bmp_header.nx

bmp_header.nx -- parse Windows BMP/DIB image headers. BMP is two concatenated structures: 1. BITMAPFILEHEADER (14 bytes, fixed): 0 2 bfType "BM" = 0x42 0x4D 2 4 bfSize total file size LE 6 2 bfReserved1 8 2 bfReserved2 10 4 bfOffBits offset of pixel data from file start 2. DIB header; the first u32 LE is its size, which discriminates: 12 = BITMAPCOREHEADER (OS/2 1.x) 40 = BITMAPINFOHEADER (Windows 3.0+, the common case) 52 = BITMAPV2INFOHEADER 56 = BITMAPV3INFOHEADER 108 = BITMAPV4HEADER 124 = BITMAPV5HEADER We parse the BMP file header and the first two u32s of the DIB (its size + width), plus the canonical BITMAPINFOHEADER fields when size >= 40. Later DIB-version extensions are surfaced only by their presence (size value). Invariants: B1 Magic "BM" (0x4D42 LE) required. B2 dib_size must be one of the canonical sizes. B3 Height may be negative in BITMAPINFOHEADER -- means top-down. We surface raw signed value; caller picks orientation.