Krdytkyu

Looking for a way to add a custom line cap (slight bulbous) like one would get when inking a line on real paper.

Can one add such arrow tips? Sure. Am I sure that my proposal is the best inky tip ever? Absolutely not. However, this should give you an idea how to approach the "best inky tip ever". ;-)

documentclass[tikz,border=3.14mm]{standalone}

usetikzlibrary{arrows.meta}

usetikzlibrary{bending}



pgfdeclarearrow{

  name=ink,

  parameters= {thepgfarrowlength},  

  setup code={

   pgfarrowssettipend{0pt}

   pgfarrowssetlineend{-pgfarrowlength}

   pgfarrowlinewidth=pgflinewidth

   pgfarrowssavethepgfarrowlength

  },

  drawing code={

   pgfpathmoveto{pgfpoint{-pgfarrowlength}{0.5pgflinewidth}}

   pgfpathcurveto{pgfpoint{-0.75pgfarrowlength}{0.6pgflinewidth}}{%

   pgfpoint{-0.01pgfarrowlength}{0.6pgflinewidth}}{%

   pgfpoint{0pt}{0pt}}

   pgfpathcurveto{pgfpoint{-0.01pgfarrowlength}{-0.5pgflinewidth}}{%

   pgfpoint{-0.05pgfarrowlength}{-0.95pgflinewidth}}{%

   pgfpoint{-0.2pgfarrowlength}{-0.9pgflinewidth}}

   pgfpathcurveto{pgfpoint{-0.3pgfarrowlength}{-0.8pgflinewidth}}{%

   pgfpoint{-0.6pgfarrowlength}{-0.55pgflinewidth}}{%

   pgfpoint{-01pgfarrowlength}{-0.5pgflinewidth}}

   pgfusepathqfill

  },

  defaults = { length = 12pt }

}

begin{document}

begin{tikzpicture}

draw [line width=1mm,{ink[bend]}-{ink[bend]},blue!80!black] (0,0) to [bend left] (3,0);

end{tikzpicture}

end{document}

Or, if you wish to have a more pronounced tip, this can also be done.

documentclass[tikz,border=3.14mm]{standalone}

usetikzlibrary{arrows.meta}

usetikzlibrary{bending}



pgfdeclarearrow{

  name=ink,

  parameters= {thepgfarrowlength},  

  setup code={

   pgfarrowssettipend{0pt}

   pgfarrowssetlineend{-pgfarrowlength}

   pgfarrowlinewidth=pgflinewidth

   pgfarrowssavethepgfarrowlength

  },

  drawing code={

   pgfpathmoveto{pgfpoint{-pgfarrowlength}{0.5pgflinewidth}}

   pgfpathcurveto{pgfpoint{-0.75pgfarrowlength}{0.6pgflinewidth}}{%

   pgfpoint{-0.01pgfarrowlength}{0.6pgflinewidth}}{%

   pgfpoint{0pt}{0pt}}

   pgfpathcurveto{pgfpoint{-0.01pgfarrowlength}{-0.5pgflinewidth}}{%

   pgfpoint{-0.2pgfarrowlength}{-1.35pgflinewidth}}{%

   pgfpoint{-0.3pgfarrowlength}{-1.1pgflinewidth}}

   pgfpathcurveto{pgfpoint{-0.4pgfarrowlength}{-0.8pgflinewidth}}{%

   pgfpoint{-0.6pgfarrowlength}{-0.55pgflinewidth}}{%

   pgfpoint{-01pgfarrowlength}{-0.5pgflinewidth}}

   pgfusepathqfill

  },

  defaults = { length = 12pt }

}

begin{document}

begin{tikzpicture}

draw [line width=1mm,{ink[bend]}-{ink[bend]},blue!80!black] (0,0) to [bend left] (3,0);

end{tikzpicture}

end{document}

And one could also add some randomness.

documentclass[tikz,border=3.14mm]{standalone}

usetikzlibrary{arrows.meta}

usetikzlibrary{bending}



pgfdeclarearrow{

  name=ink,

  parameters= {thepgfarrowlength},  

  setup code={

   pgfarrowssettipend{0pt}

   pgfarrowssetlineend{-pgfarrowlength}

   pgfarrowlinewidth=pgflinewidth

   pgfarrowssavethepgfarrowlength

  },

  drawing code={

   pgfpathmoveto{pgfpoint{-pgfarrowlength}{0.5pgflinewidth}}

   pgfpathcurveto{pgfpoint{-0.75pgfarrowlength}{0.6pgflinewidth}}{%

   pgfpoint{-0.01pgfarrowlength}{0.6pgflinewidth}}{%

   pgfpoint{0pt}{0pt}}

   pgfpathcurveto{pgfpoint{-0.01pgfarrowlength}{-0.5pgflinewidth}}{%

   pgfpoint{-0.2pgfarrowlength}{-(1+0.3*rnd)*pgflinewidth}}{%

   pgfpoint{-0.3pgfarrowlength}{-0.8*(1+0.3*rnd)*pgflinewidth}}

   pgfpathcurveto{pgfpoint{-0.4pgfarrowlength}{-0.6*(1+0.3*rnd)*pgflinewidth}}{%

   pgfpoint{-0.6pgfarrowlength}{-0.3*(1+0.3*rnd)*pgflinewidth}}{%

   pgfpoint{-1pgfarrowlength}{-0.5pgflinewidth}}

   pgfusepathqfill

  },

  defaults = { length = 12pt }

}

begin{document}

begin{tikzpicture}

draw [line width=1mm,{ink[bend]}-{ink[bend]},blue!80!black] (0,0) to [bend left] (3,0);

end{tikzpicture}

end{document}

BONUS: Some quick and dirty "inky" letters.

