type Drawer struct {
    // Dst is the destination image.
    Dst draw.Image
    // Src is the source image.
    Src image.Image
    // Face provides the glyph mask images.
    Face Face
    // Dot is the baseline location to draw the next glyph. The majority of the
    // affected pixels will be above and to the right of the dot, but some may
    // be below or to the left. For example, drawing a 'j' in an italic face
    // may affect pixels below and to the left of the dot.
    Dot fixed.Point26_6
}

type Face interface {
    io.Closer

    // Glyph returns the draw.DrawMask parameters (dr, mask, maskp) to draw r's
    // glyph at the sub-pixel destination location dot, and that glyph's
    // advance width.
    //
    // It returns !ok if the face does not contain a glyph for r.
    //
    // The contents of the mask image returned by one Glyph call may change
    // after the next Glyph call. Callers that want to cache the mask must make
    // a copy.
    Glyph(dot fixed.Point26_6, r rune) (
        dr image.Rectangle, mask image.Image, maskp image.Point, advance fixed.Int26_6, ok bool)

    // GlyphBounds returns the bounding box of r's glyph, drawn at a dot equal
    // to the origin, and that glyph's advance width.
    //
    // It returns !ok if the face does not contain a glyph for r.
    //
    // The glyph's ascent and descent equal -bounds.Min.Y and +bounds.Max.Y. A
    // visual depiction of what these metrics are is at
    // https://developer.apple.com/library/mac/documentation/TextFonts/Conceptual/CocoaTextArchitecture/Art/glyph_metrics_2x.png
    GlyphBounds(r rune) (bounds fixed.Rectangle26_6, advance fixed.Int26_6, ok bool)

    // GlyphAdvance returns the advance width of r's glyph.
    //
    // It returns !ok if the face does not contain a glyph for r.
    GlyphAdvance(r rune) (advance fixed.Int26_6, ok bool)

    // Kern returns the horizontal adjustment for the kerning pair (r0, r1). A
    // positive kern means to move the glyphs further apart.
    Kern(r0, r1 rune) fixed.Int26_6

    // Metrics returns the metrics for this Face.
    Metrics() Metrics
}

type Hinting int

const (
    HintingNone Hinting = iota
    HintingVertical
    HintingFull
)

type Metrics struct {
    // Height is the recommended amount of vertical space between two lines of
    // text.
    Height fixed.Int26_6

    // Ascent is the distance from the top of a line to its baseline.
    Ascent fixed.Int26_6

    // Descent is the distance from the bottom of a line to its baseline. The
    // value is typically positive, even though a descender goes below the
    // baseline.
    Descent fixed.Int26_6
}

type Stretch int

const (
    StretchUltraCondensed Stretch = -4
    StretchExtraCondensed Stretch = -3
    StretchCondensed      Stretch = -2
    StretchSemiCondensed  Stretch = -1
    StretchNormal         Stretch = +0
    StretchSemiExpanded   Stretch = +1
    StretchExpanded       Stretch = +2
    StretchExtraExpanded  Stretch = +3
    StretchUltraExpanded  Stretch = +4
)

type Style int

const (
    StyleNormal Style = iota
    StyleItalic
    StyleOblique
)

type Weight int

const (
    WeightThin       Weight = -3 // CSS font-weight value 100.
    WeightExtraLight Weight = -2 // CSS font-weight value 200.
    WeightLight      Weight = -1 // CSS font-weight value 300.
    WeightNormal     Weight = +0 // CSS font-weight value 400.
    WeightMedium     Weight = +1 // CSS font-weight value 500.
    WeightSemiBold   Weight = +2 // CSS font-weight value 600.
    WeightBold       Weight = +3 // CSS font-weight value 700.
    WeightExtraBold  Weight = +4 // CSS font-weight value 800.
    WeightBlack      Weight = +5 // CSS font-weight value 900.
)