2 files changed, 299 insertions, 0 deletions
diff --git a/src/renderer/heatmap.rs b/src/renderer/heatmap.rs
new file mode 100644
index 0000000..c4af7a6
--- /dev/null
+++ b/src/renderer/heatmap.rs
@@ -0,0 +1,209 @@
+//! Actual rendering functions for heatmaps.
+//!
+//! We begin the rendering by using [`render_heatcounter`] to turn a list of GPX tracks into a
+//! [`HeatCounter`], which is basically a grayscale heatmap, where each pixel represents the number
+//! of tracks that goes through this pixel.
+//!
+//! We then render the colored heatmap tiles using [`lazy_colorization`], which provides us with
+//! colorful PNG data.
+use color_eyre::{eyre::Result, Report};
+use crossbeam_channel::Sender;
+use image::{ImageBuffer, Luma, Pixel, RgbaImage};
+use nalgebra::{vector, Vector2};
+use rayon::iter::ParallelIterator;
+
+use super::{
+    super::{
+        gpx::Coordinates,
+        layer::{self, TileLayer},
+    },
+    RenderedTile,
+};
+
+/// Type for the intermediate heat counters.
+pub type HeatCounter = TileLayer<Luma<u8>>;
+
+fn render_circle<P: Pixel>(layer: &mut TileLayer<P>, center: (u64, u64), radius: u64, pixel: P) {
+    let topleft = (center.0 - radius, center.1 - radius);
+    let rad_32: u32 = radius.try_into().unwrap();
+    let mut circle = ImageBuffer::<P, Vec<P::Subpixel>>::new(rad_32 * 2 + 1, rad_32 * 2 + 1);
+    imageproc::drawing::draw_filled_circle_mut(
+        &mut circle,
+        (
+            i32::try_from(radius).unwrap(),
+            i32::try_from(radius).unwrap(),
+        ),
+        radius.try_into().unwrap(),
+        pixel,
+    );
+    layer.blit_nonzero(topleft.0, topleft.1, &circle);
+}
+
+fn direction_vector(a: (u64, u64), b: (u64, u64)) -> Vector2<f64> {
+    let dx = if b.0 > a.0 {
+        (b.0 - a.0) as f64
+    } else {
+        -((a.0 - b.0) as f64)
+    };
+    let dy = if b.1 > a.1 {
+        (b.1 - a.1) as f64
+    } else {
+        -((a.1 - b.1) as f64)
+    };
+    vector![dx, dy]
+}
+
+fn render_line<P: Pixel>(
+    layer: &mut TileLayer<P>,
+    start: (u64, u64),
+    end: (u64, u64),
+    thickness: u64,
+    pixel: P,
+) {
+    use imageproc::point::Point;
+
+    if start == end {
+        return;
+    }
+
+    fn unsigned_add(a: Vector2<u64>, b: Vector2<i32>) -> Vector2<u64> {
+        let x = if b[0] < 0 {
+            a[0] - u64::from(b[0].unsigned_abs())
+        } else {
+            a[0] + u64::try_from(b[0]).unwrap()
+        };
+        let y = if b[1] < 0 {
+            a[1] - u64::from(b[1].unsigned_abs())
+        } else {
+            a[1] + u64::try_from(b[1]).unwrap()
+        };
+        vector![x, y]
+    }
+
+    let r = direction_vector(start, end);
+    let normal = vector![r[1], -r[0]].normalize();
+
+    let start = vector![start.0, start.1];
+    let end = vector![end.0, end.1];
+
+    let displacement = normal * thickness as f64;
+    let displacement = displacement.map(|x| x as i32);
+    if displacement == vector![0, 0] {
+        return;
+    }
+    let polygon = [
+        unsigned_add(start, displacement),
+        unsigned_add(end, displacement),
+        unsigned_add(end, -displacement),
+        unsigned_add(start, -displacement),
+    ];
+    let min_x = polygon.iter().map(|p| p[0]).min().unwrap();
+    let min_y = polygon.iter().map(|p| p[1]).min().unwrap();
