src/drawing/backend.rs

use crate::style::text_anchor::{HPos, VPos};
use crate::style::{Color, FontDesc, FontError, RGBAColor, ShapeStyle, TextStyle};
use std::error::Error;

/// A coordinate in the image
pub type BackendCoord = (i32, i32);

/// The error produced by a drawing backend
#[derive(Debug)]
pub enum DrawingErrorKind<E: Error + Send + Sync>
where
    FontError: Send + Sync,
{
    /// A drawing backend error
    DrawingError(E),
    /// A font rendering error
    FontError(FontError),
}

impl<E: Error + Send + Sync> std::fmt::Display for DrawingErrorKind<E> {
    fn fmt(&self, fmt: &mut std::fmt::Formatter) -> Result<(), std::fmt::Error> {
        match self {
            DrawingErrorKind::DrawingError(e) => write!(fmt, "Drawing backend error: {}", e),
            DrawingErrorKind::FontError(e) => write!(fmt, "Font loading error: {}", e),
        }
    }
}

impl<E: Error + Send + Sync> Error for DrawingErrorKind<E> {}

/// The style data for the backend drawing API
pub trait BackendStyle {
    /// The underlying type represents the color for this style
    type ColorType: Color;

    /// Convert the style into the underlying color
    fn as_color(&self) -> RGBAColor;

    // TODO: In the future we should support stroke width, line shape, etc....
    fn stroke_width(&self) -> u32 {
        1
    }
}

impl<T: Color> BackendStyle for T {
    type ColorType = T;
    fn as_color(&self) -> RGBAColor {
        self.to_rgba()
    }
}

impl BackendStyle for ShapeStyle {
    type ColorType = RGBAColor;
    fn as_color(&self) -> RGBAColor {
        self.color.clone()
    }
    fn stroke_width(&self) -> u32 {
        self.stroke_width
    }
}

///  The drawing backend trait, which implements the low-level drawing APIs.
///  This trait has a set of default implementation. And the minimal requirement of
///  implementing a drawing backend is implementing the `draw_pixel` function.
///
///  If the drawing backend supports vector graphics, the other drawing APIs should be
///  override by the backend specific implementation. Otherwise, the default implementation
///  will use the pixel-based approach to draw other types of low-level shapes.
pub trait DrawingBackend: Sized {
    /// The error type reported by the backend
    type ErrorType: Error + Send + Sync;

    /// Get the dimension of the drawing backend in pixels
    fn get_size(&self) -> (u32, u32);

    /// Ensure the backend is ready to draw
    fn ensure_prepared(&mut self) -> Result<(), DrawingErrorKind<Self::ErrorType>>;

    /// Finalize the drawing step and present all the changes.
    /// This is used as the real-time rendering support.
    /// The backend may implement in the following way, when `ensure_prepared` is called
    /// it checks if it needs a fresh buffer and `present` is called rendering all the
    /// pending changes on the screen.
    fn present(&mut self) -> Result<(), DrawingErrorKind<Self::ErrorType>>;

    /// Draw a pixel on the drawing backend
    /// - `point`: The backend pixel-based coordinate to draw
    /// - `color`: The color of the pixel
    fn draw_pixel(
        &mut self,
        point: BackendCoord,
        color: &RGBAColor,
    ) -> Result<(), DrawingErrorKind<Self::ErrorType>>;

    /// Draw a line on the drawing backend
    /// - `from`: The start point of the line
    /// - `to`: The end point of the line
    /// - `style`: The style of the line
    fn draw_line<S: BackendStyle>(
        &mut self,
        from: BackendCoord,
        to: BackendCoord,
        style: &S,
    ) -> Result<(), DrawingErrorKind<Self::ErrorType>> {
        super::rasterizer::draw_line(self, from, to, style)
    }

    /// Draw a rectangle on the drawing backend
    /// - `upper_left`: The coordinate of the upper-left corner of the rect
    /// - `bottom_right`: The coordinate of the bottom-right corner of the rect
    /// - `style`: The style
    /// - `fill`: If the rectangle should be filled
    fn draw_rect<S: BackendStyle>(
        &mut self,
        upper_left: BackendCoord,
        bottom_right: BackendCoord,
        style: &S,
        fill: bool,
    ) -> Result<(), DrawingErrorKind<Self::ErrorType>> {
        super::rasterizer::draw_rect(self, upper_left, bottom_right, style, fill)
    }

    /// Draw a path on the drawing backend
    /// - `path`: The iterator of key points of the path
    /// - `style`: The style of the path
    fn draw_path<S: BackendStyle, I: IntoIterator<Item = BackendCoord>>(
        &mut self,
        path: I,
        style: &S,
    ) -> Result<(), DrawingErrorKind<Self::ErrorType>> {
        if style.as_color().alpha() == 0.0 {
            return Ok(());
        }

        if style.stroke_width() == 1 {
            let mut begin: Option<BackendCoord> = None;
            for end in path.into_iter() {
                if let Some(begin) = begin {
                    let result = self.draw_line(begin, end, style);
                    if result.is_err() {
                        return result;
                    }
                }
                begin = Some(end);
            }
        } else {
            let p: Vec<_> = path.into_iter().collect();
            let v = super::rasterizer::polygonize(&p[..], style.stroke_width());
            return self.fill_polygon(v, &style.as_color());
        }
        Ok(())
    }

