ENH: Switch to new Slicer logo in module panel

Switch to the new Slicer logo: it is more compact than the old one and works with both dark and bright background (see #5367).

For crisp rendering of the new icon, Qt::AA_UseHighDpiPixmaps application option is now enabled.
If higher-resolution variants of icons are provided by ...@2x.png, ...@3x.png, ... names then they are now used on high-DPI displays.

Move the logo to the module panel dockable widget's header to save space. See discussion at #5820 for more details.

Co-authored-by: James Butler <jbutler@sonovol.com>

Applications/SlicerApp/Resources/SlicerApp.qrc

@@ -8,5 +8,6 @@
     <file alias="SplashScreen.png">Images/Slicer-SplashScreen.png</file>
     <file alias="Logo.png">Images/Slicer-Logo.png</file>
     <file alias="ModulePanelLogo.png">Images/Slicer-ModulePanelLogo.png</file>
+    <file alias="ModulePanelLogo@2x.png">Images/Slicer-ModulePanelLogo@2x.png</file>
   </qresource>
 </RCC>

Applications/SlicerApp/qSlicerAppMainWindow.cxx

@@ -23,6 +23,7 @@
 
 // Qt includes
 #include <QDesktopServices>
+#include <QLabel>
 #include <QPixmap>
 #include <QStyle>
 #include <QUrl>
@@ -117,7 +118,19 @@ void qSlicerAppMainWindowPrivate::setupUi(QMainWindow * mainWindow)
   mainWindow->setWindowTitle("3D Slicer");
   mainWindow->setWindowIcon(QIcon(":/Icons/Medium/DesktopIcon.png"));
 
-  this->LogoLabel->setPixmap(QPixmap(":/ModulePanelLogo.png"));
+  QLabel* logoLabel = new QLabel();
+  logoLabel->setObjectName("LogoLabel");
+  // QIcon stores multiple versions of the image (in different sizes)
+  // and uses the most suitable one (depending on DevicePixelRatio).
+  // QLabel cannot take a QIcon, therefore we need to get the most suitable
+  // QPixmap from the QIcon (base.png, base@2x, ...).
+  // To achieve this, we first determine the pixmap size in device independent units,
+  // which is the size of the base image (icon.availableSizes().first(), because for that
+  // DevicePixelRatio=1.0), and then we retieve the pixmap for this size.
+  QIcon icon = QIcon(":/ModulePanelLogo.png");
+  QPixmap logo = icon.pixmap(icon.availableSizes().first());
+  logoLabel->setPixmap(logo);
+  this->PanelDockWidget->setTitleBarWidget(logoLabel);
 
   this->HelpMenu->addAction(helpKeyboardShortcutsAction);
   this->HelpMenu->addAction(helpInterfaceDocumentationAction);

Base/QTApp/Resources/UI/qSlicerMainWindow.ui

@@ -45,13 +45,6 @@
      <property name="bottomMargin">
       <number>0</number>
      </property>
-     <item>
-      <widget class="QLabel" name="LogoLabel">
-       <property name="alignment">
-        <set>Qt::AlignLeading|Qt::AlignLeft|Qt::AlignTop</set>
-       </property>
-      </widget>
-     </item>
      <item>
       <widget class="qSlicerModulePanel" name="ModulePanel"/>
      </item>
@@ -66,6 +59,9 @@
        <property name="text">
         <string>Data Probe</string>
        </property>
+       <property name="collapsedHeight">
+        <number>0</number>
+       </property>
        <layout class="QHBoxLayout" name="horizontalLayout"/>
       </widget>
      </item>
@@ -217,7 +213,7 @@
      <x>0</x>
      <y>0</y>
      <width>835</width>
-     <height>25</height>
+     <height>22</height>
     </rect>
    </property>
    <widget class="QMenu" name="FileMenu">

Libs/MRML/Widgets/qMRMLWidget.cxx

@@ -130,6 +130,15 @@ void qMRMLWidget::preInitializeApplication()
   // Enable automatic scaling based on the pixel density of the monitor
   QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
 
+  // Enable QIcon to provide higher-resolution pixmaps than the size in device independent units.
+  // These pixmaps render sharply on a high-dpi display.
+  // If Qt::AA_UseHighDpiPixmaps is enabled then when an icon is loaded by the filename "base.png" then the icon
+  // class will also look for higher-resolution variants of this image by the names base@2x.png, base@3x.png, etc.
+  // On a high-DPI monitor (with DevicePixelRatio > 1) QIcon will actually load a higher-resolution pixmap
+  // that best matches the DevicePixelRatio of the display, resulting in crisp rendering of the icon.
+  // See https://doc.qt.io/qt-5/qicon.html#pixmap for more details.
+  QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
+
   // Enables resource sharing between the OpenGL contexts used by classes like QOpenGLWidget and QQuickWidget
   QApplication::setAttribute(Qt::AA_ShareOpenGLContexts);
 }