Gadgetbridge

Table of Contents

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Overview
Header

Character Section
Bitmap Section

Font Editor

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⚠ The wiki has been replaced by the new website - this page has been moved: https://gadgetbridge.org/internals/specifics/mi-band-2/#font-format

The Mi Band 2 uses a special Font format, which is documented below:

Overview

The HMZK Format has three sections:

Header
Character section, an array of the characters available in the font in order
Bitmap section, a list of 16x16 bitmaps in the same order as the Character Section

Header

The file starts with a 16 byte header: 484d 5a4b 01ff ffff ffff ffff ffff 7438

The first two bytes (484d 5a4b) are the "magic number" and spell out "HMZK" in ascii (possibly means HuaMi ZiKu, HuaMi character library)

The next five bytes (01ff ffff ffff ffff ffff in the chinese font, 01ff ffff ff00 ffff ffff in the english font) are of an unknown purpose, likely just padding.

The last two bytes are a 16bit little-endian integer describing the offset of the bitmap section from the character section in bytes (to find the absolute location of the bitmap section, add 16 to this number). This value can also be interpreted as length of the character section in bytes.

Character Section

This Section contains an array of 16bit (widechar) values:

2000 2100 2200 2300 2400 2500 2600 2700

Each of these elements is a little endian utf-16 like encoded character (note: UTF-16 specifies prefix bytes e.g. 0xFF 0xFE. These are omitted here)

Bitmap Section

0000 0000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 0000 0000 1000 1000

Each Bitmap consists of 16 16bit big endian (!!!) values. Each 16bit value represents one row of the bitmap. The above thus expands to:

................
................
...#............
...#............
...#............
...#............
...#............
...#............
...#............
...#............
...#............
...#............
................
................
...#............
...#............

Font Editor

CBiX/bitsnpicas-hmzk; modified version of kreativekorp/bitsnpicas to support importing and exporting HMZK fonts based on this specification. This can be used to either extend the original fonts with custom characters or convert existing bitmap fonts from the supported file formats for the Mi Band 2.

If you want to use other (more powerful) bitmap font editors (e.g. FontForge), you could use .bdf as an exchange format. However, be careful to export it with as close to 16x16 as possible and with little to no ascent/descent, as HMZK is fixed to 16x16 and fonts might look odd or cut off otherwise. Also the amount of possible characters is limited. Not sure about the hard limit, but try to stay within the size of the original chinese font, which contains 7226 characters. This is also used as a hard limit for export in bitsnpicas-hmzk for security reasons.