⟵ Back 8 min · 2022-04-08

Building cursed_font

As part of my relentless drive to realize the Year of the Linux Desktop, I had created a new bitmapped font about a year back, cursed_font. Because there is a terrible famine for readable, elegant fonts on Linux, obviously.

Why bitmapped fonts? While BDF fonts (and bitmapped fonts in general) are inferior to vector fonts in many ways (no variadic widths, no ligatures, no colors for emojis, etc), bitmapped fonts appear much sharper on low-DPI monitors. On my particular monitor, just about every vector font I'd like to use (Cascadia, Iosevka, Hack, Fira, Source Code Pro) is unacceptably blurry.

After its release, I had a few folks asking me what tools I used to create the font, as well as generate the font screenshots and character maps. Building BDF fonts is actually much easier than creating a vector font (since you're basically just placing dots on a grid), and due to the simplicity of the BDF format (which is a text format) creating the image generation tools was trivial.

The font editor

I started off creating BDF fonts using fnt, which is an ncurses utility. It was horrible: being a "suckless" project, it omits every feature which could possibly be useful in editing a font, instead presenting you with a bare grid which you have to use arrow keys (mouse support is bloated, didn't you know) to manipulate. Not to mention that it doesn't even create a BDF font, it stores the font in a textual format devoid of any metadata (it expects the font height/width to be in the directory name), which you must then manually convert to a BDF font via a provided script. Needless to say, it was a pain to use.

I now use gbdfed to edit the fonts. It's nothing close to perfect. It's a bit unpolished, lacks a few features, making editing font properties unnecessarily clumsy, and is a bit crash-prone.

gbdfed screenshot

However, gbdfed much easier to get the hang of than fontforge (at least, that was my experience.) You literally just start gbdfed, click on a glyph, and click on a grid to place dots. It's just Nice and Intuitive™. With fontforge, my monkey brain couldn't even figure out how to create a new bitmapped font without reaching for a guide. That said, if you're familiar with fontforge, go ahead using it -- many other bitmapped fonts I've seen out there (such as scientifica and cozette) do so; bitmapping with fontforge is hardly uncharted territory.

Screenshot generation

Let's focus on the charmap.lua script, which creates an image of all the glyphs in the font in a nice grid layout.

I'm going to be assuming that you know Lua for this section; if you don't, feel free to take a look at how Lua's gmatch/match functions work (which is used for splitting strings).

Now, we're just going to read the BDF file, line by line. The font file looks like this:

STARTFONT 2.1
FONT -kiedtl-cursed-medium-r-normal--18-180-75-75-C-90-ISO10646-1
SIZE 18 75 75
// *** font properties elided ***
ENDPROPERTIES
CHARS 654
STARTCHAR char0
ENCODING 0
SWIDTH 480 0
DWIDTH 9 0
BBX 9 18 0 -3
BITMAP
// *** some numbers here, we'll get to that. ***
ENDCHAR
STARTCHAR char0
ENCODING 1
SWIDTH 480 0
DWIDTH 9 0
BBX 9 18 0 -3
BITMAP
// *** more numbers... ***
ENDCHAR
STARTCHAR char0
ENCODING 2
SWIDTH 480 0
DWIDTH 9 0
BBX 9 18 0 -3
BITMAP
// *** more numbers... ***
ENDCHAR

Each directive is a single word, followed by one or more space-separated values. Splitting each line and grabbing the first two words is enough parsing for our purposes.

for line in file_data:gmatch("([^\n]+)\n?") do
    local cmd, arg = line:match("([^%s]+)%s+(.+)")
end

Firstly we need the CHARS directive, which tells us how many glyphs are in the font.

if cmd == "CHARS" then
    local total = tonumber(arg)
    max = math.floor(math.sqrt(total))
end

(The max global variable is the maximum width of a row of characters we should output before starting a new row. You could just make the row a fixed width and ignore CHARS entirely.)

