Detailed Description
The QFontMetrics class provides font metrics information.
QFontMetrics functions calculate the size of characters and strings for a given font. There are three ways you can create a QFontMetrics object:
- Calling the QFontMetrics constructor with a QFont creates a font metrics object for a screen-compatible font, i.e. the font cannot be a printer font. If the font is changed later, the font metrics object is not updated.
(Note: If you use a printer font the values returned may be inaccurate. Printer fonts are not always accessible so the nearest screen font is used if a printer font is supplied.)
- QWidget::fontMetrics() returns the font metrics for a widget's font. This is equivalent to QFontMetrics(widget->font()). If the widget's font is changed later, the font metrics object is not updated.
- QPainter::fontMetrics() returns the font metrics for a painter's current font. If the painter's font is changed later, the font metrics object is not updated.
Once created, the object provides functions to access the individual metrics of the font, its characters, and for strings rendered in the font.
There are several functions that operate on the font: ascent(), descent(), height(), leading() and lineSpacing() return the basic size properties of the font. The underlinePos(), overlinePos(), strikeOutPos() and lineWidth() functions, return the properties of the line that underlines, overlines or strikes out the characters. These functions are all fast.
There are also some functions that operate on the set of glyphs in the font: minLeftBearing(), minRightBearing() and maxWidth(). These are by necessity slow, and we recommend avoiding them if possible.
For each character, you can get its width(), leftBearing() and rightBearing() and find out whether it is in the font using inFont(). You can also treat the character as a string, and use the string functions on it.
The string functions include width(), to return the width of a string in pixels (or points, for a printer), boundingRect(), to return a rectangle large enough to contain the rendered string, and size(), to return the size of that rectangle.
Example:
QFont font("times", 24);
QFontMetrics fm(font);
int pixelsWide = fm.width("What's the width of this text?");
int pixelsHigh = fm.height();
See also QFont, QFontInfo, and QFontDatabase.
Member Function Documentation
QFontMetrics::QFontMetrics ( const QFont & font )
Constructs a font metrics object for font.
The font metrics will be compatible with the paintdevice used to create font.
The font metrics object holds the information for the font that is passed in the constructor at the time it is created, and is not updated if the font's attributes are changed later.
Use QFontMetrics(const QFont &, QPaintDevice *) to get the font metrics that are compatible with a certain paint device.
QFontMetrics::QFontMetrics ( const QFont & font, QPaintDevice * paintdevice )
Constructs a font metrics object for font and paintdevice.
The font metrics will be compatible with the paintdevice passed. If the paintdevice is 0, the metrics will be screen-compatible, ie. the metrics you get if you use the font for drawing text on a widgets or pixmaps, not on a QPicture or QPrinter.
The font metrics object holds the information for the font that is passed in the constructor at the time it is created, and is not updated if the font's attributes are changed later.
QFontMetrics::QFontMetrics ( const QFontMetrics & fm )
Constructs a copy of fm.
QFontMetrics::~QFontMetrics ()
Destroys the font metrics object and frees all allocated resources.
int QFontMetrics::ascent () const
Returns the ascent of the font.
The ascent of a font is the distance from the baseline to the highest position characters extend to. In practice, some font designers break this rule, e.g. when they put more than one accent on top of a character, or to accommodate an unusual character in an exotic language, so it is possible (though rare) that this value will be too small.
See also descent().
QRect QFontMetrics::boundingRect ( QChar ch ) const
Returns the rectangle that is covered by ink if the character where to be drawn at the origin of the coordinate system.
Note that the bounding rectangle may extend to the left of (0, 0), e.g. for italicized fonts, and that the text output may cover all pixels in the bounding rectangle. For a space character the rectangle will usually be empty.
Note that the rectangle usually extends both above and below the base line.
Warning: The width of the returned rectangle is not the advance width of the character. Use boundingRect(const QString &) or width() instead.
See also width().
QRect QFontMetrics::boundingRect ( const QString & str ) const
This is an overloaded member function, provided for convenience.
Returns the bounding rectangle of the characters in the string specified by str, which is the set of pixels the text would cover if drawn at (0, 0).
