Trezor Optimized Image Format (TOIF)
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 4 variants:
f: full-color big endianF: full-color little endiang: gray-scale odd highG: gray-scale even high
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 |
The data is stored according to endianness.
Gray-scale
Each pixel is encoded using a 4-bit value. Each byte contains color of two pixels:
Odd high:
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|
| Po | Po | Po | Po | Pe | Pe | Pe | Pe |
Even high:
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|
| Pe | Pe | Pe | Pe | Po | Po | Po | Po |
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()
TOIF tool
Tool for converting PNGs into TOI format and back, see the following links for more: