skrifa/outline/path.rs
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//! TrueType style outline to path conversion.
use super::pen::{OutlinePen, PathStyle};
use core::fmt;
use raw::{
tables::glyf::{PointCoord, PointFlags},
types::Point,
};
/// Errors that can occur when converting an outline to a path.
#[derive(Clone, Debug)]
pub enum ToPathError {
/// Contour end point at this index was less than its preceding end point.
ContourOrder(usize),
/// Expected a quadratic off-curve point at this index.
ExpectedQuad(usize),
/// Expected a quadratic off-curve or on-curve point at this index.
ExpectedQuadOrOnCurve(usize),
/// Expected a cubic off-curve point at this index.
ExpectedCubic(usize),
/// Expected number of points to == number of flags
PointFlagMismatch { num_points: usize, num_flags: usize },
}
impl fmt::Display for ToPathError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
match self {
Self::ContourOrder(ix) => write!(
f,
"Contour end point at index {ix} was less than preceding end point"
),
Self::ExpectedQuad(ix) => write!(f, "Expected quadatic off-curve point at index {ix}"),
Self::ExpectedQuadOrOnCurve(ix) => write!(
f,
"Expected quadatic off-curve or on-curve point at index {ix}"
),
Self::ExpectedCubic(ix) => write!(f, "Expected cubic off-curve point at index {ix}"),
Self::PointFlagMismatch {
num_points,
num_flags,
} => write!(
f,
"Number of points ({num_points}) and flags ({num_flags}) must match"
),
}
}
}
/// Converts a `glyf` outline described by points, flags and contour end points
/// to a sequence of path elements and invokes the appropriate callback on the
/// given pen for each.
///
/// The input points can have any coordinate type that implements
/// [`PointCoord`]. Output points are always generated in `f32`.
///
/// This is roughly equivalent to [`FT_Outline_Decompose`](https://freetype.org/freetype2/docs/reference/ft2-outline_processing.html#ft_outline_decompose).
///
/// See [`contour_to_path`] for a more general function that takes an iterator
/// if your outline data is in a different format.
pub(crate) fn to_path<C: PointCoord>(
points: &[Point<C>],
flags: &[PointFlags],
contours: &[u16],
path_style: PathStyle,
pen: &mut impl OutlinePen,
) -> Result<(), ToPathError> {
for contour_ix in 0..contours.len() {
let start_ix = (contour_ix > 0)
.then(|| contours[contour_ix - 1] as usize + 1)
.unwrap_or_default();
let end_ix = contours[contour_ix] as usize;
if end_ix < start_ix || end_ix >= points.len() {
return Err(ToPathError::ContourOrder(contour_ix));
}
let points = &points[start_ix..=end_ix];
if points.is_empty() {
continue;
}
let flags = flags
.get(start_ix..=end_ix)
.ok_or(ToPathError::PointFlagMismatch {
num_points: points.len(),
num_flags: flags.len(),
})?;
let last_point = points.last().unwrap();
let last_flags = flags.last().unwrap();
let last_point = ContourPoint {
x: last_point.x,
y: last_point.y,
flags: *last_flags,
};
contour_to_path(
points.iter().zip(flags).map(|(point, flags)| ContourPoint {
x: point.x,
y: point.y,
flags: *flags,
}),
last_point,
path_style,
pen,
)
.map_err(|e| match &e {
ToPathError::ExpectedCubic(ix) => ToPathError::ExpectedCubic(ix + start_ix),
ToPathError::ExpectedQuad(ix) => ToPathError::ExpectedQuad(ix + start_ix),
ToPathError::ExpectedQuadOrOnCurve(ix) => {
ToPathError::ExpectedQuadOrOnCurve(ix + start_ix)
}
_ => e,
})?
}
Ok(())
}
/// Combination of point coordinates and flags.
#[derive(Copy, Clone, Default, Debug)]
pub(crate) struct ContourPoint<T> {
pub x: T,
pub y: T,
pub flags: PointFlags,
}
impl<T> ContourPoint<T>
where
T: PointCoord,
{
fn point_f32(&self) -> Point<f32> {
Point::new(self.x.to_f32(), self.y.to_f32())
}
fn midpoint(&self, other: Self) -> ContourPoint<T> {
let (x, y) = (self.x.midpoint(other.x), self.y.midpoint(other.y));
Self {
x,
y,
flags: other.flags,
}
}
}
/// Generates a path from an iterator of contour points.
