rsvg/

length.rs

//! CSS length values.
//!
//! [`CssLength`] is the struct librsvg uses to represent CSS lengths.
//! See its documentation for examples of how to construct it.
//!
//! `CssLength` values need to know whether they will be normalized with respect to the width,
//! height, or both dimensions of the current viewport.  `CssLength` values can be signed or
//! unsigned.  So, a `CssLength` has two type parameters, [`Normalize`] and [`Validate`];
//! the full type is `CssLength<N: Normalize, V: Validate>`.  We provide [`Horizontal`],
//! [`Vertical`], and [`Both`] implementations of [`Normalize`]; these let length values know
//! how to normalize themselves with respect to the current viewport.  We also provide
//! [`Signed`] and [`Unsigned`] implementations of [`Validate`].
//!
//! For ease of use, we define two type aliases [`Length`] and [`ULength`] corresponding to
//! signed and unsigned.
//!
//! For example, the implementation of [`Circle`][crate::shapes::Circle] defines this
//! structure with fields for the `(center_x, center_y, radius)`:
//!
//! ```
//! # use rsvg::doctest_only::{Length,ULength,Horizontal,Vertical,Both};
//! pub struct Circle {
//!     cx: Length<Horizontal>,
//!     cy: Length<Vertical>,
//!     r: ULength<Both>,
//! }
//! ```
//!
//! This means that:
//!
//! * `cx` and `cy` define the center of the circle, they can be positive or negative, and
//!   they will be normalized with respect to the current viewport's width and height,
//!   respectively.  If the SVG document specified `<circle cx="50%" cy="30%">`, the values
//!   would be normalized to be at 50% of the the viewport's width, and 30% of the viewport's
//!   height.
//!
//! * `r` is non-negative and needs to be resolved against the [normalized diagonal][diag]
//!   of the current viewport.
//!
//! The `N` type parameter of `CssLength<N, I>` is enough to know how to normalize a length
//! value; the [`CssLength::to_user`] method will handle it automatically.
//!
//! [diag]: https://www.w3.org/TR/SVG/coords.html#Units

use cssparser::{Parser, Token, match_ignore_ascii_case};
use std::f64::consts::*;
use std::fmt;
use std::marker::PhantomData;

use crate::dpi::Dpi;
use crate::drawing_ctx::Viewport;
use crate::error::*;
use crate::parsers::{Parse, finite_f32};
use crate::properties::{ComputedValues, FontSize, TextOrientation, WritingMode};
use crate::rect::Rect;
use crate::viewbox::ViewBox;

/// Units for length values.
// This needs to be kept in sync with `rsvg.h:RsvgUnit`.
#[non_exhaustive]
#[repr(C)]
#[derive(Debug, PartialEq, Copy, Clone)]
pub enum LengthUnit {
    /// `1.0` means 100%
    Percent,

    /// Pixels, or the CSS default unit
    Px,

    /// Size of the current font
    Em,

    /// x-height of the current font
    Ex,

    /// Inches (25.4 mm)
    In,

    /// Centimeters
    Cm,

    /// Millimeters
    Mm,

    /// Points (1/72 inch)
    Pt,

    /// Picas (12 points)
    Pc,

    /// Advance measure of a '0' character (depends on the text orientation)
    Ch,
}

/// A CSS length value.
///
/// This is equivalent to [CSS lengths].
///
/// [CSS lengths]: https://www.w3.org/TR/CSS22/syndata.html#length-units
///
/// It is up to the calling application to convert lengths in non-pixel units (i.e. those
/// where the [`unit`][RsvgLength::unit] field is not [`LengthUnit::Px`]) into something
/// meaningful to the application.  For example, if your application knows the
/// dots-per-inch (DPI) it is using, it can convert lengths with [`unit`] in
/// [`LengthUnit::In`] or other physical units.
// Keep this in sync with rsvg.h:RsvgLength
#[repr(C)]
#[derive(Debug, PartialEq, Copy, Clone)]
pub struct RsvgLength {
    /// Numeric part of the length
    pub length: f64,

