How to change logical DPI for QML app?

Question

I want to control the pointSize-to-pixel scaling of all Text elements in my Qt 5.2.1 QML/C++ app.

I have a QML singleton component with target display properties like width, height and dot pitch that calculates appropriate pixel sizes for common dimensions such as the recommended size in pixels for a touchscreen button. This works fine for controls I write, but the Text element has a perfectly good font.pointSize that I would like to use if I could just set the logical DPI used for text scaling.

I use this to simulate target devices with very different screen DPI while debugging on my 96 DPI development screen. For example, I would like to run my app that targets a 1280x720 133DPI display and has an element like:

Text { font.pointSize: 72; text: “Xy” }

display 133 pixels tall, not the 96 pixel tall text I get because the OS tells Qt that I have a 96 DPI monitor attached. I want to override the logical DPI scaling for my application.

I can see the logicalDotsPerInchX through the QGuiApplication QScreen list. There is tons of documentation on how to get the logical DPI. But I cannot find any information on how to change it.

How can I change the logical DPI for my Qt app?

Answer 1

I don't think you can simply change the DPI values in the QScreen class (there are only public getter methods). Maybe there are some "hacks" for that problem if you modify the QScreen class and add a public setter or something yourself and then recompile Qt, but that might be some work and takes a lot of time...

Another solution might be to just add a scale factor to your font sizes? I don't know how many you have but that can be simply done in QML and I use something like that even for production setting to scale the fonts and other sizes properly to the device. You could use Screen.pixelDensityor just define your custom QML property and multiply that to all font sizes? if you do that you can dynamically change the sizes while the app is running.