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70 lines
1.9 KiB
Markdown
70 lines
1.9 KiB
Markdown
# Trezor Optimized Image Format (TOIF)
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All multibyte integer values are little endian!
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## Header
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| offset | length | name | description |
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|-------:|-------:|------|-------------|
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| 0x0000 | 3 | magic | `TOI` |
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| 0x0003 | 1 | fmt | data format: `f` or `g` (see below) |
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| 0x0004 | 2 | width | width of the image |
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| 0x0006 | 2 | height | height of the image |
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| 0x0008 | 4 | datasize | length of the compressed data |
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| 0x000A | ? | data | compressed data (see below) |
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## Format
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TOI currently supports 4 variants:
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* `f`: full-color big endian
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* `F`: full-color little endian
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* `g`: gray-scale odd high
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* `G`: gray-scale even high
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### Full-color
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For each pixel a 16-bit value is used.
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First 5 bits are used for red component, next 6 bits are green,
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final 5 bits are blue:
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| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
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|----|----|----|----|----|----|---|---|---|---|---|---|---|---|---|---|
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| R | R | R | R | R | G | G | G | G | G | G | B | B | B | B | B |
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The data is stored according to endianness.
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### Gray-scale
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Each pixel is encoded using a 4-bit value.
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Each byte contains color of two pixels:
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#### Odd high:
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| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
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|---|---|---|---|---|---|---|---|
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| Po | Po | Po | Po | Pe | Pe | Pe | Pe |
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#### Even high:
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| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
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|-----|-----|-----|-----|-----|-----|-----|-----|
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| Pe | Pe | Pe | Pe | Po | Po | Po | Po |
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Where Po is odd pixel and Pe is even pixel.
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## Compression
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Pixel data is compressed using DEFLATE algorithm with 10-bit sliding window
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and no header. This can be achieved with ZLIB library by using the following:
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```python
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import zlib
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z = zlib.compressobj(level=9, wbits=-10)
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zdata = z.compress(pixeldata) + z.flush()
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```
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## Tools
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* [toif_convert](https://github.com/trezor/trezor-firmware/blob/master/core/tools/toif_convert.py) - tool for converting PNGs into TOI format and back
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