    /// Unit part of the length
    pub unit: LengthUnit,
}

impl RsvgLength {
    /// Constructs a CSS length value.
    pub fn new(l: f64, unit: LengthUnit) -> RsvgLength {
        RsvgLength { length: l, unit }
    }
}

/// Used for the `N` type parameter of `CssLength<N: Normalize, V: Validate>`.
pub trait Normalize {
    /// Computes an orientation-based scaling factor.
    ///
    /// This is used in the [`CssLength::to_user`] method to resolve lengths with percentage
    /// units; they need to be resolved with respect to the width, height, or [normalized
    /// diagonal][diag] of the current viewport.
    ///
    /// [diag]: https://www.w3.org/TR/SVG/coords.html#Units
    fn normalize(x: f64, y: f64) -> f64;
}

/// Allows declaring `CssLength<Horizontal>`.
#[derive(Debug, PartialEq, Copy, Clone)]
pub struct Horizontal;

/// Allows declaring `CssLength<Vertical>`.
#[derive(Debug, PartialEq, Copy, Clone)]
pub struct Vertical;

/// Allows declaring `CssLength<Both>`.
#[derive(Debug, PartialEq, Copy, Clone)]
pub struct Both;

impl Normalize for Horizontal {
    #[inline]
    fn normalize(x: f64, _y: f64) -> f64 {
        x
    }
}

impl Normalize for Vertical {
    #[inline]
    fn normalize(_x: f64, y: f64) -> f64 {
        y
    }
}

impl Normalize for Both {
    #[inline]
    fn normalize(x: f64, y: f64) -> f64 {
        viewport_percentage(x, y)
    }
}

/// Used for the `V` type parameter of `CssLength<N: Normalize, V: Validate>`.
pub trait Validate {
    /// Checks if the specified value is acceptable
    ///
    /// This is used when parsing a length value
    fn validate(v: f64) -> Result<f64, ValueErrorKind> {
        Ok(v)
    }
}

/// Used to implement `CssLength<N, Signed>`.
///
/// Signed lengths do not require validation, so they use the default implementation of of
/// [Validate].
#[derive(Debug, PartialEq, Copy, Clone)]
pub struct Signed;

impl Validate for Signed {}

/// Used to implement `CssLength<N, Unsigned>`.
///
/// Unsigned lengths need validation to ensure that their value is non-negative,
/// so they have a custom implementation of [Validate].
#[derive(Debug, PartialEq, Copy, Clone)]
pub struct Unsigned;

impl Validate for Unsigned {
    fn validate(v: f64) -> Result<f64, ValueErrorKind> {
        if v >= 0.0 {
            Ok(v)
        } else {
            Err(ValueErrorKind::Value(
                "value must be non-negative".to_string(),
            ))
        }
    }
}

/// A CSS length value.
///
/// This is equivalent to [CSS lengths].
///
/// [CSS lengths]: https://www.w3.org/TR/CSS22/syndata.html#length-units
///
/// `CssLength` implements the [`Parse`] trait, so it can be parsed out of a
/// [`cssparser::Parser`].
///
/// This type will be normally used through the type aliases [`Length`] and [`ULength`]
///
/// Examples of construction:
///
/// ```
/// # use rsvg::doctest_only::{Length,ULength,LengthUnit,Horizontal,Vertical,Both};
/// # use rsvg::doctest_only::Parse;
/// // Explicit type
/// let width: Length<Horizontal> = Length::new(42.0, LengthUnit::Cm);
///
/// // Inferred type
/// let height = Length::<Vertical>::new(42.0, LengthUnit::Cm);
///
/// // Parsed
/// let radius = ULength::<Both>::parse_str("5px").unwrap();
/// ```
///
/// During the rendering phase, a `CssLength` needs to be converted to user-space
/// coordinates with the [`CssLength::to_user`] method.
#[derive(Debug, PartialEq, Copy, Clone)]
pub struct CssLength<N: Normalize, V: Validate> {
    /// Numeric part of the length
    pub length: f64,

