for x = 0, width-1 do
    for y = 0, height-1 do
        local pos = (y * width + x) * stride

        colors = {callback(x, y, pos)}

        for i = 1, stride do
            buffer[pos+i-1] = (colors[i] or 1) * 255
        end
    end
end

...............

local res = 16
local _res = res/2

font:Fill(function(x, y, i)

    if i%res > _res then
        return 1,1,1
    else
        return 0,0,0
    end
end)