Source File
	aat_layout_trak.go

Belonging Package
	github.com/go-text/typesetting/harfbuzz

package harfbuzz

// https://developer.apple.com/documentation/coretext/1508745-ctfontcreatewithgraphicsfont
const coretextDefaultFontSize = 12.

func (font *Font) getHTracking(track float32) Position {
	ptem := font.Ptem
	if ptem <= 0. {
		ptem = coretextDefaultFontSize
	}
	tr := font.face.Trak.Horiz.GetTracking(ptem, track)
	return font.emScalefX(tr)
}

func (font *Font) getVTracking(track float32) Position {
	ptem := font.Ptem
	if ptem <= 0. {
		ptem = coretextDefaultFontSize
	}
	tr := font.face.Trak.Vert.GetTracking(ptem, track)
	return font.emScalefY(tr)
}

// track default to 0
func (c *aatApplyContext) applyTrak(  track float32) {
	ptem := c.font.Ptem
	if ptem <= 0. {
		// https://developer.apple.com/documentation/coretext/1508745-ctfontcreatewithgraphicsfont
		ptem = coretextDefaultFontSize
	}

	buffer := c.buffer
	if buffer.Props.Direction.isHorizontal() {
		advanceToAdd := c.font.getHTracking(track)
		iter, count := buffer.graphemesIterator()
		for start, _ := iter.next(); start < count; start, _ = iter.next() {
			buffer.Pos[start].XAdvance += advanceToAdd
		}

	} else {
		advanceToAdd := c.font.getVTracking(track)
		iter, count := buffer.graphemesIterator()
		for start, _ := iter.next(); start < count; start, _ = iter.next() {
			buffer.Pos[start].YAdvance += advanceToAdd
		}
	}
}

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