    /// Unit part of the length
    pub unit: LengthUnit,

    /// Dummy; used internally for the type parameter `N`
    orientation: PhantomData<N>,

    /// Dummy; used internally for the type parameter `V`
    validation: PhantomData<V>,
}

impl<N: Normalize, V: Validate> From<CssLength<N, V>> for RsvgLength {
    fn from(l: CssLength<N, V>) -> RsvgLength {
        RsvgLength {
            length: l.length,
            unit: l.unit,
        }
    }
}

impl<N: Normalize, V: Validate> Default for CssLength<N, V> {
    fn default() -> Self {
        CssLength::new(0.0, LengthUnit::Px)
    }
}

pub const POINTS_PER_INCH: f64 = 72.0;
const CM_PER_INCH: f64 = 2.54;
const MM_PER_INCH: f64 = 25.4;
const PICA_PER_INCH: f64 = 6.0;

impl<N: Normalize, V: Validate> Parse for CssLength<N, V> {
    fn parse<'i>(parser: &mut Parser<'i, '_>) -> Result<CssLength<N, V>, ParseError<'i>> {
        let l_value;
        let l_unit;

        let token = parser.next()?.clone();

        match token {
            Token::Number { value, .. } => {
                l_value = value;
                l_unit = LengthUnit::Px;
            }

            Token::Percentage { unit_value, .. } => {
                l_value = unit_value;
                l_unit = LengthUnit::Percent;
            }

            Token::Dimension {
                value, ref unit, ..
            } => {
                l_value = value;

                l_unit = match_ignore_ascii_case! {unit.as_ref(),
                    "px" => LengthUnit::Px,
                    "em" => LengthUnit::Em,
                    "ex" => LengthUnit::Ex,
                    "in" => LengthUnit::In,
                    "cm" => LengthUnit::Cm,
                    "mm" => LengthUnit::Mm,
                    "pt" => LengthUnit::Pt,
                    "pc" => LengthUnit::Pc,
                    "ch" => LengthUnit::Ch,

                    _ => return Err(parser.new_unexpected_token_error(token)),
                };
            }

            _ => return Err(parser.new_unexpected_token_error(token)),
        }

        let l_value = f64::from(finite_f32(l_value).map_err(|e| parser.new_custom_error(e))?);

        <V as Validate>::validate(l_value)
            .map_err(|e| parser.new_custom_error(e))
            .map(|l_value| CssLength::new(l_value, l_unit))
    }
}

/// Parameters for length normalization extracted from [`ComputedValues`].
///
/// This is a precursor to [`NormalizeParams::from_values`], for cases where it is inconvenient
/// to keep a [`ComputedValues`] around.
pub struct NormalizeValues {
    font_size: FontSize,
    is_vertical_text: bool,
}

impl NormalizeValues {
    pub fn new(values: &ComputedValues) -> NormalizeValues {
        let is_vertical_text = matches!(
            (values.writing_mode(), values.text_orientation()),
            (WritingMode::VerticalLr, TextOrientation::Upright)
                | (WritingMode::VerticalRl, TextOrientation::Upright)
        );

        NormalizeValues {
            font_size: values.font_size(),
            is_vertical_text,
        }
    }
}

/// Parameters to normalize [`Length`] values to user-space distances.
pub struct NormalizeParams {
    vbox: ViewBox,
    font_size: f64,
    dpi: Dpi,
    is_vertical_text: bool,
}

impl NormalizeParams {
    /// Extracts the information needed to normalize [`Length`] values from a set of
    /// [`ComputedValues`] and the viewport size in [`Viewport`].
    pub fn new(values: &ComputedValues, viewport: &Viewport) -> NormalizeParams {
        let v = NormalizeValues::new(values);
        NormalizeParams::from_values(&v, viewport)
    }

    pub fn from_values(v: &NormalizeValues, viewport: &Viewport) -> NormalizeParams {
        NormalizeParams {
            vbox: viewport.vbox,
            font_size: font_size_from_values(v, viewport.dpi),
            dpi: viewport.dpi,
            is_vertical_text: v.is_vertical_text,
        }
    }