+    let max_x = polygon.iter().map(|p| p[0]).max().unwrap();
+    let max_y = polygon.iter().map(|p| p[1]).max().unwrap();
+
+    let mut overlay = ImageBuffer::<P, Vec<P::Subpixel>>::new(
+        (max_x - min_x).try_into().unwrap(),
+        (max_y - min_y).try_into().unwrap(),
+    );
+    let adjusted_poly = polygon
+        .into_iter()
+        .map(|p| Point::new((p[0] - min_x) as i32, (p[1] - min_y) as i32))
+        .collect::<Vec<_>>();
+    imageproc::drawing::draw_polygon_mut(&mut overlay, &adjusted_poly, pixel);
+
+    layer.blit_nonzero(min_x, min_y, &overlay);
+}
+
+fn merge_heat_counter(base: &mut HeatCounter, overlay: &HeatCounter) {
+    for (tx, ty, source) in overlay.enumerate_tiles() {
+        let target = base.tile_mut(tx, ty);
+        for (x, y, source) in source.enumerate_pixels() {
+            let target = target.get_pixel_mut(x, y);
+            target[0] += source[0];
+        }
+    }
+}
+
+fn colorize_tile(tile: &ImageBuffer<Luma<u8>, Vec<u8>>, max: u32) -> RgbaImage {
+    let gradient = colorgrad::yl_or_rd();
+    let mut result = ImageBuffer::from_pixel(tile.width(), tile.height(), [0, 0, 0, 0].into());
+    for (x, y, pixel) in tile.enumerate_pixels() {
+        if pixel[0] > 0 {
+            let alpha = pixel[0] as f64 / max as f64;
+            let color = gradient.at(1.0 - alpha);
+            let target = result.get_pixel_mut(x, y);
+            *target = color.to_rgba8().into();
+        }
+    }
+    result
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct Renderer;
+
+impl super::Renderer for Renderer {
+    type Prepared = HeatCounter;
+
+    /// Renders the heat counter for the given zoom level and track points.
+    ///
+    /// The given callback will be called when a track has been rendered and merged into the
+    /// accumulator, to allow for UI feedback. The passed parameter is the number of tracks that have
+    /// been rendered since the last call.
+    fn prepare(zoom: u32, tracks: &[Vec<Coordinates>], tick: Sender<()>) -> Result<HeatCounter> {
+        let mut heatcounter = TileLayer::from_pixel([0].into());
+
+        for track in tracks {
+            let mut layer = TileLayer::from_pixel([0].into());
+
+            let points = track
+                .iter()
+                .map(|coords| coords.web_mercator(zoom))
+                .collect::<Vec<_>>();
+
+            for point in points.iter() {
+                render_circle(&mut layer, *point, (zoom as u64 / 4).max(2) - 1, [1].into());
+            }
+
+            for (a, b) in points.iter().zip(points.iter().skip(1)) {
+                render_line(&mut layer, *a, *b, (zoom as u64 / 4).max(1), [1].into());
+            }
+
+            merge_heat_counter(&mut heatcounter, &layer);
+            tick.send(()).unwrap();
+        }
+        Ok(heatcounter)
+    }
+
+    /// Lazily colorizes a [`HeatCounter`] by colorizing it tile-by-tile and saving a tile before
+    /// rendering the next one.
+    ///
+    /// This function calls the given callback with each rendered tile, and the function is responsible
+    /// for saving it. If the callback returns an `Err(...)`, the error is passed through.
+    ///
+    /// Note that this function internally uses `rayon` for parallization. If you want to limit the
+    /// number of threads used, set up the global [`rayon::ThreadPool`] first.
