getTransparency());
         if (value != Transparency.getValue()) {
-            float trans = (float)Transparency.getValue() / 100.0f;
+            float trans = fromPercent(Transparency.getValue());
             pcShapeMaterial->transparency = trans;
             ShapeAppearance.setTransparency(trans);
         }
@@ -149,7 +162,7 @@ void ViewProviderGeometryObject::onChanged(const App::Property* prop)
             getObject()->touch(true);
         }
         const App::Material& Mat = ShapeAppearance[0];
-        long value = (long)(100.0 * ShapeAppearance.getTransparency());
+        long value = toPercent(ShapeAppearance.getTransparency());
         if (value != Transparency.getValue()) {
             Transparency.setValue(value);
         }

src/Mod/Part/Gui/ViewProviderExt.cpp

@@ -96,6 +96,19 @@ FC_LOG_LEVEL_INIT("Part", true, true)
 
 using namespace PartGui;
 
+// Helper functions to consistently convert between float and long
+namespace {
+float fromPercent(long value)
+{
+    return static_cast<float>(value) / 100.0F;
+}
+
+long toPercent(float value)
+{
+    return static_cast<long>(100.0 * value + 0.5);
+}
+}
+
 PROPERTY_SOURCE(PartGui::ViewProviderPartExt, Gui::ViewProviderGeometryObject)
 
 
@@ -321,15 +334,19 @@ void ViewProviderPartExt::onChanged(const App::Property* prop)
     else if (prop == &ShapeAppearance) {
         pcFaceBind->value = SoMaterialBinding::OVERALL;
         ViewProviderGeometryObject::onChanged(prop);
-        App::Color c = ShapeAppearance.getDiffuseColor();
-        c.a = Transparency.getValue()/100.0f;
-        DiffuseColor.setValue(c);
+        // While restoring a document do not override the
+        // DiffuseColor that has already been restored
+        if (!isRestoring()) {
+            App::Color c = ShapeAppearance.getDiffuseColor();
+            c.a = fromPercent(Transparency.getValue());
+            DiffuseColor.setValue(c);
+        }
     }
     else if (prop == &Transparency) {
         const App::Material& Mat = ShapeAppearance[0];
-        long value = (long)(100*Mat.transparency);
+        long value = toPercent(Mat.transparency);
         if (value != Transparency.getValue()) {
-            float trans = Transparency.getValue()/100.0f;
+            float trans = fromPercent(Transparency.getValue());
             auto colors = DiffuseColor.getValues();
             for (auto &c : colors)
                 c.a = trans;
@@ -847,6 +864,21 @@ void ViewProviderPartExt::updateData(const App::Property* prop)
     Gui::ViewProviderGeometryObject::updateData(prop);
 }
 
+void ViewProviderPartExt::startRestoring()
+{
+    Gui::ViewProviderGeometryObject::startRestoring();
+}
+
+void ViewProviderPartExt::finishRestoring()
+{
+    // The ShapeAppearance property is restored after DiffuseColor
+    // and currently sets a single color.
+    // In case DiffuseColor has defined multiple colors they will
+    // be passed to the scene graph now.
+    DiffuseColor.touch();
+    Gui::ViewProviderGeometryObject::finishRestoring();
+}
+
 void ViewProviderPartExt::setupContextMenu(QMenu* menu, QObject* receiver, const char* member)
 {
     QIcon iconObject = mergeGreyableOverlayIcons(Gui::BitmapFactory().pixmap("Part_ColorFace.svg"));

src/Mod/Part/Gui/ViewProviderExt.h

@@ -99,6 +99,12 @@ public:
 
     void updateData(const App::Property*) override;
 
+    /** @name Restoring view provider from document load */
+    //@{
+    void startRestoring() override;
+    void finishRestoring() override;
+    //@}
+
     /** @name Selection handling
      * This group of methods do the selection handling.
      * Here you can define how the selection for your ViewProfider