ezu-paint 0.4.0

//! Pure (non-async, no HTTP) RGBA raster-tile decoding + 3×3 mosaic
//! stitching — the imagery twin of [`dem_decode`](super::dem_decode).
//!
//! The HTTP-fetching path in [`raster`](super::raster) uses these
//! primitives after retrieving bytes; the wasm renderer uses them
//! directly once JS hands the raw tile bytes in. Lives outside the
//! `http` feature gate so it's available on `wasm32` without
//! `reqwest`.

use std::collections::HashMap;

use ezu_graph::{CanvasInfo, RasterBuf};

use crate::host::decode_image_bytes;

/// One decoded raster tile at the source's native pixel size,
/// premultiplied RGBA8.
pub struct RasterTile {
    pub size: u32,
    pub pixels: Vec<u8>,
}

/// Decode a single raster tile (PNG / WebP / JPEG bytes, sniffed from
/// content) into a premultiplied RGBA8 grid. `(z, x, y)` is used only
/// for error messages.
pub fn decode_raster_tile(bytes: &[u8], z: u8, x: u32, y: u32) -> Result<RasterTile, String> {
    let buf = decode_image_bytes(bytes).map_err(|e| format!("decode {z}/{x}/{y}: {e}"))?;
    if buf.width != buf.height {
        return Err(format!(
            "decode {z}/{x}/{y}: non-square tile {}x{}",
            buf.width, buf.height
        ));
    }
    Ok(RasterTile {
        size: buf.width,
        pixels: buf.pixels,
    })
}

/// Stitch a 3×3 mosaic of decoded raster tiles onto the canvas's
/// padded grid. `tiles` is keyed by `(dx, dy)` with `(0, 0)` being the
/// rendered tile; missing slots edge-clamp to the centre so absent
/// neighbours degrade gracefully. Returns `None` when the centre tile
/// itself is missing.
pub fn stitch_padded_raster(
    tiles: &HashMap<(i32, i32), &RasterTile>,
    canvas: CanvasInfo,
) -> Option<RasterBuf> {
    let centre = tiles.get(&(0, 0))?;
    let padded_size = canvas.padded_size();
    let pad = canvas.pad as f32;
    let tile_px = canvas.tile_size as f32;

    let mut out = RasterBuf::new(padded_size, padded_size);
    for py in 0..padded_size {
        let ty = (py as f32 - pad) / tile_px;
        let (dy_off, ty_local) = split_fraction(ty);
        for px in 0..padded_size {
            let tx = (px as f32 - pad) / tile_px;
            let (dx_off, tx_local) = split_fraction(tx);
            let sample_tile = tiles
                .get(&(dx_off, dy_off))
                .copied()
                .or_else(|| {
                    tiles
                        .get(&(dx_off.clamp(-1, 1), dy_off.clamp(-1, 1)))
                        .copied()
                })
                .unwrap_or(centre);
            let size = sample_tile.size as f32;
            let sx = (tx_local * size).clamp(0.0, size - 1.0001);
            let sy = (ty_local * size).clamp(0.0, size - 1.0001);
            let rgba = bilinear_rgba(&sample_tile.pixels, sample_tile.size, sx, sy);
            let i = ((py * padded_size + px) * 4) as usize;
            out.pixels[i..i + 4].copy_from_slice(&rgba);
        }
    }
    Some(out)
}

/// Bilinear-upsample the sub-rectangle of `ancestor` covered by tile
/// `(x, y)` at zoom offset `shift` levels below the ancestor's zoom.
/// Used for `max-zoom` overzoom and the `on-missing: upsample` parent
/// walk.
pub fn upsample_subregion_raster(
    ancestor: &RasterTile,
    shift: u8,
    x: u32,
    y: u32,
    ax: u32,
    ay: u32,
) -> RasterTile {
    let scale = 1u32 << shift;
    let sub_size = ancestor.size as f32 / scale as f32;
    let origin_x = (x - ax * scale) as f32 * sub_size;
    let origin_y = (y - ay * scale) as f32 * sub_size;
    let out_size = ancestor.size;
    let mut pixels = Vec::with_capacity((out_size * out_size * 4) as usize);
    let ancestor_max = ancestor.size as f32 - 1.000_1;
    for py in 0..out_size {
        let sy = (origin_y + sub_size * (py as f32 + 0.5) / out_size as f32 - 0.5)
            .clamp(0.0, ancestor_max);
        for px in 0..out_size {
            let sx = (origin_x + sub_size * (px as f32 + 0.5) / out_size as f32 - 0.5)
                .clamp(0.0, ancestor_max);
            pixels.extend_from_slice(&bilinear_rgba(&ancestor.pixels, ancestor.size, sx, sy));
        }
    }
    RasterTile {
        size: out_size,
        pixels,
    }
}