    /// Draw a circle on the drawing backend
    /// - `center`: The center coordinate of the circle
    /// - `radius`: The radius of the circle
    /// - `style`: The style of the shape
    /// - `fill`: If the circle should be filled
    fn draw_circle<S: BackendStyle>(
        &mut self,
        center: BackendCoord,
        radius: u32,
        style: &S,
        fill: bool,
    ) -> Result<(), DrawingErrorKind<Self::ErrorType>> {
        super::rasterizer::draw_circle(self, center, radius, style, fill)
    }

    fn fill_polygon<S: BackendStyle, I: IntoIterator<Item = BackendCoord>>(
        &mut self,
        vert: I,
        style: &S,
    ) -> Result<(), DrawingErrorKind<Self::ErrorType>> {
        let vert_buf: Vec<_> = vert.into_iter().collect();

        super::rasterizer::fill_polygon(self, &vert_buf[..], style)
    }

    /// Draw a text on the drawing backend
    /// - `text`: The text to draw
    /// - `style`: The text style
    /// - `pos` : The text anchor point
    fn draw_text(
        &mut self,
        text: &str,
        style: &TextStyle,
        pos: BackendCoord,
    ) -> Result<(), DrawingErrorKind<Self::ErrorType>> {
        let font = &style.font;
        let color = &style.color;
        if color.alpha() == 0.0 {
            return Ok(());
        }

        let layout = font.layout_box(text).map_err(DrawingErrorKind::FontError)?;
        let ((min_x, min_y), (max_x, max_y)) = layout;
        let width = (max_x - min_x) as i32;
        let height = (max_y - min_y) as i32;
        let dx = match style.pos.h_pos {
            HPos::Left => 0,
            HPos::Right => -width,
            HPos::Center => -width / 2,
        };
        let dy = match style.pos.v_pos {
            VPos::Top => 0,
            VPos::Center => -height / 2,
            VPos::Bottom => -height,
        };
        let trans = font.get_transform();
        let (w, h) = self.get_size();
        match font.draw(text, (0, 0), |x, y, v| {
            let (x, y) = trans.transform(x + dx - min_x, y + dy - min_y);
            let (x, y) = (pos.0 + x, pos.1 + y);
            if x >= 0 && x < w as i32 && y >= 0 && y < h as i32 {
                self.draw_pixel((x, y), &color.mix(f64::from(v)))
            } else {
                Ok(())
            }
        }) {
            Ok(drawing_result) => drawing_result,
            Err(font_error) => Err(DrawingErrorKind::FontError(font_error)),
        }
    }

    /// Estimate the size of the horizontal text if rendered on this backend.
    /// This is important because some of the backend may not have font ability.
    /// Thus this allows those backend reports proper value rather than ask the
    /// font rasterizer for that.
    ///
    /// - `text`: The text to estimate
    /// - `font`: The font to estimate
    /// - *Returns* The estimated text size
    fn estimate_text_size<'a>(
        &self,
        text: &str,
        font: &FontDesc<'a>,
    ) -> Result<(u32, u32), DrawingErrorKind<Self::ErrorType>> {
        let layout = font.layout_box(text).map_err(DrawingErrorKind::FontError)?;
        Ok((
            ((layout.1).0 - (layout.0).0) as u32,
            ((layout.1).1 - (layout.0).1) as u32,
        ))
    }

    /// Blit a bitmap on to the backend.
    ///
    /// - `text`: pos the left upper conner of the bitmap to blit
    /// - `src`: The source of the image
    ///
    /// TODO: The default implementation of bitmap blitting assumes that the bitmap is RGB, but
    /// this may not be the case. But for bitmap backend it's actually ok if we use the bitmap
    /// element that matches the pixel format, but we need to fix this.
    fn blit_bitmap<'a>(
        &mut self,
        pos: BackendCoord,
        (iw, ih): (u32, u32),
        src: &'a [u8],
    ) -> Result<(), DrawingErrorKind<Self::ErrorType>> {
        let (w, h) = self.get_size();

        for dx in 0..iw {
            if pos.0 + dx as i32 >= w as i32 {
                break;
            }
            for dy in 0..ih {
                if pos.1 + dy as i32 >= h as i32 {
                    break;
                }
                // FIXME: This assume we have RGB image buffer
                let r = src[(dx + dy * w) as usize * 3];
                let g = src[(dx + dy * w) as usize * 3 + 1];
                let b = src[(dx + dy * w) as usize * 3 + 2];
                let color = crate::style::RGBColor(r, g, b);
                let result =
                    self.draw_pixel((pos.0 + dx as i32, pos.1 + dy as i32), &color.to_rgba());
                if result.is_err() {
                    return result;
                }
            }
        }

        Ok(())
    }
}