letratag-render

Bounded, deterministic image rendering for the DYMO LetraTag 200B: PNG or 8-bit grayscale input becomes a 32-row binary raster in the wire format of letratag-protocol, and rasters can be encoded back to PNG previews. No Bluetooth, CUPS, or hardware dependency.

Part of letratag-linux.

Install

[dependencies]
letratag-render = "0.1.1"

Rust 1.98 or newer. No default features; there are no feature flags. PNG support uses the image crate; letratag-protocol provides the raster type.

Examples

Render grayscale samples

render_gray takes row-major 8-bit grayscale (0 is black, 255 is white). Images keep their aspect ratio (rounded down to whole pixels, one-pixel minimum) and are centered on the canvas; an odd spare pixel goes to the right or bottom. Scaling samples nearest neighbors at pixel centers.

use letratag_render::{render_gray, RenderOptions};

let pixels = vec![0u8; 4 * 2]; // 4x2 black source
let raster = render_gray(4, 2, &pixels, RenderOptions::default()).unwrap();
assert_eq!(raster.width(), 400); // the default output width
// The 4x2 source scales to 60x30 and is centered on the 400x32 canvas.
assert!(raster.get(200, 16));
assert!(!raster.get(0, 0));

Render a PNG and build a preview

render_png decodes only PNG (header dimensions are checked before decoder construction, and decoder buffers are bounded). preview_png encodes the actual full-height raster, without rescaling or trimming, as a grayscale PNG.

use letratag_render::{preview_png, render_gray, render_png, RenderOptions};

let raster = render_gray(2, 2, &[0, 255, 255, 0], RenderOptions::default()).unwrap();
let png = preview_png(&raster).unwrap();
assert_eq!(&png[..8], b"\x89PNG\r\n\x1a\n");

// The preview is a full-height grayscale PNG that decodes again.
let roundtrip = render_png(&png, RenderOptions::default()).unwrap();
assert_eq!(roundtrip.width(), raster.width());

Options and validation errors

RenderOptions selects the output width (feed columns), the number of active rows across the tape, the luminance threshold, and optional deterministic 4x4 Bayer dithering. Invalid options and mismatched input are rejected with structured errors before any rendering.

use letratag_render::{render_gray, Error, RenderOptions};

let mut options = RenderOptions::default();
options.width = 0;
assert!(matches!(
    render_gray(1, 1, &[0], options),
    Err(Error::InvalidOutputWidth { width: 0 })
));
assert!(matches!(
    render_gray(1, 1, &[0, 0], RenderOptions::default()),
    Err(Error::GrayscaleLength { .. })
));

API overview

Limitations

• PNG only. No JPEG, text, font, or PDF rendering. For APNG, only the default image is rendered, not animation frames.
• Rendering is aspect-ratio preserving with thresholding or Bayer dithering. It never stretches, crops silently, or fits to page; content outside the active rows is a design choice, not a physical printhead calibration.
• The 30 active rows are a design convention (S1's canvas), not a measured printhead limit; the full 32-row wire frame is always transmitted.
• This crate renders images; it does not print. Feeding a raster to a printer is the job of a transport (see letratag-transport), and physical print completion is never proven by a raster or a preview.

License

MIT. See the main repository for the full license text and the reference-notice inventory in docs/legal/.