.CTB File Format – Image encoding

CTB layers: 7-bit greyscale runs

Each entry starts with a byte whose low 7 bits are a grey level (0 to 127).

• High bit clear: one pixel of that grey.
• High bit set: a run of that grey, whose length follows in 1 to 4 bytes, big-endian, with the length of the field given by its first bits:

A grey level g other than 0 becomes the 8-bit pixel (g << 1) | 1, so 127 is 255 (white); 0 stays black. Longer runs than 268 435 455 pixels are split. Black runs at the end may be left out, since readers start from a black image.

Worked example

CTB layer encryption

When the header’s EncryptionKey is not 0, each layer’s encoded bytes are XORed with a keystream made from that seed and the layer number n (all arithmetic on unsigned 32-bit numbers, wrapping):

init = seed × 0x2D83CDAC + 0xD8A83423
key  = (n × 0x1E1530CD + 0xEC3D47CD) × init
for each whole 4-byte word w (little-endian):
    w   = w XOR key
    key = key + init
remaining 1–3 bytes: XOR with the low, then next, bytes of key

With seed 1, init is 0x062C01CF and layer 0’s first key is 0x6810DBC3, so the 3-byte layer FF 81 2C is stored as 3C 5A 3C (XOR with C3 DB 10). Chitubox picks a random seed; CBDDLP files are never encrypted.

CBDDLP layers: 1-bit planes

Each byte is one run: the high bit is the pixel state (1 lit, 0 dark) and the low 7 bits the run length, 1 to 125 in files written by Chitubox. Runs fill the image row by row; a plane ends when all pixels are covered.

For antialiasing a layer is stored as N planes (AntiAliasLevel, 1, 2, 4 or 8), each with its own entry in the layer table. Plane b is lit where the original grey is at or above a threshold, so a pixel is lit in more planes the brighter it is. To rebuild the grey, count the planes in which a pixel is lit, c, and take c × (256 / N) − 1 (0 when c is 0). With one plane the threshold is 127.

Example: FD FD 8A 05 is 125 lit, 125 lit, 10 lit, then 5 dark pixels.

Previews: 16-bit colour runs

Each pixel is a 16-bit little-endian word in RGB 5-6-5 form whose lowest green bit (bit 5, mask 0x0020) is reused as a repeat flag:

Without the flag the word is one pixel. With it, a second 16-bit word follows whose low 12 bits are the number of extra copies (so up to 4096 pixels in all); Chitubox sets its bits 12 and 13 (| 0x3000). To get 8-bit channels, shift each 5-bit value left by 3.

Example: FF FF 63 30 is white (0xFFFF, flag set) repeated 0x063 + 1 = 100 times; 00 00 is one black pixel.