Implement support for Scale directory key according to Icon Theme spec

Qt already supports high DPI icons using the “@nx” approach, where the
device pixel ratio that the image was designed for is in the file
name. However, our implementation of the freedekstop.org Icon Theme
specification did not support the Scale directory key:

https://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html#directory_layout

This meant that users creating icons via QIcon::fromTheme() did not
get high DPI support. This patch fixes that.

[ChangeLog][QtGui][QIcon] Implemented support for Scale directory key
according to Icon Theme Spec. Icons created via QIcon::fromTheme()
now have high DPI support by specifying the Scale in the appropriate
entry of the relevant index.theme file.

Task-number: QTBUG-49820
Change-Id: If442fbc551034166d88defe607109de1c6ca1d28
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 49 insertions, 3 deletions

diff --git a/src/gui/image/qicon.cpp b/src/gui/image/qicon.cpp
index e8f2c878c8..17734f05f3 100644
--- a/src/gui/image/qicon.cpp
+++ b/src/gui/image/qicon.cpp
@@ -605,6 +605,51 @@ QFactoryLoader *qt_iconEngineFactoryLoader()
 
   \note QIcon needs a QGuiApplication instance before the icon is created.
 
+  \section1 High DPI Icons
+
+  There are two ways that QIcon supports \l {High DPI Displays}{high DPI}
+  icons: via \l addFile() and \l fromTheme().
+
+  \l addFile() is useful if you have your own custom directory structure and do
+  not need to use the \l {Icon Theme Specification}{freedesktop.org Icon Theme
+  Specification}. Icons created via this approach use Qt's \l {High Resolution
+  Versions of Images}{"@nx" high DPI syntax}.
+
+  Using \l fromTheme() is necessary if you plan on following the Icon Theme
+  Specification. To make QIcon use the high DPI version of an image, add an
+  additional entry to the appropriate \c index.theme file:
+
+  \badcode
+  [Icon Theme]
+  Name=Test
+  Comment=Test Theme
+
+  Directories=32x32/actions,32x32@2/actions
+
+  [32x32/actions]
+  Size=32
+  Context=Actions
+  Type=Fixed
+
+  # High DPI version of the entry above.
+  [32x32@2/actions]
+  Size=32
+  Scale=2
+  Type=Fixed
+  \endcode
+
+  Your icon theme directory would then look something like this:
+
+  \badcode
+    ├── 32x32
+    │   └── actions
+    │       └── appointment-new.png
+    ├── 32x32@2
+    │   └── actions
+    │       └── appointment-new.png
+    └── index.theme
+  \endcode
+
   \sa {fowler}{GUI Design Handbook: Iconic Label}, {Icons Example}
 */
 
@@ -847,9 +892,10 @@ QPixmap QIcon::pixmap(QWindow *window, const QSize &size, Mode mode, State state
     }
 
     // Try get a pixmap that is big enough to be displayed at device pixel resolution.
-    QPixmap pixmap = d->engine->pixmap(size * devicePixelRatio, mode, state);
-    pixmap.setDevicePixelRatio(d->pixmapDevicePixelRatio(devicePixelRatio, size, pixmap.size()));
-    return pixmap;
+    QIconEngine::ScaledPixmapArgument scalePixmapArg = { size * devicePixelRatio, mode, state, devicePixelRatio, QPixmap() };
+    d->engine->virtual_hook(QIconEngine::ScaledPixmapHook, reinterpret_cast<void*>(&scalePixmapArg));
+    scalePixmapArg.pixmap.setDevicePixelRatio(d->pixmapDevicePixelRatio(devicePixelRatio, size, scalePixmapArg.pixmap.size()));
+    return scalePixmapArg.pixmap;
 }
 
 /*!