(Sdl|Glfw)Application: _commandLineDpiScalingPolicy always default, app-defined scaling never takes effect

[rune-scape]

If there is no --magnum-dpi-scaling specified on the command line, dpi-scaling is set to "default"

Which according to this code, which is run by every ctor of GlfwApplication:

magnum/src/Magnum/Platform/GlfwApplication.cpp

Lines 91 to 108 in 7cfa170

	/* Save command-line arguments */
	if(args.value("log") == "verbose") _verboseLog = true;
	const std::string dpiScaling = args.value("dpi-scaling");
	if(dpiScaling == "default")
	_commandLineDpiScalingPolicy = Implementation::GlfwDpiScalingPolicy::Default;
	#ifdef CORRADE_TARGET_APPLE
	else if(dpiScaling == "framebuffer")
	_commandLineDpiScalingPolicy = Implementation::GlfwDpiScalingPolicy::Framebuffer;
	#else
	else if(dpiScaling == "virtual")
	_commandLineDpiScalingPolicy = Implementation::GlfwDpiScalingPolicy::Virtual;
	else if(dpiScaling == "physical")
	_commandLineDpiScalingPolicy = Implementation::GlfwDpiScalingPolicy::Physical;
	#endif
	else if(dpiScaling.find_first_of(" \t\n") != std::string::npos)
	_commandLineDpiScaling = args.value<Vector2>("dpi-scaling");
	else
	_commandLineDpiScaling = Vector2{args.value<Float>("dpi-scaling")};

_commandLineDpiScalingPolicy is set here and only here to GlfwDpiScalingPolicy::Default, which is just an alias to the platform-dependant default policy.

Crucially GlfwDpiScalingPolicy::Default is not 0
so... when this code comes along:

magnum/src/Magnum/Platform/GlfwApplication.cpp

Lines 137 to 150 in 7cfa170

	/* Use values from the configuration only if not overriden on command line.
	In any case explicit scaling has a precedence before the policy. */
	Implementation::GlfwDpiScalingPolicy dpiScalingPolicy{};
	if(!_commandLineDpiScaling.isZero()) {
	Debug{verbose} << "Platform::GlfwApplication: user-defined DPI scaling" << _commandLineDpiScaling.x();
	return _commandLineDpiScaling;
	} else if(UnsignedByte(_commandLineDpiScalingPolicy)) {
	dpiScalingPolicy = _commandLineDpiScalingPolicy;
	} else if(!configuration.dpiScaling().isZero()) {
	Debug{verbose} << "Platform::GlfwApplication: app-defined DPI scaling" << _commandLineDpiScaling.x();
	return configuration.dpiScaling();
	} else {
	dpiScalingPolicy = configuration.dpiScalingPolicy();
	}

and checks if(UnsignedByte(_commandLineDpiScalingPolicy))
The other 2 cases are not possible!

[rune-scape]

I made a tiny edit in Magnum/Platform/Implementation/DpiScaling.cpp
changing this line:

magnum/src/Magnum/Platform/Implementation/DpiScaling.cpp

Line 64 in 7cfa170

args.addOption("dpi-scaling", "default")

to
args.addOption("dpi-scaling")
taking out the default option, because that seems to be what the first branch was made to parse
and it does what i would expect it to do

[rune-scape]

this is also because the glfw dpi detection always uses monitor dpi instead of glfwGetWindowContentScale()

[mosra]

Re glfwGetWindowContentScale() -- this is on my list to fix for Windows (#411), unfortunately right now i only have access to a Mac, where the DPI scaling is the weirdest of all, impatiently waiting for my real computer to get back from a repair 😅

Re the other thing -- yep there's definitely something fishy. I'll check what SDL does also, I remember at least one of the two had it working correctly at some point.

[rune-scape]

thank you!

[mosra]

The overrides should be fixed in 65743b5, thanks a lot for reporting this -- somehow it was completely broken right from the start :/

Virtual DPI scaling in GLFW on Windows is implemented in 444b925, which makes #243 almost done, except for DPI change events.