    /// Just used by rsvg-convert, where there is no font size nor viewport.
    pub fn from_dpi(dpi: Dpi) -> NormalizeParams {
        NormalizeParams {
            vbox: ViewBox::from(Rect::default()),
            font_size: 1.0,
            dpi,
            is_vertical_text: false,
        }
    }
}

impl<N: Normalize, V: Validate> CssLength<N, V> {
    /// Creates a CssLength.
    ///
    /// The compiler needs to know the type parameters `N` and `V` which represents the
    /// length's orientation and validation.
    /// You can specify them explicitly, or call the parametrized method:
    ///
    /// ```
    /// # use rsvg::doctest_only::{Length,LengthUnit,Horizontal,Vertical};
    /// // Explicit type
    /// let width: Length<Horizontal> = Length::new(42.0, LengthUnit::Cm);
    ///
    /// // Inferred type
    /// let height = Length::<Vertical>::new(42.0, LengthUnit::Cm);
    /// ```
    pub fn new(l: f64, unit: LengthUnit) -> CssLength<N, V> {
        CssLength {
            length: l,
            unit,
            orientation: PhantomData,
            validation: PhantomData,
        }
    }

    /// Convert a Length with units into user-space coordinates.
    ///
    /// Lengths may come with non-pixel units, and when rendering, they need to be normalized
    /// to pixels based on the current viewport (e.g. for lengths with percent units), and
    /// based on the current element's set of [`ComputedValues`] (e.g. for lengths with `Em`
    /// units that need to be resolved against the current font size).
    ///
    /// Those parameters can be obtained with [`NormalizeParams::new()`].
    pub fn to_user(&self, params: &NormalizeParams) -> f64 {
        match self.unit {
            LengthUnit::Px => self.length,

            LengthUnit::Percent => {
                self.length * <N as Normalize>::normalize(params.vbox.width(), params.vbox.height())
            }

            LengthUnit::Em => self.length * params.font_size,

            LengthUnit::Ex => self.length * params.font_size / 2.0,

            // how far "0" advances the text, so it varies depending on orientation
            // we're using the 0.5em or 1.0em (based on orientation) fallback from the spec
            LengthUnit::Ch => {
                if params.is_vertical_text {
                    self.length * params.font_size
                } else {
                    self.length * params.font_size / 2.0
                }
            }

            LengthUnit::In => self.length * <N as Normalize>::normalize(params.dpi.x, params.dpi.y),

            LengthUnit::Cm => {
                self.length * <N as Normalize>::normalize(params.dpi.x, params.dpi.y) / CM_PER_INCH
            }

            LengthUnit::Mm => {
                self.length * <N as Normalize>::normalize(params.dpi.x, params.dpi.y) / MM_PER_INCH
            }

            LengthUnit::Pt => {
                self.length * <N as Normalize>::normalize(params.dpi.x, params.dpi.y)
                    / POINTS_PER_INCH
            }

            LengthUnit::Pc => {
                self.length * <N as Normalize>::normalize(params.dpi.x, params.dpi.y)
                    / PICA_PER_INCH
            }
        }
    }

    /// Converts a Length to points.  Pixels are taken to be respect with the DPI.
    ///
    /// # Panics
    ///
    /// Will panic if the length is in Percent, Em, or Ex units.
    pub fn to_points(&self, params: &NormalizeParams) -> f64 {
        match self.unit {
            LengthUnit::Px => {
                self.length / <N as Normalize>::normalize(params.dpi.x, params.dpi.y) * 72.0
            }

