What is the difference between BITMAPFILEHEADER and BITMAPINFOHEADER?

BITMAPFILEHEADER is the first 14 bytes of a BMP — the BM signature, total file size and the offset where pixel data begins. BITMAPINFOHEADER (usually the next 40 bytes) describes the image itself: width, height, color planes, bit depth, compression and resolution.

Why is the BMP height sometimes negative?

A positive height means the rows are stored bottom-up (the standard for BMP). A negative height means top-down row order. The inspector reports the sign and shows the absolute pixel height.

How does an ICO store multiple sizes?

An .ico begins with a 6-byte header (reserved, type=1, image count) followed by one 16-byte directory entry per image. Each entry gives the width, height, color count, bit depth and the byte offset and size of that image, so a single file holds many resolutions.

What do the BMP compression codes mean?

0 is BI_RGB (uncompressed), 1 is BI_RLE8, 2 is BI_RLE4, 3 is BI_BITFIELDS (custom channel masks, common for 16 and 32-bit), 4 is BI_JPEG and 5 is BI_PNG. The inspector labels the code it reads from the header.

Is my image uploaded anywhere?

No. All parsing happens in your browser with FileReader and DataView. No bytes are transmitted, so the inspector works offline and keeps your files private.

BMP / ICO File Inspector

Decode the binary header of a Windows bitmap (.bmp) or icon (.ico) file — its dimensions, bit depth, compression and, for icons, every resolution packed inside. The inspector reads the BITMAPFILEHEADER, BITMAPINFOHEADER and ICO directory directly with the browser’s DataView API, entirely on your device.

How it works

BMP

A BMP starts with a 14-byte BITMAPFILEHEADER:

bytes 0-1  : "BM" signature
bytes 2-5  : total file size (LE)
bytes 10-13: offset to pixel data

then a 40-byte BITMAPINFOHEADER (the common DIB header):

biSize         (4)  header size, usually 40
biWidth        (4)  pixel width (signed)
biHeight       (4)  pixel height (signed; negative = top-down)
biPlanes       (2)  color planes, always 1
biBitCount     (2)  bits per pixel: 1, 4, 8, 16, 24, 32
biCompression  (4)  0=RGB, 1=RLE8, 2=RLE4, 3=BITFIELDS, 4=JPEG, 5=PNG
biSizeImage    (4)  pixel data size in bytes
biClrUsed      (4)  palette entries used

ICO

An icon file begins with a 6-byte header: a reserved word, a type word (1 = icon, 2 = cursor) and the image count. Each image then has a 16-byte directory entry:

width  (1)  0 means 256
height (1)  0 means 256
colors (1)  palette size, 0 if ≥256
reserved(1)
planes (2)
bitCount(2)
bytesInRes(4)  size of this image's data
imageOffset(4) where its data starts

The inspector walks the directory and lists every entry, so you can confirm a favicon really contains 16×16, 32×32 and 48×48 variants.

Tips

A 32-bit BMP with compression 3 (BITFIELDS) carries an alpha channel via custom channel masks.
ICO entries with bytesInRes beginning with the PNG signature are PNG-compressed icons, common for 256×256 sizes — the inspector flags those.
If the BMP biSizeImage is 0 it is legal for uncompressed images; the size can be recomputed from width, height, bit depth and row padding (rows pad to a 4-byte boundary).