opensteno_qmk/docs/quantum_painter_qff.md

QMK Font Format :id=qmk-font-format

QMK uses a font format ("Quantum Font Format" - QFF) specifically for resource-constrained systems.

This format is capable of encoding 1-, 2-, 4-, and 8-bit-per-pixel greyscale- and palette-based images into a font. It also includes RLE for pixel data for some basic compression.

All integer values are in little-endian format.

The QFF is defined in terms of blocks -- each block contains a header and an optional blob of data. The header contains the block's typeid, and the length of the blob that follows. Each block type is denoted by a different typeid has its own block definition below. All blocks are defined as packed structs, containing zero padding between fields.

The general structure of the file is:

• Font descriptor block
• ASCII glyph block (optional, only if ASCII glyphs are included)
• Unicode glyph block (optional, only if Unicode glyphs are included)
• Font palette block (optional, depending on frame format)
• Font data block

Block Header :id=qff-block-header

The block header is identical to QGF's block header, and is present for all blocks, including the font descriptor.

Font descriptor block :id=qff-font-descriptor

• typeid = 0x00
• length = 20

This block must be located at the start of the file contents, and can exist a maximum of once in an entire QGF file. It is always followed by either the ASCII glyph table or the Unicode glyph table, depending on which glyphs are included in the font.

Block format:

typedef struct __attribute__((packed)) qff_font_descriptor_v1_t {
    qgf_block_header_v1_t header;               // = { .type_id = 0x00, .neg_type_id = (~0x00), .length = 20 }
    uint24_t              magic;                // constant, equal to 0x464651 ("QFF")
    uint8_t               qff_version;          // constant, equal to 0x01
    uint32_t              total_file_size;      // total size of the entire file, starting at offset zero
    uint32_t              neg_total_file_size;  // negated value of total_file_size, used for detecting parsing errors
    uint8_t               line_height;          // glyph height in pixels
    bool                  has_ascii_table;      // whether the font has an ascii table of glyphs (0x20...0x7E)
    uint16_t              num_unicode_glyphs;   // the number of glyphs in the unicode table -- no table specified if zero
    uint8_t               format;               // frame format, see below.
    uint8_t               flags;                // frame flags, see below.
    uint8_t               compression_scheme;   // compression scheme, see below.
    uint8_t               transparency_index;   // palette index used for transparent pixels (not yet implemented)
} qff_font_descriptor_v1_t;
// _Static_assert(sizeof(qff_font_descriptor_v1_t) == (sizeof(qgf_block_header_v1_t) + 20), "qff_font_descriptor_v1_t must be 25 bytes in v1 of QFF");

The values for format, flags, compression_scheme, and transparency_index match QGF's frame descriptor block, with the exception that the delta flag is ignored by QFF.

ASCII glyph table :id=qff-ascii-table

• typeid = 0x01
• length = 290

If the font contains ascii characters, the ASCII glyph block must be located directly after the font descriptor block.

#define QFF_GLYPH_WIDTH_BITS 6
#define QFF_GLYPH_WIDTH_MASK ((1<<QFF_GLYPH_WIDTH_BITS)-1)
#define QFF_GLYPH_OFFSET_BITS 18
#define QFF_GLYPH_OFFSET_MASK (((1<<QFF_GLYPH_OFFSET_BITS)-1) << QFF_GLYPH_WIDTH_BITS)

typedef struct __attribute__((packed)) qff_ascii_glyph_table_v1_t {
    qgf_block_header_v1_t header;  // = { .type_id = 0x01, .neg_type_id = (~0x01), .length = 285 }
    uint24_t glyph[95];            // 95 glyphs, 0x20..0x7E, see bits/masks above for values
} qff_ascii_glyph_table_v1_t;
// _Static_assert(sizeof(qff_ascii_glyph_table_v1_t) == (sizeof(qgf_block_header_v1_t) + 285), "qff_ascii_glyph_table_v1_t must be 290 bytes in v1 of QFF");

Unicode glyph table :id=qff-unicode-table

• typeid = 0x02
• length = variable

If this font contains unicode characters, the unicode glyph block must be located directly after the ASCII glyph table block, or the font descriptor block if the font does not contain ASCII characters.

typedef struct __attribute__((packed)) qff_unicode_glyph_table_v1_t {
    qgf_block_header_v1_t header;     // = { .type_id = 0x02, .neg_type_id = (~0x02), .length = (N * 6) }
    struct __attribute__((packed)) {  // container for a single unicode glyph
        uint24_t code_point;          // the unicode code point
        uint24_t glyph;               // the glyph information, as per ASCII glyphs above
    } glyph[N];                       // N glyphs worth of data
} qff_unicode_glyph_table_v1_t;

Font palette block :id=qff-palette-descriptor

• typeid = 0x03
• length = variable

The font palette block is identical to QGF's frame palette block, retaining the same typeid of 0x03.

It is only specified in the QFF if the font is palette-based, and follows the unicode glyph block if the font contains any Unicode glyphs, or the ASCII glyph block if the font contains only ASCII glyphs.

Font data block :id=qff-data-descriptor

• typeid = 0x04
• length = variable

The font data block is the last block in the file and is identical to QGF's frame data block, however has a different typeid of 0x04 in QFF.