+    fn colorize(layer: HeatCounter, tx: Sender<RenderedTile>) -> Result<()> {
+        let max = layer.pixels().map(|l| l.0[0]).max().unwrap_or_default();
+        if max == 0 {
+            return Ok(());
+        }
+
+        layer
+            .into_parallel_tiles()
+            .try_for_each_with(tx, |tx, (tile_x, tile_y, tile)| {
+                let colorized = colorize_tile(&tile, max.into());
+                let data = layer::compress_png_as_bytes(&colorized)?;
+                tx.send(RenderedTile {
+                    x: tile_x,
+                    y: tile_y,
+                    data,
+                })?;
+                Ok::<(), Report>(())
+            })
+    }
+
+    fn tile_count(layer: &HeatCounter) -> Result<u64> {
+        Ok(layer.tile_count().try_into().unwrap())
+    }
+}
diff --git a/src/renderer/mod.rs b/src/renderer/mod.rs
new file mode 100644
index 0000000..927c6ed
--- /dev/null
+++ b/src/renderer/mod.rs
@@ -0,0 +1,90 @@
+//! Generic "tile rendering" methods.
+use std::thread;
+
+use color_eyre::Result;
+use crossbeam_channel::Sender;
+
+use super::gpx::Coordinates;
+
+pub mod heatmap;
+
+const CHANNEL_SIZE: usize = 30;
+
+/// Represents a fully rendered tile.
+#[derive(Debug, Clone)]
+pub struct RenderedTile {
+    /// The `x` coordinate of the tile.
+    pub x: u64,
+    /// The `y` coordinate of the tile.
+    pub y: u64,
+    /// The encoded (PNG) image data, ready to be saved to disk.
+    pub data: Vec<u8>,
+}
+
+/// An object that is responsible for turning raw GPX tracks into a representation.
+///
+/// This is done in two steps, preparation and actual rendering. This allows different feedback for
+/// the user.
+pub trait Renderer {
+    type Prepared: Send;
+
+    /// Prepare the rendered data.
+    ///
+    /// The `tick` channel is used to provide user-feedback, for every finished track a tick should
+    /// be sent.
+    fn prepare(zoom: u32, tracks: &[Vec<Coordinates>], tick: Sender<()>) -> Result<Self::Prepared>;
+
+    /// Actually produce the colored tiles, using the previously prepared data.
+    ///
+    /// The `saver` channel is used to send the finished tiles to a thread that is responsible for
+    /// saving them.
+    fn colorize(prepared: Self::Prepared, saver: Sender<RenderedTile>) -> Result<()>;
+
+    /// Returns the tile count of the prepared data.
+    ///
+    /// This is used for the user interface, to scale progress bars appropriately.
+    fn tile_count(prepared: &Self::Prepared) -> Result<u64>;
+}
+
+/// A convenience wrapper to call [`Renderer::prepare`].
+///
+/// This function takes the same arguments, but provides the ability to use a callback closure
+/// instead of having to set up a channel. The callback is always called on the same thread.
+pub fn prepare<R: Renderer, F: FnMut() -> Result<()>>(
+    zoom: u32,
+    tracks: &[Vec<Coordinates>],
+    mut tick: F,
+) -> Result<R::Prepared> {
+    thread::scope(|s| {
+        let (sender, receiver) = crossbeam_channel::bounded(CHANNEL_SIZE);
+
+        let preparer = s.spawn(|| R::prepare(zoom, tracks, sender));
+
+        for _ in receiver {
+            tick()?;
+        }
+
+        preparer.join().unwrap()
+    })
+}
+
+/// A convenience wrapper to call [`Renderer::colorize`].
+///
+/// This function takes the same arguments, but provides the ability to use a callback closure
+/// instead of having to set up a channel. The callback is always called on the same thread.
+pub fn colorize<R: Renderer, F: FnMut(RenderedTile) -> Result<()>>(
+    prepared: R::Prepared,
+    mut saver: F,
+) -> Result<()> {
+    thread::scope(|s| {
+        let (sender, receiver) = crossbeam_channel::bounded(CHANNEL_SIZE);
+
+        let colorizer = s.spawn(|| R::colorize(prepared, sender));
+
+        for tile in receiver {
+            saver(tile)?;
+        }
+
+        colorizer.join().unwrap()
+    })
+}