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diff --git a/docs/quantum_painter_qff.md b/docs/quantum_painter_qff.md new file mode 100644 index 0000000000..f62d59bdcb --- /dev/null +++ b/docs/quantum_painter_qff.md @@ -0,0 +1,103 @@ +# 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](quantum_painter_qgf.md#qgf-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: + +```c +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](quantum_painter_qgf.md#qgf-frame-descriptor), 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_. + +```c +#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. + +```c +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](quantum_painter_qgf.md#qgf-frame-palette-descriptor), 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](quantum_painter_qgf.md#qgf-frame-data-descriptor), however has a different _typeid_ of 0x04 in QFF. |