///
/// Note that this requires the last point of the contour to be passed
/// separately to support FreeType style path conversion when the contour
/// begins with an off curve point. The points iterator should still
/// yield the last point as well.
///
/// This is more general than [`to_path`] and exists to support cases (such as
/// autohinting) where the source outline data is in a different format.
pub(crate) fn contour_to_path<C: PointCoord>(
points: impl Iterator<Item = ContourPoint<C>>,
last_point: ContourPoint<C>,
style: PathStyle,
pen: &mut impl OutlinePen,
) -> Result<(), ToPathError> {
let mut points = points.enumerate().peekable();
let Some((_, first_point)) = points.peek().copied() else {
// This is an empty contour
return Ok(());
};
// We don't accept an off curve cubic as the first point
if first_point.flags.is_off_curve_cubic() {
return Err(ToPathError::ExpectedQuadOrOnCurve(0));
}
// For FreeType style, we may need to omit the last point if we find the
// first on curve there
let mut omit_last = false;
// For HarfBuzz style, may skip up to two points in finding the start, so
// process these at the end
let mut trailing_points = [None; 2];
// Find our starting point
let start_point = if first_point.flags.is_off_curve_quad() {
// We're starting with an off curve, so select our first move based on
// the path style
match style {
PathStyle::FreeType => {
if last_point.flags.is_on_curve() {
// The last point is an on curve, so let's start there
omit_last = true;
last_point
} else {
// It's also an off curve, so take implicit midpoint
last_point.midpoint(first_point)
}
}
PathStyle::HarfBuzz => {
// Always consume the first point
points.next();
// Then check the next point
let Some((_, next_point)) = points.peek().copied() else {
// This is a single point contour
return Ok(());
};
if next_point.flags.is_on_curve() {
points.next();
trailing_points = [Some((0, first_point)), Some((1, next_point))];
// Next is on curve, so let's start there
next_point
} else {
// It's also an off curve, so take the implicit midpoint
trailing_points = [Some((0, first_point)), None];
first_point.midpoint(next_point)
}
}
}
} else {
// We're starting with an on curve, so consume the point
points.next();
first_point
};
let point = start_point.point_f32();
pen.move_to(point.x, point.y);
let mut state = PendingState::default();
if omit_last {
while let Some((ix, point)) = points.next() {
if points.peek().is_none() {
break;
}
state.emit(ix, point, pen)?;
}
} else {
for (ix, point) in points {
state.emit(ix, point, pen)?;
}
}
for (ix, point) in trailing_points.iter().filter_map(|x| *x) {
state.emit(ix, point, pen)?;
}
state.finish(0, start_point, pen)?;
Ok(())
}
#[derive(Copy, Clone, Default)]
enum PendingState<C> {
/// No pending points.
#[default]
Empty,
/// Pending off-curve quad point.
PendingQuad(ContourPoint<C>),
/// Single pending off-curve cubic point.
PendingCubic(ContourPoint<C>),
/// Two pending off-curve cubic points.
