Anti-aliased line/circle rendering, grayscale PNG output, and Spring Boot-style actuator endpoints

[Grizaster]

Overview

This PR improves rendering quality to match commercial alternatives (e.g. Labelary) by adding anti-aliasing for all line and circle primitives, and adds Spring Boot-style management endpoints to the HTTP server.

Rendering Improvements

Anti-aliased diagonal lines (^GD)

• Thin lines (thickness ≤ 1): Replaced Bresenham (draw_line_segment_mut) with Xiaolin Wu's algorithm (draw_antialiased_line_segment_mut) for smooth edges
• Thick lines (thickness > 1): Custom parallelogram fill with per-pixel coverage at the left and right edges. The previous draw_polygon_mut approach only drew the outline, not the filled shape

Anti-aliased circles (^GC)

• Replaced draw_filled_circle_mut and the binary distance-check ring with a per-pixel coverage-based approach
• Both inner and outer ring edges use a 1px-wide coverage ramp (distance ± 0.5) for smooth anti-aliased edges

Grayscale PNG output

• Removed the binary threshold (pixel > 128 ? 255 : 0) from encode_png
• PNG output is now 8-bit grayscale, preserving anti-aliasing from the renderer
• Matches Labelary's default X-Quality: Grayscale output

Management Endpoints (Actuator)

Added Spring Boot-style actuator endpoints under /manage/:

Endpoint	Description
GET /manage/health	Health check with component status (diskSpace)
GET /manage/info	Build info: app name, version, architecture, OS, timezone, language
GET /manage/env	Filtered environment variables (LABELIZE_*, RUST_*, TZ, LANG, LC_ALL, PATH, HOME, HOSTNAME, USER, SHELL, TERM)
GET /manage/metrics	Render metrics: total/preview/PDF/failed counts, average render time, uptime

Metrics are tracked via AtomicU64 counters and Instant timer. The existing /health endpoint is kept for backwards compatibility.

Dependencies

• Added serde_json (optional, behind serve feature) for structured JSON responses

Files Changed

• src/drawers/renderer.rs — anti-aliased line and circle rendering
• src/images/monochrome.rs — grayscale PNG encoding (removed binary threshold)
• src/main.rs — actuator endpoints, metrics tracking
• Cargo.toml — serde_json dependency

[Copilot]

Pull request overview

This PR updates Labelize’s rendering pipeline to preserve anti-aliasing (by improving circle/diagonal-line rasterization and switching PNG output to grayscale), and extends the Axum HTTP server with Spring Boot-style /manage/* actuator endpoints that expose health/info/env/metrics.

Changes:

• Added /manage/health, /manage/info, /manage/env, /manage/metrics endpoints and basic request/render timing counters in the serve HTTP server.
• Refactored the renderer to expose draw_label_to_rgba, and implemented anti-aliased circle + diagonal-line drawing.
• Changed PNG encoding from thresholded black/white to 8-bit grayscale; added serde_json behind the serve feature.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
src/main.rs	Adds /manage/* endpoints, shared app state, and render/request metrics; switches HTTP PNG path to encode from an RGBA canvas.
src/images/monochrome.rs	Changes PNG encoding to grayscale (L8) by removing the binarization threshold.
src/drawers/renderer.rs	Introduces draw_label_to_rgba and implements anti-aliased circle and diagonal-line rendering.
Cargo.toml	Adds optional serde_json and wires it into the serve feature.

Comments suppressed due to low confidence (1)

src/images/monochrome.rs:20

• encode_png is publicly re-exported (src/lib.rs:20) and lives under images::monochrome, but it now encodes 8-bit grayscale (ExtendedColorType::L8) instead of a binary/monochrome image. That semantic change can break API expectations for downstream consumers who relied on 1-bit output.

Consider either (a) keeping the old thresholded encoder as encode_png_monochrome and introducing encode_png_grayscale, or (b) adding an explicit option/parameter to select the output mode.

pub fn encode_png(img: &RgbaImage, w: &mut impl Write) -> Result<(), LabelizeError> {
    let (width, height) = img.dimensions();
    let mut gray = image::GrayImage::new(width, height);

    for y in 0..height {
        for x in 0..width {
            let pixel = img.get_pixel(x, y);
            gray.put_pixel(x, y, image::Luma([pixel[0]]));
        }
    }

    let encoder = image::codecs::png::PngEncoder::new(w);
    use image::ImageEncoder;
    encoder
        .write_image(gray.as_raw(), width, height, image::ExtendedColorType::L8)
        .map_err(|e| LabelizeError::Encode(format!("PNG encode error: {}", e)))

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[GOODBOY008]

@Grizaster Thanks for your contributions. Pls fix the ci. I will take a look this feature.

[GOODBOY008]

@Grizaster This PR contains multiple features. Could you please split it into several smaller PRs?

[Grizaster]

Closed in favor of two smaller PRs as requested:

• Anti-aliased line/circle rendering and grayscale PNG output #22 — Anti-aliased line/circle rendering and grayscale PNG output
• Add Spring Boot-style actuator endpoints (/manage/*) #23 — Spring Boot-style actuator endpoints (/manage/*)