Qt QIcon SVG different sizes - c++

Question

I am using the QIcon class of Qt to set the application icon of my apps.
My icons are SVG files, which load just fine.
QIcon appIcon;
QString iconPath = "share/icons/myapp.svg;
QFile iconFile(iconPath);
if (iconFile.exists()) {
appIcon.addFile(iconPath);
}
QApplication::setWindowIcon(appIcon);
My question is:
I want to have different icons for different pusposes.
For instance, I'd like to have different icons (i.e. sizes) for the title bar and for the actual icon, showed when pressing ALT+TAB.
I found the QSize parameter of addFile(), but then, the icon would stick to one certain size, which I don't know (because different window managers could have different title bar sizes).
How can I define an icon for the title bar and one for the rest (that's pretty much the short version of what I am asking)?
Kind regards,
mefiX

Answer 1

I normally use a separate PNG image (16x16 pixels, e.g. "./Icons/app_window.png") to set the title bar icon using the setWindowIcon() function in my mainwindow class constructor:
this->setWindowIcon(QIcon(QLatin1String(":/Images/Icons/app_window.png")));
For the rest icons you can use the following excellent freeware application:
http://www.icofx.ro/
It allows you to create both a "myapp.ico" file (for MS-Windows) and a "myapp.icns" file (for Mac OSX) and if you start from a 256x256 image it can automatically create all icon sizes for you!
For Mac OS/X you must put the following line in your "MyApp.pro" file:
ICON = Icons/myapp.icns
For MS-Windows you must put the following line in your "MyApp.pro" file:
win32::RC_FILE = myapp.rc
and you have also to create a "myapp.rc" file that look like this:
IDI_ICON1 ICON DISCARDABLE "./Icons/myapp.ico"
#include <windows.h>
#include "MyAppVersion.h"
VS_VERSION_INFO VERSIONINFO
FILEVERSION VER_FILEVERSION
PRODUCTVERSION VER_PRODUCTVERSION
BEGIN
BLOCK "StringFileInfo"
BEGIN
BLOCK "040904E4"
BEGIN
VALUE "CompanyName", VER_COMPANYNAME_STR
VALUE "FileDescription", VER_FILEDESCRIPTION_STR
VALUE "FileVersion", VER_FILEVERSION_STR
VALUE "InternalName", VER_INTERNALNAME_STR
VALUE "LegalCopyright", VER_LEGALCOPYRIGHT_STR
VALUE "LegalTrademarks1", VER_LEGALTRADEMARKS1_STR
VALUE "LegalTrademarks2", VER_LEGALTRADEMARKS2_STR
VALUE "OriginalFilename", VER_ORIGINALFILENAME_STR
VALUE "ProductName", VER_PRODUCTNAME_STR
VALUE "ProductVersion", VER_PRODUCTVERSION_STR
END
END
BLOCK "VarFileInfo"
BEGIN
VALUE "Translation", 0x409, 1252
END
END
The #include "MyAppVersion.h" and the rest options is off-course optional but you can use it to define all the VER_xxx constants instead of putting them directly as a constant string like "MyAppName". The only important line for icon definition is the first one:
IDI_ICON1 ICON DISCARDABLE "./Icons/myapp.ico"
which implies that a file "MyApp.ico" resides in the folder ./Icons/
Note also that on MS-Windows if you modify your "myapp.ico" file, rebuild your project and run your application again, the icon is not always displayed right, due to the icon cache of windows-explorer.
Here is a link with various methods to clear/rebuild the windows-explorer icon cache:
Rebuild icon cache to fix incorrectly displayed icons

Answer 2

You can use one SVG-Icon in different position, it should be scaled when drawn. But the application icon (shown at ALT+TAB) is different, see the documentation here.
To learn how to convert SVG icons to the ico format, look here.