            LengthUnit::Percent => {
                panic!("Cannot convert a percentage length into an absolute length");
            }

            LengthUnit::Em => {
                panic!("Cannot convert an Em length into an absolute length");
            }

            LengthUnit::Ex => {
                panic!("Cannot convert an Ex length into an absolute length");
            }

            LengthUnit::In => self.length * POINTS_PER_INCH,

            LengthUnit::Cm => self.length / CM_PER_INCH * POINTS_PER_INCH,

            LengthUnit::Mm => self.length / MM_PER_INCH * POINTS_PER_INCH,

            LengthUnit::Pt => self.length,

            LengthUnit::Pc => self.length / PICA_PER_INCH * POINTS_PER_INCH,

            LengthUnit::Ch => {
                panic!("Cannot convert a Ch length into an absolute length");
            }
        }
    }

    pub fn to_inches(&self, params: &NormalizeParams) -> f64 {
        self.to_points(params) / POINTS_PER_INCH
    }

    pub fn to_cm(&self, params: &NormalizeParams) -> f64 {
        self.to_inches(params) * CM_PER_INCH
    }

    pub fn to_mm(&self, params: &NormalizeParams) -> f64 {
        self.to_inches(params) * MM_PER_INCH
    }

    pub fn to_picas(&self, params: &NormalizeParams) -> f64 {
        self.to_inches(params) * PICA_PER_INCH
    }
}

fn font_size_from_values(values: &NormalizeValues, dpi: Dpi) -> f64 {
    let v = values.font_size.value();

    match v.unit {
        LengthUnit::Percent => unreachable!("ComputedValues can't have a relative font size"),

        LengthUnit::Px => v.length,

        // The following implies that our default font size is 12, which
        // matches the default from the FontSize property.
        LengthUnit::Em => v.length * 12.0,
        LengthUnit::Ex => v.length * 12.0 / 2.0,
        LengthUnit::Ch => v.length * 12.0 / 2.0,

        // FontSize always is a Both, per properties.rs
        LengthUnit::In => v.length * Both::normalize(dpi.x, dpi.y),
        LengthUnit::Cm => v.length * Both::normalize(dpi.x, dpi.y) / CM_PER_INCH,
        LengthUnit::Mm => v.length * Both::normalize(dpi.x, dpi.y) / MM_PER_INCH,
        LengthUnit::Pt => v.length * Both::normalize(dpi.x, dpi.y) / POINTS_PER_INCH,
        LengthUnit::Pc => v.length * Both::normalize(dpi.x, dpi.y) / PICA_PER_INCH,
    }
}

fn viewport_percentage(x: f64, y: f64) -> f64 {
    // https://www.w3.org/TR/SVG/coords.html#Units
    // "For any other length value expressed as a percentage of the viewport, the
    // percentage is calculated as the specified percentage of
    // sqrt((actual-width)**2 + (actual-height)**2))/sqrt(2)."
    (x * x + y * y).sqrt() / SQRT_2
}

/// Alias for `CssLength` types that can have negative values
pub type Length<N> = CssLength<N, Signed>;

/// Alias for `CssLength` types that are non negative
pub type ULength<N> = CssLength<N, Unsigned>;

#[derive(Debug, Default, PartialEq, Copy, Clone)]
pub enum LengthOrAuto<N: Normalize> {
    #[default]
    Auto,
    Length(CssLength<N, Unsigned>),
}

impl<N: Normalize> Parse for LengthOrAuto<N> {
    fn parse<'i>(parser: &mut Parser<'i, '_>) -> Result<LengthOrAuto<N>, ParseError<'i>> {
        if parser
            .try_parse(|i| i.expect_ident_matching("auto"))
            .is_ok()
        {
            Ok(LengthOrAuto::Auto)
        } else {
            Ok(LengthOrAuto::Length(CssLength::parse(parser)?))
        }
    }
}

