trezor-firmware/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, file extension `.toif`
* `g`: gray-scale, file extension `.toig`

###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, so it looks like this:

| 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, so it looks like this:

| 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:

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

##Tools

* [png2toi](../tools/png2toi) - tool for converting PNG into TOI format