FNT Format

FNT Font Format Specification

Overview

FNT is a custom bitmap font format designed for ModuOS. It provides full Unicode support with efficient storage and rendering.

File Format

Header

Offset | Size | Type     | Description
-------|------|----------|---------------------------
0x00   | 4    | char[4]  | Magic: "FNT1"
0x04   | 2    | uint16   | Version (currently 1)
0x06   | 2    | uint16   | Font name length
0x08   | N    | char[]   | Font name (UTF-8)
N+8    | 2    | uint16   | Maximum glyph width
N+10   | 2    | uint16   | Glyph height
N+12   | 2    | uint16   | Baseline position
N+14   | 4    | uint32   | Number of glyphs

Note: There are no padding bytes between baseline and glyph count - they are consecutive.

Glyph Entry (repeated for each glyph)

Offset | Size | Type     | Description
-------|------|----------|---------------------------
0x00   | 4    | uint32   | Unicode codepoint
0x04   | 2    | uint16   | Actual character width (for spacing)
0x06   | 2    | uint16   | Bitmap width
0x08   | 2    | uint16   | Bitmap height
0x0A   | N    | uint8[]  | Packed bitmap data (1 bit per pixel)

Bitmap Data

• Packed Format: 8 pixels per byte, MSB first
• Row Padding: Each row is padded to byte boundary
• Bytes per row: (bitmap_width + 7) / 8
• Total size: bytes_per_row * bitmap_height

Features

• Unicode Support: Store any Unicode codepoint
• Variable Width: Each glyph can have different width
• Efficient Lookup: Binary search for non-ASCII, direct array for ASCII
• Baseline: Proper vertical alignment support
• Compact: 1-bit bitmap, minimal overhead

Usage in ModuOS

Loading a Font

#include "moduos/drivers/graphics/fnt_font.h"

void *font_data = /* load from file */;
size_t font_size = /* file size */;

fnt_font_t *font = fnt_load_font(font_data, font_size);
if (font) {
    fbcon_set_fnt_font(&console, font);
}

Getting a Glyph

fnt_glyph_t *glyph = fnt_get_glyph(font, 0x0041); // 'A'
if (glyph) {
    // Render glyph->bitmap
}

Rendering

// Manual rendering
for (int y = 0; y < glyph->bitmap_height; y++) {
    for (int x = 0; x < glyph->bitmap_width; x++) {
        if (fnt_get_pixel(glyph, x, y)) {
            // Draw pixel at (x, y)
        }
    }
}

// Or use built-in renderer
fnt_render_glyph(glyph, framebuffer, x, y, pitch, bpp, fg_color, bg_color);

Font Creation

Use the custom FNT font editor (Python/tkinter GUI) to create fonts:

• Visual bitmap glyph editing
• Unicode codepoint support
• Real-time preview
• Export to .fnt format

Default Font Location

ModuOS looks for fonts at: /ModuOS/shared/assets/fonts/Unicode.fnt

Advantages over PF2

• No GNU dependencies: Custom format, no licensing concerns
• Simpler parsing: Straightforward binary format
• Better performance: Optimized for OS needs
• Easier tooling: Custom editor tailored for ModuOS

Notes

• All multi-byte values are little-endian
• Glyphs should be sorted by codepoint for binary search
• ASCII glyphs (0-127) are cached for fast access
• Missing glyphs fall back to '?' character