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