Note that the bounding rectangle may extend to the left of (0, 0), e.g. for italicized fonts, and that the text output may cover all pixels in the bounding rectangle.
Newline characters are processed as normal characters, not as linebreaks.
Due to the different actual character heights, the height of the bounding rectangle of e.g. "Yes" and "yes" may be different.
See also width() and QPainter::boundingRect().
QRect QFontMetrics::boundingRect ( int x, int y, int width, int height, int flags, const QString & text, int tabstops = 0, int * tabarray = 0 ) const
This is an overloaded member function, provided for convenience.
Returns the bounding rectangle for the given text within the rectangle specified by the x and y coordinates, width, and height.
If Qt::TextExpandTabs is set in flags and tabarray is non-null, it specifies a 0-terminated sequence of pixel-positions for tabs; otherwise, if tabstops is non-zero, it is used as the tab spacing (in pixels).
QRect QFontMetrics::boundingRect ( const QRect & r, int flgs, const QString & str, int tabstops = 0, int * tabarray = 0 ) const
This is an overloaded member function, provided for convenience.
Returns the bounding rectangle of the characters in the string specified by str, which is the set of pixels the text would cover if drawn at (0, 0). The drawing, and hence the bounding rectangle, is constrained to the rectangle r.
The flgs argument is the bitwise OR of the following flags:
Qt::Horizontal alignment defaults to Qt::AlignLeft and vertical alignment defaults to Qt::AlignTop.
If several of the horizontal or several of the vertical alignment flags are set, the resulting alignment is undefined.
These flags are defined in qnamespace.h.
If Qt::TextExpandTabs is set in flgs, then: if tabarray is non-null, it specifies a 0-terminated sequence of pixel-positions for tabs; otherwise if tabstops is non-zero, it is used as the tab spacing (in pixels).
Note that the bounding rectangle may extend to the left of (0, 0), e.g. for italicized fonts, and that the text output may cover all pixels in the bounding rectangle.
Newline characters are processed as linebreaks.
Despite the different actual character heights, the heights of the bounding rectangles of "Yes" and "yes" are the same.
The bounding rectangle returned by this function is somewhat larger than that calculated by the simpler boundingRect() function. This function uses the maximum left and right font bearings as is necessary for multi-line text to align correctly. Also, fontHeight() and lineSpacing() are used to calculate the height, rather than individual character heights.
See also width(), QPainter::boundingRect(), and Qt::Alignment.
int QFontMetrics::charWidth ( const QString & str, int pos ) const
Returns the width of the character at position pos in the string str.
The whole string is needed, as the glyph drawn may change depending on the context (the letter before and after the current one) for some languages (e.g. Arabic).
This function also takes non spacing marks and ligatures into account.
int QFontMetrics::descent () const
Returns the descent of the font.
The descent is the distance from the base line to the lowest point characters extend to. (Note that this is different from X, which adds 1 pixel.) In practice, some font designers break this rule, e.g. to accommodate an unusual character in an exotic language, so it is possible (though rare) that this value will be too small.
See also ascent().
int QFontMetrics::height () const
Returns the height of the font.
This is always equal to ascent()+descent()+1 (the 1 is for the base line).
See also leading() and lineSpacing().
bool QFontMetrics::inFont ( QChar ch ) const
Returns true if character ch is a valid character in the font; otherwise returns false.
int QFontMetrics::leading () const
Returns the leading of the font.
This is the natural inter-line spacing.
See also height() and lineSpacing().
int QFontMetrics::leftBearing ( QChar ch ) const
Returns the left bearing of character ch in the font.
The left bearing is the right-ward distance of the left-most pixel of the character from the logical origin of the character. This value is negative if the pixels of the character extend to the left of the logical origin.
See width(QChar) for a graphical description of this metric.
See also rightBearing(), minLeftBearing(), and width().
int QFontMetrics::lineSpacing () const
Returns the distance from one base line to the next.
This value is always equal to leading()+height().
See also height() and leading().
int QFontMetrics::lineWidth () const
Returns the width of the underline and strikeout lines, adjusted for the point size of the font.
