Check if Image is BMP

No. The check runs entirely in your browser. Only the first 256 bytes are read – with File.slice().arrayBuffer() – so even huge files are never fully loaded or transferred.

The tool reads the first two bytes and looks for 42 4D (ASCII BM). OS/2 bitmap-array variants (BA, CI, CP, IC, PT) are also recognised and labelled.

Yes. The extension is ignored – only the magic bytes matter. A BMP file saved as logo.jpg is still reported as BMP.

It’s reported as “Not a BMP” with the reason “Magic bytes X do not match”. The extension is unreliable – always trust the bytes.

BITMAPCOREHEADER (V2, 12 bytes), BITMAPINFOHEADER (V3, 40 bytes), BITMAPV4HEADER (108 bytes), and BITMAPV5HEADER (124 bytes). Other header sizes are still labelled as “BMP” with the size shown so you can research further.

The number of bits per pixel: 1 (monochrome), 4 (16-colour), 8 (256-colour), 16 (5-5-5 or 5-6-5), 24 (RGB888), or 32 (RGBA8888). Combined with compression, this describes the pixel storage format.

Most BMP files store rows bottom-up, which is why the height field is positive. A negative height flags the file as top-down – rows read from first to last line. The tool shows this with a “Top-down DIB” row.

BITMAPINFOHEADER supports BI_BITFIELDS (code 3) which explicitly sets the red/green/blue masks. It is common for 16-bit and 32-bit BMPs. The pixel data is still uncompressed; only the channel layout is custom.

No. Only the first 256 bytes are read, so file size is irrelevant for the check itself.

After the page loads, yes. HTML, CSS, and JS are self-contained, so you can disconnect Wi-Fi and keep checking files.