Scaled Fonts

Scaled Fonts — Caching metrics for a particular font size

Synopsis




typedef     cairo_scaled_font_t;
            cairo_font_extents_t;
            cairo_text_extents_t;

Description

Details

cairo_scaled_font_t

typedef struct _cairo_scaled_font cairo_scaled_font_t;

A cairo_scaled_font_t is a font scaled to a particular size and device resolution. A cairo_scaled_font_t is most useful for low-level font usage where a library or application wants to cache a reference to a scaled font to speed up the computation of metrics.


cairo_font_extents_t

typedef struct {
    double ascent;
    double descent;
    double height;
    double max_x_advance;
    double max_y_advance;
} cairo_font_extents_t;

The cairo_text_extents_t structure stores metric information for a font. Values are given in the current user-space coordinate system.

Because font metrics are in user-space coordinates, they are mostly, but not entirely, independent of the current transformation matrix. If you call cairo_scale(cr, 2.0, 2.0), text will be drawn twice as big, but the reported text extents will not be doubled. They will change slightly due to hinting (so you can't assume that metrics are independent of the transformation matrix), but otherwise will remain unchanged.

double ascent; the distance that the font extends above the baseline. Note that this is not always exactly equal to the maximum of the extents of all the glyphs in the font, but rather is picked to express the font designer's intent as to how the font should align with elements above it.
double descent; the distance that the font extends below the baseline. This value is positive for typical fonts that include portions below the baseline. Note that this is not always exactly equal to the maximum of the extents of all the glyphs in the font, but rather is picked to express the font designer's intent as to how the the font should align with elements below it.
double height; the recommended vertical distance between baselines when setting consecutive lines of text with the font. This is greater than ascent+descent by a quantity known as the line spacing or external leading. When space is at a premium, most fonts can be set with only a distance of ascent+descent between lines.
double max_x_advance; the maximum distance in the X direction that the the origin is advanced for any glyph in the font.
double max_y_advance; the maximum distance in the Y direction that the the origin is advanced for any glyph in the font. this will be zero for normal fonts used for horizontal writing. (The scripts of East Asia are sometimes written vertically.)

cairo_text_extents_t

typedef struct {
    double x_bearing;
    double y_bearing;
    double width;
    double height;
    double x_advance;
    double y_advance;
} cairo_text_extents_t;

The cairo_text_extents_t structure stores the extents of a single glyph or a string of glyphs in user-space coordinates. Because text extents are in user-space coordinates, they are mostly, but not entirely, independent of the current transformation matrix. If you call cairo_scale(cr, 2.0, 2.0), text will be drawn twice as big, but the reported text extents will not be doubled. They will change slightly due to hinting (so you can't assume that metrics are independent of the transformation matrix), but otherwise will remain unchanged.

double x_bearing; the horizontal distance from the origin to the leftmost part of the glyphs as drawn. Positive if the glyphs lie entirely to the right of the origin.
double y_bearing; the vertical distance from the origin to the topmost part of the glyphs as drawn. Positive only if the glyphs lie completely below the origin; will usually be negative.
double width; width of the glyphs as drawn
double height; height of the glyphs as drawn
double x_advance; distance to advance in the X direction after drawing these glyphs
double y_advance; distance to advance in the Y direction after drawing these glyphs. Will typically be zero except for vertical text layout as found in East-Asian languages.