/// Bilinear sample of premultiplied RGBA8 at fractional pixel
/// coordinates. Blending premultiplied values channel-wise is the
/// correct path (no halos at transparent edges).
#[inline]
fn bilinear_rgba(pixels: &[u8], size: u32, x: f32, y: f32) -> [u8; 4] {
    let x0 = x.floor() as u32;
    let y0 = y.floor() as u32;
    let x1 = (x0 + 1).min(size - 1);
    let y1 = (y0 + 1).min(size - 1);
    let fx = x - x0 as f32;
    let fy = y - y0 as f32;
    let idx = |xx: u32, yy: u32| ((yy * size + xx) * 4) as usize;
    let (i00, i10, i01, i11) = (idx(x0, y0), idx(x1, y0), idx(x0, y1), idx(x1, y1));
    let mut out = [0u8; 4];
    for c in 0..4 {
        let a = pixels[i00 + c] as f32 * (1.0 - fx) + pixels[i10 + c] as f32 * fx;
        let b = pixels[i01 + c] as f32 * (1.0 - fx) + pixels[i11 + c] as f32 * fx;
        out[c] = (a * (1.0 - fy) + b * fy).round().clamp(0.0, 255.0) as u8;
    }
    out
}

/// Split a tile-fractional coordinate `t` into a neighbour offset
/// `n ∈ {-1, 0, 1}` and a position inside that neighbour `∈ [0, 1)`.
#[inline]
fn split_fraction(t: f32) -> (i32, f32) {
    let n = t.floor() as i32;
    let local = t - n as f32;
    (n.clamp(-1, 1), local.clamp(0.0, 0.999_999))
}

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

    fn solid_tile(size: u32, rgba: [u8; 4]) -> RasterTile {
        let mut pixels = Vec::with_capacity((size * size * 4) as usize);
        for _ in 0..(size * size) {
            pixels.extend_from_slice(&rgba);
        }
        RasterTile { size, pixels }
    }

    #[test]
    fn stitch_requires_centre() {
        let canvas = CanvasInfo {
            tile_size: 8,
            pad: 2,
        };
        assert!(stitch_padded_raster(&HashMap::new(), canvas).is_none());
    }

    #[test]
    fn stitch_fills_pad_from_neighbours_and_clamps_missing() {
        let canvas = CanvasInfo {
            tile_size: 8,
            pad: 2,
        };
        let centre = solid_tile(8, [255, 0, 0, 255]);
        let left = solid_tile(8, [0, 255, 0, 255]);
        let mut tiles: HashMap<(i32, i32), &RasterTile> = HashMap::new();
        tiles.insert((0, 0), &centre);
        tiles.insert((-1, 0), &left);
        let out = stitch_padded_raster(&tiles, canvas).unwrap();
        let px = |x: u32, y: u32| {
            let i = ((y * out.width + x) * 4) as usize;
            [
                out.pixels[i],
                out.pixels[i + 1],
                out.pixels[i + 2],
                out.pixels[i + 3],
            ]
        };
        // Left pad comes from the left neighbour; right pad edge-clamps
        // to the centre (no right neighbour bound).
        assert_eq!(px(0, 6), [0, 255, 0, 255]);
        assert_eq!(px(6, 6), [255, 0, 0, 255]);
        assert_eq!(px(out.width - 1, 6), [255, 0, 0, 255]);
    }

    #[test]
    fn upsample_quarters_an_ancestor() {
        // 2x2 ancestor: distinct quadrant colors; child (1, 1) of the
        // ancestor's 2x2 split should read mostly the bottom-right color.
        let mut pixels = vec![0u8; 2 * 2 * 4];
        let colors = [
            [10u8, 0, 0, 255],
            [0, 20, 0, 255],
            [0, 0, 30, 255],
            [40, 40, 40, 255],
        ];
        for (i, c) in colors.iter().enumerate() {
            pixels[i * 4..i * 4 + 4].copy_from_slice(c);
        }
        let ancestor = RasterTile { size: 2, pixels };
        let child = upsample_subregion_raster(&ancestor, 1, 1, 1, 0, 0);
        assert_eq!(child.size, 2);
        // Bottom-right sample of the child leans to the [40,40,40] cell.
        let i = ((child.size + 1) * 4) as usize;
        assert!(child.pixels[i] > 20, "got {:?}", &child.pixels[i..i + 4]);
    }
}