How to understand logical DPI and physical DPI in QScreen?

[willxli]

Hi all:
I can't understand some concepts in QScreen.

What is logical DPI and physical DPI ?
What is difference between logical DPI and physical DPI ?

Please give some hints， Thank you.

[raven-worx]

@willxli
read the docs or precise your question

[willxli]

@raven-worx I update the question, thank you for reply.

[raven-worx]

@willxli said in How to understand logical DPI and physical DPI in QScreen?:

@raven-worx I update the question, thank you for reply.

in my opinion you didnt read the docs nor you rewrote your question to be more precise :)

[Pl45m4]

@willxli said in How to understand logical DPI and physical DPI in QScreen?:

What is logical DPI and physical DPI ?

Physical DPI:
DPI of your screen/device

Logical DPI:
Appearance of your font or anything you display.

• https://doc.qt.io/qt-5/qscreen.html#details

Let's say you have a line with a length of 3px (3x1)... and it actually takes 3 physical pixels of "space" on your screen. If you double your avaiable physical pixels, your line and anything else would appear way smaller on the screen (-> resolution). Therefore you need to map logical pixels of your content to correct pixels of your screen (new width and height).
To stay with the example of a 3px line... If you double your physical pixels, you need to increase the logical pixels the line takes from 3 to 6 to make it look the same.
[Edit: You don't increase the logical pixels.. You just map the 3 (3x1) logical pixels to 6 (6x2) physical pixels of your new screen]

I hope this is understandable... Correct me if I said something wrong here :)

[SimonSchroeder]

If I am not mistaken the relation between the physical and logical pixels is the scaling factor (usually set by the operating system). For example, Apple's Retina displays usually have a scaling of 200%. Which means that for every logical pixel there are 2 physical pixels (actually 2 in x- and y-direction each; which makes 4 in total) inside your display.

You should try to change the scaling in your OS and then write a small toy program to see what physical and logical DPI you get. The physical DPI should be fixed.