trezor-firmware/core/docs/toif.md

Trezor Optimized Image Format

All multibyte integer values are little endian!

Header

offset	length	name	description
0x0000	3	magic	TOI
0x0003	1	fmt	data format: f or g (see below)
0x0004	2	width	width of the image
0x0006	2	height	height of the image
0x0008	4	datasize	length of the compressed data
0x000A	?	data	compressed data (see below)

Format

TOI currently supports 2 variants:

• f: full-color
• g: gray-scale

Full-color

For each pixel a 16-bit value is used. First 5 bits are used for red component, next 6 bits are green, final 5 bits are blue:

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
R	R	R	R	R	G	G	G	G	G	G	B	B	B	B	B

Gray-scale

Each pixel is encoded using a 4-bit value. Each byte contains color of two pixels:

7	6	5	4	3	2	1	0
Po	Po	Po	Po	Pe	Pe	Pe	Pe

Where Po is odd pixel and Pe is even pixel.

Compression

Pixel data is compressed using DEFLATE algorithm with 10-bit sliding window and no header. This can be achieved with ZLIB library by using the following:

import zlib
z = zlib.compressobj(level=9, wbits=10)
zdata = z.compress(pixeldata) + z.flush()
zdata = zdata[2:-4] # strip header and checksum

Tools

• toif_convert - tool for converting PNGs into TOI format and back