[Development] QFontMetrics and QScriptLine height calculation

[Konstantin Ritt]

First of all, which Qt version you're talking about?
I assume it is 4.8 since QAccessibleTextWidget has been reworked alot
in 5.0 and the problem you've described seems to gone there (see
QAccessibleTextWidget::characterRect() impl).
If it is 5.0, then you're probably subclassing the accessibility
widgets, are you?

Briefly:
the QTextDocumentLayout sets the leadingIncluded to true for every
line in a blocks (re-)layouting code and thus the line's height equals
to QFontMetrics::lineSpacing(), whilst QFontMetrics::height() better
suites to what characterRect() should return as a rect's height.

Anyways, more info required...


Konstantin


2012/10/4 Rafael Roquetto <rafael.roquetto at kdab.com>:
> Hello,
>
> I am trying to fix tst_QAccessibility for QNX, and I have a problem on line
> 2750:
>
> QCOMPARE(iface->textInterface()->characterRect(0,
> QAccessible2::RelativeToParent).size(), QSize(fm.width("h"), fm.height()));
>
> This comparison is failing, because the caracter rect height is bigger than
> the height returned by QFontMetrics. And...?
>
> Well, ultimately the height of the character rect comes from
> QScriptLine::height(). Here is how it is calculated (simplified):
>
>     height = ascent + descent + 1 (leadingIncluded ? leading : QFixed())
>
> While QFontMetrics does not use 'leading' to compute 'height' (simplified):
>
>     height = ascent + descent + 1
>
>
> Which leads to a few questions:
>
> 1. Is this meant to be like that or is either class implementation incorrect?
>
> 2. If this is correct, then is the test erroneusly assuming that
> QFontMetrics::height() == QScriptLine::height() for every use case? Or...
>
> 3. Should the 'leading' be '0' in the context of the test (what happens in Qt
> built for Linux, but not for QNX)?
>
> Well, I will continue to investigate it, but since I do not know much about
> this corner of Qt, I thought it wouldn't hurt to ask :)
>
> Thanks a lot in advance,
> Rafael
>
> --
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