Scalable Screen Font

Scalable Screen Font is a compressed font format which comes with a small, free, MIT licensed rendering library. It was designed specially for hobby OS developers, therefore it has minimal dependencies.

Supported Font Types

• Bitmap - you can convert PC Screen Font with UNICODE table into SSFN directly. X11 Bitmap Distribution Format also supported.
• Pixmap - to have colorful emoji icons, you can convert TGA files into SSFN fonts.
• Vector - vector based fonts can be converted from OpenType and TrueType fonts.

Fonts are compressed, and in case of vector fonts, with a data-loss compression. You can set the compression ratio to balance between quality and font size. With the same quality as a typical OpenType file, the SSFN font usually takes only the half the file size.

Library

The SSFN package comes with a single ANSI C/C++ header file, which has two renderers. The header contains everything, no shared library nor static linking needed.

Simple Renderer

Designed specially for OS consoles, has only one function. It can render unscaled bitmap fonts directly to the framebuffer. Has absolutely no dependencies, and compiles to less than a kilobyte of code.

#define SSFN_NOIMPLEMENTATION               /* don't include the normal renderer implementation */
#define SSFN_CONSOLEBITMAP_HICOLOR          /* use the special renderer for hicolor packed pixels */
#include <ssfn.h>

/* set up context by global variables */
ssfn_font = &_binary_console_sfn_start;     /* the bitmap font to use */
ssfn_dst_ptr = 0xE0000000;                  /* framebuffer address and bytes per line */
ssfn_dst_pitch = 4096;
ssfn_fg = 0xFFFF;                           /* colors, white on black */
ssfn_bg = 0;
ssfn_x = 100;                               /* coordinates to draw to */
ssfn_y = 200;

/* render one glyph directly to the screen and then adjust ssfn_x and ssfn_y */
ssfn_putc(0x41);

Normal Renderer

There's another for user space applications. This one can render all three types of fonts, it can scale, antialias, autohint and kern glyphs. Has minimal libc dependencies (memset, memcmp, realloc, free) and compiles to about 22 kilobytes of code.

#include <ssfn.h>

ssfn_t ctx;                                         /* the renderer context */
ssfn_glyph_t *glyph;                                /* the returned rasterized bitmap */

/* you don't need to initialize the library, just make sure the context is zerod out */
memset(&ctx, 0, sizeof(ssfn_t));

/* add one or more fonts to the context. Fonts must be already in memory */
ssfn_load(&ctx, &_binary_times_sfn_start);          /* you can add different styles... */
ssfn_load(&ctx, &_binary_timesbold_sfn_start);
ssfn_load(&ctx, &_binary_timesitalic_sfn_start);
ssfn_load(&ctx, &_binary_emoji_sfn_start);          /* ...or different UNICODE ranges */
ssfn_load(&ctx, &_binary_cjk_sfn_start);

/* select the typeface to use */
ssfn_select(&ctx,
    SSFN_FAMILY_SERIF, NULL,                        /* family */
    SSFN_STYLE_REGULAR | SSFN_STYLE_UNDERLINE, 64,  /* style and size */
    SSFN_MODE_BITMAP                                /* rendering mode */
);

/* rasterize a glyph for the 0x41 UNICODE code point into a newly allocated bitmap */
glyph = ssfn_render(&ctx, 0x41);

/* display the bitmap on your screen */
my_draw_glyph(
    pen_x, pen_y - glyph->baseline,                 /* coordinates to draw to */
    glyph->w, glyph->h, glyph->pitch,               /* bitmap dimensions */
    &glyph->data                                    /* bitmap data */
);
pen_x += glyph->adv_x;                              /* adjust the cursor */
pen_y += glyph->adv_y;                              /* ssfn handles vertical fonts too */

/* free resources */
free(glyph);                                        /* no special treatment for freeing glyphs */
ssfn_free(&ctx);                                    /* free the renderer context's internal buffers */

Left-to-right text

This is the default. Left to right requires nothing special, only to subtract baseline and add the advance value to your current pen position.

my_draw_glyph(pen_x, pen_y - glyph->baseline, ...); /* position the glyph vertically */

pen_x += glyph->adv_x;                              /* adjust X for the next glyph */

Vertical text

To display vertical fonts properly, you should subtract the baseline from the X coordinate before you draw.

my_draw_glyph(pen_x - glyph->baseline, pen_y, ...); /* position the glyph horizontally */

pen_y += glyph->adv_y;                              /* adjust the Y coordinate this time */

Right-to-left text

It is not as simple as one would think, because it's not enough to know if the given UNICODE character belongs to a right-to-left scripting system, BiDi requires a minimal state machine too. That has to be implemented in the text renderer (or text shaping library) that's built on top of the low level rasterizer. The algorithm to properly display bidirectional texts is specified by UNICODE. But once you have decided that you need to draw a glyph in right-to-left direction, these are the required steps:

my_draw_glyph(pen_x - glyph->w, pen_y, ...);        /* subtract the glyph's width first */

pen_x -= glyph->adv_x;                              /* and subtract the advance instead of adding */

Putting every direction variations all together:

if(glyph->adv_y) {                                  /* things to temporarily adjust before drawing */
    x = pen_x - glyph->baseline;
    y = pen_y;
} else {
    x = pen_x - (rtl ? glyph->w : 0);
    y = pen_y - glyph->baseline;
}

my_draw_glyph(x, y, ...);

if(rtl)                                             /* set up cursor position for the next glyph */
    pen_x -= glyph->adv_x;
else
    pen_x += glyph->adv_x;
pen_y += glyph->adv_y;

Links

See Also

• VGA Fonts
• PC Screen Font

External Links

• OSDEV forum on Scalable Screen Fonts
• https://gitlab.com/bztsrc/scalable-font source code repository
• .sfn file format description
• SSFN API documentation