See also underlinePos(), overlinePos(), and strikeOutPos().
int QFontMetrics::maxWidth () const
Returns the width of the widest character in the font.
int QFontMetrics::minLeftBearing () const
Returns the minimum left bearing of the font.
This is the smallest leftBearing(char) of all characters in the font.
Note that this function can be very slow if the font is large.
See also minRightBearing() and leftBearing().
int QFontMetrics::minRightBearing () const
Returns the minimum right bearing of the font.
This is the smallest rightBearing(char) of all characters in the font.
Note that this function can be very slow if the font is large.
See also minLeftBearing() and rightBearing().
int QFontMetrics::overlinePos () const
Returns the distance from the base line to where an overline should be drawn.
See also underlinePos(), strikeOutPos(), and lineWidth().
int QFontMetrics::rightBearing ( QChar ch ) const
Returns the right bearing of character ch in the font.
The right bearing is the left-ward distance of the right-most pixel of the character from the logical origin of a subsequent character. This value is negative if the pixels of the character extend to the right of the width() of the character.
See width() for a graphical description of this metric.
See also leftBearing(), minRightBearing(), and width().
QSize QFontMetrics::size ( int flgs, const QString & text, int tabstops = 0, int * tabarray = 0 ) const
Returns the size in pixels of text.
The flgs argument is the bitwise OR of the following flags:
These flags are defined in qnamespace.h.
If Qt::TextExpandTabs is set in flgs, then: if tabarray is non-null, it specifies a 0-terminated sequence of pixel-positions for tabs; otherwise if tabstops is non-zero, it is used as the tab spacing (in pixels).
Newline characters are processed as linebreaks.
Despite the different actual character heights, the heights of the bounding rectangles of "Yes" and "yes" are the same.
See also boundingRect().
int QFontMetrics::strikeOutPos () const
Returns the distance from the base line to where the strikeout line should be drawn.
See also underlinePos(), overlinePos(), and lineWidth().
int QFontMetrics::underlinePos () const
Returns the distance from the base line to where an underscore should be drawn.
See also overlinePos(), strikeOutPos(), and lineWidth().
int QFontMetrics::width ( const QString & str, int len = -1 ) const
Returns the width in pixels of the first len characters of str. If len is negative (the default), the entire string is used.
Note that this value is not equal to boundingRect().width(); boundingRect() returns a rectangle describing the pixels this string will cover whereas width() returns the distance to where the next string should be drawn.
See also boundingRect().
int QFontMetrics::width ( QChar ch ) const
This is an overloaded member function, provided for convenience.
Returns the logical width of character ch in pixels. This is a distance appropriate for drawing a subsequent character after ch.
Some of the metrics are described in the image to the right. The central dark rectangles cover the logical width() of each character. The outer pale rectangles cover the leftBearing() and rightBearing() of each character. Notice that the bearings of "f" in this particular font are both negative, while the bearings of "o" are both positive.
Warning: This function will produce incorrect results for Arabic characters or non-spacing marks in the middle of a string, as the glyph shaping and positioning of marks that happens when processing strings cannot be taken into account. Use charWidth() instead if you aren't looking for the width of isolated characters.
See also boundingRect() and charWidth().
int QFontMetrics::xHeight () const
Returns the 'x' height of the font. This is often but not always the same as the height of the character 'x'.
bool QFontMetrics::operator!= ( const QFontMetrics & other )
Returns true if other is not equal to this object; otherwise returns false.
Two font metrics are considered equal if they were constructed from the same QFont and the paint devices they were constructed for are considered compatible.
See also operator==().
QFontMetrics & QFontMetrics::operator= ( const QFontMetrics & fm )
Assigns the font metrics fm.
bool QFontMetrics::operator== ( const QFontMetrics & other )
Returns true if other is equal to this object; otherwise returns false.
Two font metrics are considered equal if they were constructed from the same QFont and the paint devices they were constructed for are considered compatible.
See also operator!=().
bool QFontMetrics::operator== ( const QFontMetrics & other ) const
This is an overloaded member function, provided for convenience.
Returns true if other is equal to this object; otherwise returns false.
Two font metrics are considered equal if they were constructed from the same QFont and the paint devices they were constructed for are considered compatible.
See also operator!=().