Then, we take a look at each ENCODING directive, which will appear once for each character definition and holds the Unicode codepoint for that character (STARTCHAR is irrelevant as it just holds the "name" of the glyph, which is pretty useless as far as I can tell...). Then, we can simply output that character and write a space after it.

elseif cmd == "ENCODING" then
    local ch = tonumber(arg)
    if ch ~= 10 then
        io.stdout:write(utf8.char(ch))
        io.stdout:write(" ")
        col = col + 1
    end
end

...and then check if we should output a newline and start a new row.

if col >= max then
    io.stdout:write("\n")
    col = 0
end

(Link to full script.)

I pipe the output of this script to tools/draw.lua, which takes a stream of text (in this case, our charmap) and does the actual work of turning it into an image.

To do this, tools/draw.lua reads the font again. Except this time, we need to read in more information about the font, such as the height and width.

Start off by declaring some variables to hold the font data:

font = {}
font.current = 0
font.data = {}

Then, we'll parse the font, line by line, and split it by spaces:

for unparsed in font_data:gmatch("([^\n]+)\n?") do
    local line = collect(unparsed:gmatch("([^%s]+)%s?"))
end
collect() is a helper function I use to collect an iterator's values (in this case, gmatch) into an array:

function collect(...)
    local function _collect_helper(vals, i_f, i_s, i_v)
        local values = { i_f(i_s, i_v) }
        i_v = values[1]
        if not i_v then return vals end
        vals[#vals + 1] = table.unpack(values)
        return _collect_helper(vals, i_f, i_s, i_v)
    end
    return _collect_helper({}, ...)
end

FONTBOUNDINGBOX is parsed as normal, stuffing the values into the font table. ENCODING is stored in font.current to keep track of which font glyph we're reading (recall that a BDF glyph definition takes the form of STARTCHAR ... ENCODING ... <glyph data> ... ENDCHAR). Then, for all other numerical directives, we assume it's a piece of font data, and store it in the array at font.data[font.current]:

if line[1] == "FONTBOUNDINGBOX" then
    font.width  = tonumber(line[2])
    font.height = tonumber(line[3])
elseif line[1] == "ENCODING" then
    font.current = tonumber(line[2])
    font.data[font.current] = {}
elseif tonumber(line[1], 16) then
    local nm = tonumber(line[1], 16)
    local len = #font.data[font.current]
    font.data[font.current][len + 1] = nm
end

Those numbers which we had read as part of the symbol definition (which were, by the way, hexadecimal) need to be decoded to reveal the pixel layout: each number represents a row of pixels, with each bit in the 16-bit number representing a single pixel (with the highest bits being the leftmost pixels). Just a few lines of code:

for _, n in ipairs(font.data[<glyph>]) do
    -- <snip>

    for i = 1, 16 do
        local pixel_row = <font symbol data>
        if (pixel_row & (1 << (16 - i))) ~= 0 then
            canvas[y][x] = "1"
        else
            canvas[y][x] = "0"
        end

        -- <snip>
    end

    -- <snip>
end
In most fonts I've seen, the pixel data is 16 bits long; however in many other fonts that are less than 8 pixels wide, it is only 8 bits long, as it only needs to encode 8 bits of pixel data. Taking this into account will complicate the above code a bit, and anyway gbdfed has only creates 16-bit-wide pixel data as far as I can tell, so I'm ignoring that possibility here.

The rest of draw.lua simply creates a canvas matrix, iterates over each character piped to it from charmap.lua, and draws each character via the aforementioned method. It then outputs the canvas as a stream of ones and zeros, which is then turned into a PNG image after some more processing.

That's it! The simplicity of the BDF format makes it very easy to write helper scripts, like the ones I've reviewed here. I actually have a few other BDF scripts I use, like one that converts a font to C/Rust/Zig file that I can import into projects. If this were a vector font I'd have to instead start a terminal, pipe text to it, and then take a screenshot of it (or something like that) with an xdotool incantation. Yuck.

The Lua scripts are licensed under the Unlicense, so feel free to reuse it for whatever purposes you need -- especially if you're also creating a font! I'd encourage you to try creating your own BDF font as well, as a fun exercise.



Kiëd Llaentenn © 2019-2022 — CC BY-NC-ND 4.0