ringa/src/render.rs

//! Ring light glow rendering.
//!
//! The ring is a thick stroke centered on a single rounded rectangle SDF.
//! This guarantees uniform width everywhere, including at corners.
//! Glow bleeds outward and inward from the stroke edges.

use tiny_skia::Pixmap;

// The rounded rect centerline is inset this far from the screen edge.
// The visible ring extends RING_HALF_WIDTH on either side of this line.
const CENTERLINE_INSET: f32 = 80.0;
// Half the stroke width. Total ring thickness = 2 * RING_HALF_WIDTH.
const RING_HALF_WIDTH: f32 = 30.0;
// Corner radius of the centerline rounded rect.
const CORNER_RADIUS: f32 = 30.0;

// Soft glow spread outward and inward from the ring edges.
const GLOW_SPREAD: f32 = 50.0;
const RING_ALPHA: f32 = 1.00;

// Cursor avoidance.
const CURSOR_AVOID_RADIUS: f32 = 120.0;
const CURSOR_AVOID_FALLOFF: f32 = 60.0;

/// Render the ring light glow into `pixmap`, avoiding the cursor at `(cx, cy)`.
pub fn render_ring_light(pixmap: &mut Pixmap, cx: f64, cy: f64) {
    let w = pixmap.width() as f32;
    let h = pixmap.height() as f32;
    let cx = cx as f32;
    let cy = cy as f32;
    let wi = pixmap.width() as usize;
    let hi = pixmap.height() as usize;

    let data = pixmap.data_mut();
    data.fill(0);

    let screen_cx = w / 2.0;
    let screen_cy = h / 2.0;

    // Single rounded rect centerline.
    let hw = w / 2.0 - CENTERLINE_INSET;
    let hh = h / 2.0 - CENTERLINE_INSET;

    // The affected band: from (CENTERLINE_INSET - RING_HALF_WIDTH - GLOW_SPREAD)
    // to (CENTERLINE_INSET + RING_HALF_WIDTH + GLOW_SPREAD) from each screen edge.
    let band_start = (CENTERLINE_INSET - RING_HALF_WIDTH - GLOW_SPREAD)
        .max(0.0)
        .floor() as usize;
    let band_end = (CENTERLINE_INSET + RING_HALF_WIDTH + GLOW_SPREAD).ceil() as usize + 1;

    // Top band
    for py in band_start..band_end.min(hi) {
        for px in 0..wi {
            let a = pixel_alpha(px as f32, py as f32, screen_cx, screen_cy, hw, hh, cx, cy);
            write_pixel(data, py * wi + px, a);
        }
    }

    // Bottom band
    let bottom_start = hi.saturating_sub(band_end);
    let bottom_end = hi.saturating_sub(band_start);
    for py in bottom_start..bottom_end.min(hi) {
        if py < band_end {
            continue;
        }
        for px in 0..wi {
            let a = pixel_alpha(px as f32, py as f32, screen_cx, screen_cy, hw, hh, cx, cy);
            write_pixel(data, py * wi + px, a);
        }
    }

    // Left band (between top and bottom)
    let vert_start = band_end.min(hi);
    let vert_end = hi.saturating_sub(band_end);
    for py in vert_start..vert_end {
        for px in band_start..band_end.min(wi) {
            let a = pixel_alpha(px as f32, py as f32, screen_cx, screen_cy, hw, hh, cx, cy);
            write_pixel(data, py * wi + px, a);
        }
    }

    // Right band (between top and bottom)
    let right_start = wi.saturating_sub(band_end);
    let right_end = wi.saturating_sub(band_start);
    for py in vert_start..vert_end {
        for px in right_start..right_end.min(wi) {
            if px < band_end {
                continue;
            }
            let a = pixel_alpha(px as f32, py as f32, screen_cx, screen_cy, hw, hh, cx, cy);
            write_pixel(data, py * wi + px, a);
        }
    }
}

/// Convert a tiny-skia `Pixmap` (premultiplied RGBA) to Wayland's ARGB8888
/// format (premultiplied BGRA in memory on little-endian).
pub fn pixmap_to_argb8888(pixmap: &Pixmap, out: &mut [u8]) {
    let src = pixmap.data();
    assert_eq!(out.len(), src.len());
    for (dst, src) in out.chunks_exact_mut(4).zip(src.chunks_exact(4)) {
        dst[0] = src[2]; // B
        dst[1] = src[1]; // G
        dst[2] = src[0]; // R
        dst[3] = src[3]; // A
    }
}

// ---- Private helpers --------------------------------------------------------

/// Compute glow alpha for a single pixel using a single rounded rect SDF.
///
/// The ring is all pixels where |sdf| <= RING_HALF_WIDTH (a thick stroke).
/// Glow fades from the ring edges outward/inward over GLOW_SPREAD pixels.
#[allow(clippy::too_many_arguments)]
fn pixel_alpha(
    px: f32,
    py: f32,
    scx: f32,
    scy: f32,
    hw: f32,
    hh: f32,
    cursor_x: f32,
    cursor_y: f32,
) -> u8 {
    // Signed distance to the rounded rect centerline.
    let sd = rounded_rect_sdf(px, py, scx, scy, hw, hh, CORNER_RADIUS);
    let abs_sd = sd.abs();

    let glow = if abs_sd <= RING_HALF_WIDTH {
        // Inside the ring stroke — full brightness.
        RING_ALPHA
    } else {
        // Distance from the nearest ring edge.
        let dist = abs_sd - RING_HALF_WIDTH;
        if dist >= GLOW_SPREAD {
            return 0;
        }
        let t = dist / GLOW_SPREAD;
        smoothstep(1.0 - t) * RING_ALPHA
    };

    // Cursor avoidance.
    let dx = px - cursor_x;
    let dy = py - cursor_y;
    let cursor_dist = (dx * dx + dy * dy).sqrt();
    let cursor_factor =
        smoothstep(((cursor_dist - CURSOR_AVOID_RADIUS) / CURSOR_AVOID_FALLOFF).clamp(0.0, 1.0));

    let alpha = glow * cursor_factor;
    (alpha * 255.0) as u8
}

/// Signed distance field for a rounded rectangle.
///
/// Returns negative inside, positive outside, zero on the boundary.
fn rounded_rect_sdf(px: f32, py: f32, cx: f32, cy: f32, hw: f32, hh: f32, r: f32) -> f32 {
    let dx = (px - cx).abs() - (hw - r);
    let dy = (py - cy).abs() - (hh - r);
    let outside = dx.max(0.0);
    let outside_y = dy.max(0.0);
    let outside_len = (outside * outside + outside_y * outside_y).sqrt();
    let inside = dx.max(dy).min(0.0);
    outside_len + inside - r
}

/// Write a premultiplied white pixel with the given alpha.
fn write_pixel(data: &mut [u8], idx: usize, alpha: u8) {
    if alpha == 0 {
        return;
    }
    let offset = idx * 4;
    data[offset] = alpha;
    data[offset + 1] = alpha;
    data[offset + 2] = alpha;
    data[offset + 3] = alpha;
}

/// Hermite smoothstep: 3t^2 - 2t^3
fn smoothstep(t: f32) -> f32 {
    let t = t.clamp(0.0, 1.0);
    t * t * (3.0 - 2.0 * t)
}