// rasterize returns the glyph mask and integer-pixel offset to render the
// given glyph at the given sub-pixel offsets.
// The 24.8 fixed point arguments fx and fy must be in the range [0, 1).
func (c *Context) rasterize(glyph truetype.Index, fx, fy raster.Fix32) (*image.Alpha, image.Point, error) {
if err := c.glyphBuf.Load(c.font, glyph); err != nil {
return nil, image.ZP, err
}
// Calculate the integer-pixel bounds for the glyph.
xmin := int(fx+c.FUnitToFix32(+int(c.glyphBuf.B.XMin))) >> 8
ymin := int(fy+c.FUnitToFix32(-int(c.glyphBuf.B.YMax))) >> 8
xmax := int(fx+c.FUnitToFix32(+int(c.glyphBuf.B.XMax))+0xff) >> 8
ymax := int(fy+c.FUnitToFix32(-int(c.glyphBuf.B.YMin))+0xff) >> 8
if xmin > xmax || ymin > ymax {
return nil, image.ZP, errors.New("freetype: negative sized glyph")
}
// A TrueType's glyph's nodes can have negative co-ordinates, but the
// rasterizer clips anything left of x=0 or above y=0. xmin and ymin
// are the pixel offsets, based on the font's FUnit metrics, that let
// a negative co-ordinate in TrueType space be non-negative in
// rasterizer space. xmin and ymin are typically <= 0.
fx += raster.Fix32(-xmin << 8)
fy += raster.Fix32(-ymin << 8)
// Rasterize the glyph's vectors.
c.r.Clear()
e0 := 0
for _, e1 := range c.glyphBuf.End {
c.drawContour(c.glyphBuf.Point[e0:e1], fx, fy)
e0 = e1
}
a := image.NewAlpha(image.Rect(0, 0, xmax-xmin, ymax-ymin))
c.r.Rasterize(raster.NewAlphaSrcPainter(a))
return a, image.Point{xmin, ymin}, nil
}
// FUnitToFix32 converts the given number of FUnits into fixed point units,
// rounding to nearest.
func (c *Context) FUnitToFix32(x int) raster.Fix32 {
return raster.Fix32((x*c.scale + 128) >> 8)
}
// Pt converts from a co-ordinate pair measured in pixels to a raster.Point
// co-ordinate pair measured in raster.Fix32 units.
func Pt(x, y int) raster.Point {
return raster.Point{raster.Fix32(x << 8), raster.Fix32(y << 8)}
}
// PointToFix32 converts the given number of points (as in ``a 12 point font'')
// into fixed point units.
func (c *Context) PointToFix32(x float64) raster.Fix32 {
return raster.Fix32(x * float64(c.dpi) * (256.0 / 72.0))
}