Get the coordinate of specific character in a text givent font and string

[Violet Giraffe]

I have a text (QString), and a QFont that I will use to render this text on the screen. How can I found out either the center coordinate or the bounding rect for a character with index n in the QString?

[A Former User]

@Violet-Giraffe Hi! QFontMetricsF provides the functionality you need.

[Violet Giraffe]

@Wieland, thanks for the quick reply, but I don't see which QFontMetricsF method does that (nor do I see any difference from the regular QFontMetrics, other than using floating-point values).

[A Former User]

@Violet-Giraffe Yeah, it doesn't provide a ready-to-use function for exactly what you want but it has QRectF boundingRect(const QString &text) const and with this you can incrementally compute all said coordinates.

[Violet Giraffe]

I see! So iterate the text one QChar at a time and add up the bounding rects for each one up until the target character?

[kshegunov]

@Violet-Giraffe
I think you misunderstood, there's no need for iteration. You can get the character rectangle directly, there's only one additional step to get that rectangle's offset (provided your string doesn't have newlines, tabs and such). For example:

QFont font; //< The font you're using.
QFontMetrics metrics(font);

QString string = "mystring";
int n = 4;

QRect left = metrics.boundingRect(string.mid(0, n));
QRect character = metrics.boundingRect(string.at(n));

qint32 centerX = left.width() + (character.width() >> 1);
qint32 centerY = left.height() >> 1;  // Or character.height() divided by two, they should be the same though

Also you can look up this thread.

[Violet Giraffe]

@kshegunov said:

I think you misunderstood, there's no need for iteration.

It only occurred to me after I went to sleep last night. That's why coding after midnight is not always a great idea :)

@kshegunov said:

qint32 centerY = left.height() >> 1;  // Or character.height() divided by two, they should be the same though

Isn't any modern C++ compiler capable of doing that optimization implicitly (with optimizations enabled)?

[kshegunov]

@Violet-Giraffe said:

Isn't any modern C++ compiler capable of doing that optimization implicitly (with optimizations enabled)?

In all probability, yes; it's just a stupid habit. Compilers weren't so smart before, but they are improving by the minute. You can safely replace it with left.height() / 2.