Rendering to physical sizes, how to let it work on all platforms?

[RaDq1]

I'm creating an application that should render things to physical sizes, i.e. an intermediate goal would be to render a square that is 4x3 cm on all displays (using both iOS & windows).

(I understand that its not really possible to do this always correct as the real world size of screens varies)

the problem I ran into today is that on iOS there is a 2x factor which I can't identify by using generic calls.
QGuiApplication::primaryScreen()->devicePixelRatio()
returns 1 on a iPad (incorrect? - iOS 9.3) and 2 on a iPhone (correct? - also iOS 9.3)
or am i missing something?

using a QQuickPaintedItem derived class in the paint(QPainter *painter) function:

auto physicalpixelsperinch = painter->device->physicalDpiX() * QGuiApplication::primaryScreen()->devicePixelRatio();

is (more or less) correct for windows and a iPhone, but not for an iPad.
what I am I missing here?

[johngod]

Hi
On Desktop and Android, something like this should work:

import QtQuick.Window 2.2

Rectangle {
    property real mm: Screen.pixelDensity

    width: 40*mm
    height: 30*mm
}

The problem is that, on Android, depending on the devices, the hardware vendors dont supply reliable firmware, and usually Screen.pixelDensity doesnt give acurate values. On desktop, I usualy get correct values. Not sure abou t IOS, but please check it and let me know :)

[Lorenz]

You could check out the pixelToInches function that is included in the V-Play SDK.

[RaDq1]

@johngod Thank you so much, you're right Screen.pixelDensity would be the complete solution if it was in qml,
sifting trough the qt source i was able that it was derived from:

double pixelpermm = QGuiApplication::primaryScreen()->physicalDotsPerInch() / 25.4
//physicalDotsPerInchX() & physicalDotsPerInchY() also exist

and it seems to be correct on both my iPhone & iPad quite wel (better than all other ways I found so far)

@Lorenz thanks, but for the time being I want to stick to Qt as it is a paid sdk as soon as you go commercial