Incorrect line rendering with very sharp angles

[edwloef]

Is your issue REALLY a bug?

• [X] My issue is indeed a bug!
• [X] I am not crazy! I will not fill out this form just to ask a question or request a feature. Pinky promise.

Is there an existing issue for this?

• [X] I have searched the existing issues.

Is this issue related to iced?

• [X] My hardware is compatible and my graphics drivers are up-to-date.

What happened?

In the following code example, the second path is rendered incorrectly:

struct State;

#[derive(Debug)]
enum Message {}

impl State {
    fn new() -> Self {
        Self
    }

    fn update(&mut self, _: Message) {}

    fn view(&self) -> iced::Element<'_, Message> {
        iced::Element::from(
            iced::widget::Canvas::new(self)
                .width(iced::Length::Fill)
                .height(iced::Length::Fill),
        )
    }
}

impl Default for State {
    fn default() -> Self {
        Self::new()
    }
}

impl iced::widget::canvas::Program<Message> for State {
    type State = ();

    fn draw(
        &self,
        _state: &Self::State,
        renderer: &iced::Renderer,
        theme: &iced::Theme,
        bounds: iced::Rectangle,
        _cursor: iced::mouse::Cursor,
    ) -> Vec<iced::widget::canvas::Geometry> {
        let mut frame = iced::widget::canvas::Frame::new(renderer, bounds.size());

        let path1 = iced::widget::canvas::Path::new(|path| {
            path.line_to(iced::Point::new(100.0, 100.0));
            path.line_to(iced::Point::new(100.0, 200.0));
        });
        frame.stroke(
            &path1,
            iced::widget::canvas::Stroke::default().with_color(theme.palette().text),
        );

        let path2 = iced::widget::canvas::Path::new(|path| {
            path.line_to(iced::Point::new(200.0, 100.0));
            path.line_to(iced::Point::new(200.0, 200.0));
            path.line_to(iced::Point::new(201.0, 100.0));
        });
        frame.stroke(
            &path2,
            iced::widget::canvas::Stroke::default().with_color(theme.palette().text),
        );

        vec![frame.into_geometry()]
    }
}

fn main() -> iced::Result {
    iced::application("", State::update, State::view).run()
}

This occurs on both the crates.io release and the master branch.

What is the expected behavior?

The right path shouldn't be cut off higher than the left path.

Version

master

Operating System

Linux

Do you have any log output?

No response

[edwloef]

Additional info: this only occurs with the wgpu backend. Using tiny-skia results in the lines rendering as expected.

[edwloef]

Even more additional info: this is probably down to floating-point precision. The output of the following code snippet illustrates that:

        let path1 = iced::widget::canvas::Path::new(|path| {
            path.line_to(iced::Point::new(200.0, 100.0));
            path.line_to(iced::Point::new(200.0, 200.0));
            path.line_to(iced::Point::new(201.00004, 100.0));
        });
        frame.stroke(
            &path1,
            iced::widget::canvas::Stroke::default().with_color(theme.palette().text),
        );

        let path2 = iced::widget::canvas::Path::new(|path| {
            path.line_to(iced::Point::new(200.0, 300.0));
            path.line_to(iced::Point::new(200.0, 400.0));
            path.line_to(iced::Point::new(201.00003, 300.0));
        });
        frame.stroke(
            &path2,
            iced::widget::canvas::Stroke::default().with_color(theme.palette().text),
        );

this code snippet also creates some interesting results:

        let path2 = iced::widget::canvas::Path::new(|path| {
            path.line_to(iced::Point::new(200.0, 400.0));
            path.line_to(iced::Point::new(200.0, 600.0));
            path.line_to(iced::Point::new(202.0, 400.0));
        });
        frame.stroke(
            &path2,
            iced::widget::canvas::Stroke::default().with_color(theme.palette().text),
        );