Dims

Probe remote image dimensions (width × height) from just a URL — without downloading the image.

Image formats keep their dimensions in the file header, so Dims fetches only the first ~128 KB via an HTTP Range request, parses the JPEG / PNG / WebP / GIF header, and discards the bytes. Milliseconds and a few KB per image instead of megabytes.

Dims.probe("https://cdn.example.com/page1.jpg")
#=> %{width: 800, height: 8000}

Dims.probe_all(urls)
#=> [%{url: ..., width: 800, height: 1200}, ...]   # input order preserved

Useful anywhere you deal with images you don't host: reserving layout space (aspect-ratio, <img width height>) before anything loads, link previews, gallery imports, comic/manga readers deciding page layouts, CMS ingestion.

Install

def deps do
  [
    {:dims, "~> 0.1"}
  ]
end

Batch probing, built for the real world

Parallel — probe_all/2 sweeps the list with bounded concurrency (default 8), preserving order.
Median backfill — a probe that fails or times out gets the median dimensions of its successful siblings, not a constant fallback. Image sets (chapter pages, galleries) are near-uniform, so the estimate is close — and relative layouts built from the results never explode.
Sampling — probe_sampled/2 probes ~20 evenly-distributed URLs of a huge list and median-fills the rest: accuracy within a percent or two for uniform sets at ~25× less traffic. probe_auto/2 switches between full and sampled by list length (default threshold 80).
Caching — results cache in ETS with a 30-day TTL (a URL's bytes don't change, so its dimensions don't either). Bypass per call with cache: false.

Options

Dims.probe(url,
  headers: [{"referer", "https://source.example/"}],  # CDNs that check referers
  headers_fun: &MyApp.headers_for/1,                  # per-URL headers
  probe_bytes: 131_071,
  receive_timeout: 8_000,
  cache: true,
  cache_ttl: :timer.hours(24 * 30)
)

Batch calls also take :max_concurrency, :sample_size, and :full_threshold.

Parsing bytes you already have

Dims.Parser.parse/1 is the pure header parser — hand it the leading bytes of an upload or a cached chunk and get %{width:, height:} back with no I/O.

Formats

JPEG (all SOFn markers, so progressive too), PNG, WebP (VP8 / VP8L / VP8X), GIF (87a/89a).

License

MIT

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