favicon.website

FWC-1§2Normative

Terminology

Precise vocabulary. Later sections use these words with exactly these meanings.

Term	Definition
payload	The original bytes the author wants delivered — for FWC-1 always UTF-8 text, in practice an HTML document.
body	The payload after optional compression and optional encryption. This is what the CRC covers and what the length field counts.
header	The fixed 12-byte prefix described in §3.
container	header ‖ body — the complete byte stream written into the carrier.
carrier	The RGBA pixel buffer of a square PNG image, together with a mapping rule (§4) that says which bits of which channels hold container bytes.
mode	The carrier mapping in use: `noise` or `lsb`.
capacity	The maximum body length a given image size and mode can hold, i.e. total carrier bytes minus 12.
cover	In `lsb` mode, the visible image whose two least-significant bits per channel are overwritten.
host page	The tiny HTML document that references the favicon and embeds the loader (§8).
loader	The dependency-free script in the host page that decodes the container and replaces the document.
encoder / decoder	Any software that produces, respectively consumes, an FWC-1 PNG. The Studio and the loader are one encoder and one decoder among many possible.

Notation

a ‖ b denotes byte concatenation.
Multi-byte integers are big-endian (network byte order) unless stated otherwise.
Bit 0 is the least-significant bit. flags & 0b01 therefore tests bit 0.
Pixel data is addressed as the flat Uint8ClampedArray returned by getImageData: index i is channel i % 4 (0 = R, 1 = G, 2 = B, 3 = A) of pixel ⌊i / 4⌋, pixels in row-major order from the top-left.