Lusamine.ImageIdentifier

Identify an image's format and pixel dimensions by reading only its header. The full image is never loaded into memory. Works over any readable stream, including non-seekable ones (network, compression).

Supported formats: PNG, JPEG, GIF, BMP, WebP (VP8/VP8L/VP8X), TIFF, ICO.

Usage

using Lusamine.ImageIdentifier;

var identifier = new ImageIdentifier();

using var stream = File.OpenRead("photo.jpg");
ImageInfo? info = identifier.Identify(stream);

if (info is not null)
    Console.WriteLine($"{info.Format} {info.Width}x{info.Height}");

Identify returns null when the data is unrecognized or the header is truncated.

Why

Only the leading header bytes are read (typically well under 1 KB), so identifying a multi-megabyte image is allocation-light and near-instant. For non-seekable streams the reader skips forward by reading and discarding, so it still never buffers the whole file.

A few formats keep their dimensions past the first header bytes; these are reached by skipping forward, never by loading the file:

• JPEG : the SOFn marker is located by walking the marker chain, scanning up to 2 MB to clear large APPn metadata (a max-size EXIF block plus stacked ICC-profile segments).
• TIFF / BigTIFF : both the classic (magic 42, 32-bit offsets) and BigTIFF (magic 43, 64-bit offsets) variants are supported. The first IFD is read wherever it sits, up to a 128 MB offset cap. Backward IFD offsets are rejected since the reader is forward-only.
• SVG : width/height are read first (bare or px values), falling back to viewBox. Relative units (%, em, ...) can't be resolved to pixels, so a file with only relative sizes and no viewBox returns null.

These caps bound work on crafted input; ordinary files resolve within the first read.

Performance

Benchmarks run against ImageSharp's Image.Identify on .NET 10, Windows 11, Intel Core i7-11800H:

Library	Format	Mean	Ratio	Allocated
Lusamine	bmp	58.71 ns	1.00	256 B
ImageSharp	bmp	618.13 ns	10.57	1,144 B
Lusamine	gif	39.53 ns	1.00	224 B
ImageSharp	gif	25,894,502.47 ns	658,221.37	6,224 B
Lusamine	jpeg	116.84 ns	1.00	256 B
ImageSharp	jpeg	1,883.00 ns	16.13	10,768 B
Lusamine	png	42.09 ns	1.00	256 B
ImageSharp	png	5,390.30 ns	128.68	1,520 B
Lusamine	webp	64.48 ns	1.00	256 B
ImageSharp	webp	1,584.73 ns	24.66	1,424 B
Lusamine	tiff	131.42 ns	1.00	256 B
ImageSharp	tiff	62,062.19 ns	472.65	220,437 B

And on .NET 10, macOS Tahoe 26.4, Apple M3:

Library	Format	Mean	Ratio	Allocated
Lusamine	bmp	35.88 ns	1.00	256 B
ImageSharp	bmp	459.39 ns	12.81	1,144 B
Lusamine	gif	26.55 ns	1.00	224 B
ImageSharp	gif	41,702,047.39 ns	1,571,280.33	6,224 B
Lusamine	jpeg	64.35 ns	1.00	224 B
ImageSharp	jpeg	1,026.48 ns	15.96	10,768 B
Lusamine	png	26.64 ns	1.00	256 B
ImageSharp	png	7,844.73 ns	294.64	1,520 B
Lusamine	webp	39.98 ns	1.00	256 B
ImageSharp	webp	1,030.49 ns	25.78	1,424 B
Lusamine	tiff	78.10 ns	1.00	256 B
ImageSharp	tiff	36,514.15 ns	467.55	220,440 B

Lusamine reads only the image header (typically under 1 KB), while ImageSharp decodes significantly more of the file. The result is consistently one to several orders of magnitude faster across formats, from ~11–13x for BMP up to hundreds of thousands of times faster for GIF, with a fraction of the allocations.

The advantage is just as large on the failure paths : unrecognized, empty, or corrupt input. Lusamine returns null cheaply and never allocates an exception, whereas ImageSharp throws and pays for it (.NET 10, macOS Tahoe 26.4, Apple M3):

Library	Scenario	Mean	Ratio	Allocated
Lusamine	corrupt-jpeg	85.79 ns	1.00	192 B
ImageSharp	corrupt-jpeg	6,180.97 ns	72.05	10,696 B
Lusamine	empty	37.55 ns	1.00	152 B
ImageSharp	empty	3,475.82 ns	92.57	2,185 B
Lusamine	random	55.63 ns	1.00	184 B
ImageSharp	random	5,295.91 ns	95.22	1,368 B
Lusamine	text	47.98 ns	1.00	152 B
ImageSharp	text	3,593.08 ns	74.92	2,313 B
Lusamine	truncated-png	30.53 ns	1.00	256 B
ImageSharp	truncated-png	5,532.30 ns	181.27	1,600 B

Extensibility

The decoder set is injectable for custom formats or testing:

var identifier = new ImageIdentifier(new IImageFormatDecoder[] { new MyFormatDecoder() });