impl fmt::Display for LengthUnit {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let unit = match &self {
            LengthUnit::Percent => "%",
            LengthUnit::Px => "px",
            LengthUnit::Em => "em",
            LengthUnit::Ex => "ex",
            LengthUnit::In => "in",
            LengthUnit::Cm => "cm",
            LengthUnit::Mm => "mm",
            LengthUnit::Pt => "pt",
            LengthUnit::Pc => "pc",
            LengthUnit::Ch => "ch",
        };

        write!(f, "{unit}")
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    use crate::properties::{ParsedProperty, SpecifiedValue, SpecifiedValues};
    use crate::{assert_approx_eq_cairo, float_eq_cairo::ApproxEqCairo};

    #[test]
    fn parses_default() {
        assert_eq!(
            Length::<Horizontal>::parse_str("42").unwrap(),
            Length::<Horizontal>::new(42.0, LengthUnit::Px)
        );

        assert_eq!(
            Length::<Horizontal>::parse_str("-42px").unwrap(),
            Length::<Horizontal>::new(-42.0, LengthUnit::Px)
        );
    }

    #[test]
    fn parses_percent() {
        assert_eq!(
            Length::<Horizontal>::parse_str("50.0%").unwrap(),
            Length::<Horizontal>::new(0.5, LengthUnit::Percent)
        );
    }

    #[test]
    fn parses_font_em() {
        assert_eq!(
            Length::<Vertical>::parse_str("22.5em").unwrap(),
            Length::<Vertical>::new(22.5, LengthUnit::Em)
        );
    }

    #[test]
    fn parses_font_ex() {
        assert_eq!(
            Length::<Vertical>::parse_str("22.5ex").unwrap(),
            Length::<Vertical>::new(22.5, LengthUnit::Ex)
        );
    }

    #[test]
    fn parses_font_ch() {
        assert_eq!(
            Length::<Vertical>::parse_str("22.5ch").unwrap(),
            Length::<Vertical>::new(22.5, LengthUnit::Ch)
        );
    }

    #[test]
    fn parses_physical_units() {
        assert_eq!(
            Length::<Both>::parse_str("72pt").unwrap(),
            Length::<Both>::new(72.0, LengthUnit::Pt)
        );

        assert_eq!(
            Length::<Both>::parse_str("-22.5in").unwrap(),
            Length::<Both>::new(-22.5, LengthUnit::In)
        );

        assert_eq!(
            Length::<Both>::parse_str("-254cm").unwrap(),
            Length::<Both>::new(-254.0, LengthUnit::Cm)
        );

        assert_eq!(
            Length::<Both>::parse_str("254mm").unwrap(),
            Length::<Both>::new(254.0, LengthUnit::Mm)
        );

        assert_eq!(
            Length::<Both>::parse_str("60pc").unwrap(),
            Length::<Both>::new(60.0, LengthUnit::Pc)
        );
    }

    #[test]
    fn parses_unsigned() {
        assert_eq!(
            ULength::<Horizontal>::parse_str("42").unwrap(),
            ULength::<Horizontal>::new(42.0, LengthUnit::Px)
        );

        assert_eq!(
            ULength::<Both>::parse_str("0pt").unwrap(),
            ULength::<Both>::new(0.0, LengthUnit::Pt)
        );

        assert!(ULength::<Horizontal>::parse_str("-42px").is_err());
    }

    #[test]
    fn empty_length_yields_error() {
        assert!(Length::<Both>::parse_str("").is_err());
    }

    #[test]
    fn invalid_unit_yields_error() {
        assert!(Length::<Both>::parse_str("8furlong").is_err());
    }

    #[test]
    fn normalize_default_works() {
        let viewport = Viewport::new(Dpi::new(40.0, 40.0), 100.0, 100.0);
        let values = ComputedValues::default();
        let params = NormalizeParams::new(&values, &viewport);

        assert_approx_eq_cairo!(
            Length::<Both>::new(10.0, LengthUnit::Px).to_user(&params),
            10.0
        );
    }