TwoPendingCubics(ContourPoint<C>, ContourPoint<C>),
}
impl<C> PendingState<C>
where
C: PointCoord,
{
#[inline(always)]
fn emit(
&mut self,
ix: usize,
point: ContourPoint<C>,
pen: &mut impl OutlinePen,
) -> Result<(), ToPathError> {
let flags = point.flags;
match *self {
Self::Empty => {
if flags.is_off_curve_quad() {
*self = Self::PendingQuad(point);
} else if flags.is_off_curve_cubic() {
*self = Self::PendingCubic(point);
} else {
let p = point.point_f32();
pen.line_to(p.x, p.y);
}
}
Self::PendingQuad(quad) => {
if flags.is_off_curve_quad() {
let c0 = quad.point_f32();
let p = quad.midpoint(point).point_f32();
pen.quad_to(c0.x, c0.y, p.x, p.y);
*self = Self::PendingQuad(point);
} else if flags.is_off_curve_cubic() {
return Err(ToPathError::ExpectedQuadOrOnCurve(ix));
} else {
let c0 = quad.point_f32();
let p = point.point_f32();
pen.quad_to(c0.x, c0.y, p.x, p.y);
*self = Self::Empty;
}
}
Self::PendingCubic(cubic) => {
if flags.is_off_curve_cubic() {
*self = Self::TwoPendingCubics(cubic, point);
} else {
return Err(ToPathError::ExpectedCubic(ix));
}
}
Self::TwoPendingCubics(cubic0, cubic1) => {
if flags.is_off_curve_quad() {
return Err(ToPathError::ExpectedCubic(ix));
} else if flags.is_off_curve_cubic() {
let c0 = cubic0.point_f32();
let c1 = cubic1.point_f32();
let p = cubic1.midpoint(point).point_f32();
pen.curve_to(c0.x, c0.y, c1.x, c1.y, p.x, p.y);
*self = Self::PendingCubic(point);
} else {
let c0 = cubic0.point_f32();
let c1 = cubic1.point_f32();
let p = point.point_f32();
pen.curve_to(c0.x, c0.y, c1.x, c1.y, p.x, p.y);
*self = Self::Empty;
}
}
}
Ok(())
}
fn finish(
mut self,
start_ix: usize,
mut start_point: ContourPoint<C>,
pen: &mut impl OutlinePen,
) -> Result<(), ToPathError> {
match self {
Self::Empty => {}
_ => {
// We always want to end with an explicit on-curve
start_point.flags = PointFlags::on_curve();
self.emit(start_ix, start_point, pen)?;
}
}
pen.close();
Ok(())
}
}
#[cfg(test)]
mod tests {
use super::{super::pen::SvgPen, *};
use raw::types::F26Dot6;
fn assert_off_curve_path_to_svg(expected: &str, path_style: PathStyle, all_off_curve: bool) {
fn pt(x: i32, y: i32) -> Point<F26Dot6> {
Point::new(x, y).map(F26Dot6::from_bits)
}
let mut flags = [PointFlags::off_curve_quad(); 4];
if !all_off_curve {
flags[1] = PointFlags::on_curve();
}
let contours = [3];
// This test is meant to prevent a bug where the first move-to was computed improperly
// for a contour consisting of all off curve points.
// In this case, the start of the path should be the midpoint between the first and last points.
// For this test case (in 26.6 fixed point): [(640, 128) + (128, 128)] / 2 = (384, 128)
// which becomes (6.0, 2.0) when converted to floating point.
let points = [pt(640, 128), pt(256, 64), pt(640, 64), pt(128, 128)];
let mut pen = SvgPen::with_precision(1);
to_path(&points, &flags, &contours, path_style, &mut pen).unwrap();
assert_eq!(pen.as_ref(), expected);
}
#[test]
fn all_off_curve_to_path_scan_backward() {
assert_off_curve_path_to_svg(
"M6.0,2.0 Q10.0,2.0 7.0,1.5 Q4.0,1.0 7.0,1.0 Q10.0,1.0 6.0,1.5 Q2.0,2.0 6.0,2.0 Z",
PathStyle::FreeType,
true,
);
}
#[test]
fn all_off_curve_to_path_scan_forward() {
assert_off_curve_path_to_svg(
"M7.0,1.5 Q4.0,1.0 7.0,1.0 Q10.0,1.0 6.0,1.5 Q2.0,2.0 6.0,2.0 Q10.0,2.0 7.0,1.5 Z",
PathStyle::HarfBuzz,
true,
);
}
#[test]
fn start_off_curve_to_path_scan_backward() {
assert_off_curve_path_to_svg(
"M6.0,2.0 Q10.0,2.0 4.0,1.0 Q10.0,1.0 6.0,1.5 Q2.0,2.0 6.0,2.0 Z",
PathStyle::FreeType,
false,
);
}
#[test]
fn start_off_curve_to_path_scan_forward() {
assert_off_curve_path_to_svg(
"M4.0,1.0 Q10.0,1.0 6.0,1.5 Q2.0,2.0 6.0,2.0 Q10.0,2.0 4.0,1.0 Z",
PathStyle::HarfBuzz,
false,
);
}
}