    #[test]
    fn normalize_absolute_units_works() {
        let viewport = Viewport::new(Dpi::new(40.0, 50.0), 100.0, 100.0);
        let values = ComputedValues::default();
        let params = NormalizeParams::new(&values, &viewport);

        assert_approx_eq_cairo!(
            Length::<Horizontal>::new(10.0, LengthUnit::In).to_user(&params),
            400.0
        );
        assert_approx_eq_cairo!(
            Length::<Vertical>::new(10.0, LengthUnit::In).to_user(&params),
            500.0
        );

        assert_approx_eq_cairo!(
            Length::<Horizontal>::new(10.0, LengthUnit::Cm).to_user(&params),
            400.0 / CM_PER_INCH
        );
        assert_approx_eq_cairo!(
            Length::<Horizontal>::new(10.0, LengthUnit::Mm).to_user(&params),
            400.0 / MM_PER_INCH
        );
        assert_approx_eq_cairo!(
            Length::<Horizontal>::new(10.0, LengthUnit::Pt).to_user(&params),
            400.0 / POINTS_PER_INCH
        );
        assert_approx_eq_cairo!(
            Length::<Horizontal>::new(10.0, LengthUnit::Pc).to_user(&params),
            400.0 / PICA_PER_INCH
        );
    }

    #[test]
    fn normalize_percent_works() {
        let viewport = Viewport::new(Dpi::new(40.0, 40.0), 100.0, 200.0);
        let values = ComputedValues::default();
        let params = NormalizeParams::new(&values, &viewport);

        assert_approx_eq_cairo!(
            Length::<Horizontal>::new(0.05, LengthUnit::Percent).to_user(&params),
            5.0
        );
        assert_approx_eq_cairo!(
            Length::<Vertical>::new(0.05, LengthUnit::Percent).to_user(&params),
            10.0
        );
    }

    #[test]
    fn normalize_font_em_ex_ch_works() {
        let mut values = ComputedValues::default();
        let viewport = Viewport::new(Dpi::new(40.0, 40.0), 100.0, 200.0);
        let mut params = NormalizeParams::new(&values, &viewport);

        // These correspond to the default size for the font-size
        // property and the way we compute Em/Ex from that.

        assert_approx_eq_cairo!(
            Length::<Vertical>::new(1.0, LengthUnit::Em).to_user(&params),
            12.0
        );

        assert_approx_eq_cairo!(
            Length::<Vertical>::new(1.0, LengthUnit::Ex).to_user(&params),
            6.0
        );

        assert_approx_eq_cairo!(
            Length::<Vertical>::new(1.0, LengthUnit::Ch).to_user(&params),
            6.0
        );

        // check for vertical upright text
        let mut specified = SpecifiedValues::default();
        specified.set_parsed_property(&ParsedProperty::TextOrientation(SpecifiedValue::Specified(
            TextOrientation::Upright,
        )));
        specified.set_parsed_property(&ParsedProperty::WritingMode(SpecifiedValue::Specified(
            WritingMode::VerticalLr,
        )));
        specified.to_computed_values(&mut values);
        params = NormalizeParams::new(&values, &viewport);
        assert_approx_eq_cairo!(
            Length::<Vertical>::new(1.0, LengthUnit::Ch).to_user(&params),
            12.0
        );
    }

    #[test]
    fn to_points_works() {
        let params = NormalizeParams::from_dpi(Dpi::new(40.0, 96.0));

        assert_approx_eq_cairo!(
            Length::<Horizontal>::new(80.0, LengthUnit::Px).to_points(&params),
            2.0 * 72.0
        );
        assert_approx_eq_cairo!(
            Length::<Vertical>::new(192.0, LengthUnit::Px).to_points(&params),
            2.0 * 72.0
        );
    }
}