documentclass[tikz,border=3.14mm]{standalone}

usetikzlibrary{arrows.meta}

usetikzlibrary{bending}

usetikzlibrary{positioning}

pgfdeclarearrow{

  name=ink,

  parameters= {thepgfarrowlength},  

  setup code={

   pgfarrowssettipend{0pt}

   pgfarrowssetlineend{-pgfarrowlength}

   pgfarrowlinewidth=pgflinewidth

   pgfarrowssavethepgfarrowlength

  },

  drawing code={

   pgfpathmoveto{pgfpoint{-pgfarrowlength}{0.5pgflinewidth}}

   pgfpathcurveto{pgfpoint{-0.75pgfarrowlength}{0.6pgflinewidth}}{%

   pgfpoint{-0.01pgfarrowlength}{0.6pgflinewidth}}{%

   pgfpoint{0pt}{0pt}}

   pgfpathcurveto{pgfpoint{-0.01pgfarrowlength}{-0.5pgflinewidth}}{%

   pgfpoint{-0.2pgfarrowlength}{-(1+0.3*rnd)*pgflinewidth}}{%

   pgfpoint{-0.3pgfarrowlength}{-0.8*(1+0.3*rnd)*pgflinewidth}}

   pgfpathcurveto{pgfpoint{-0.4pgfarrowlength}{-0.6*(1+0.3*rnd)*pgflinewidth}}{%

   pgfpoint{-0.6pgfarrowlength}{-0.3*(1+0.3*rnd)*pgflinewidth}}{%

   pgfpoint{-1pgfarrowlength}{-0.5pgflinewidth}}

   pgfusepathqfill

  },

  defaults = { length = 12pt }

}

begin{document}

tikzset{pics/.cd,

  A/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] 

  (0,-0.55) -- (0.3,0.4) -- (0.6,-0.55);

  draw[line width=5](0.1,1/3-0.45) --

  (0.5,1/3-0.45);

  path (0.7,0);}},

  B/.style={code={draw[line width=5] (0,-0.45) -- (0,0.45)

  to[out=0,in=0,looseness=2.5]  (0,0)  to[out=0,in=0,looseness=3] cycle;}},

  C/.style={code={draw[line width=5,-{ink[bend]}]

   (0,0) to[out=90,in=110,looseness=2]  (0.5,0.25);

   draw[line width=5,-{ink[bend]}](0,0) to[out=-90,in=-110,looseness=2]  (0.5,-0.25);

   path (0.7,0);}},

  D/.style={code={draw[line width=5] (0,-0.45) -- (0,0.45) 

  to[out=0,in=0,looseness=2.25]   cycle;

  path (0.7,0);}},

  E/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] 

  (0.5,-0.45) --(0,-0.45) -- (0,0.45)  -- (0.5,0.45);

  draw[line width=5,-{ink[bend]}] (0,0) -- (0.5,0);

  path (0.7,0);}},

  F/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] 

  (0,-0.45) -- (0,0.45)  -- (0.5,0.45);

  draw[line width=5,-{ink[bend]}] (0,0) -- (0.5,0);

  path (0.7,0);}},

  G/.style={code={draw[line width=5,-{ink[bend]}]

   (0,0) to[out=90,in=110,looseness=2]  (0.5,0.25);

   draw[line width=5] (0,0) to[out=-90,in=-110,looseness=2]  

   (0.5,-0.25);

   draw[line width=5,-{ink[bend]}] (0.54,-0.25) to (0.3,-0.25);

   path (0.7,0);}},

  H/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] 

  (0,-0.5) -- (0,0.5);  

  draw[line width=5,{ink[bend]}-{ink[bend]}] (0.5,-0.5) -- (0.5,0.5);

  draw[line width=5] (0,0) -- (0.5,0);

  path (0.7,0);}},

  I/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] (0,-0.45) -- (0,0.45);

  path (0.25,0);}},

  J/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] (0.2,0.45) -- (0.2,-0.35) to[out=-90,in=0]

  (0.1,-0.45) to[out=180,in=-90] (0,-0.35);

  path (0.45,0);}},

  K/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] 

  (0,-0.45) -- (0,0.45); 

  draw[line width=5,{ink[bend]}-{ink[bend]}] (0.4,0.45) -- (0.02,0) --  (0.4,-0.45);

  path (0.6,0);}},

  L/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] 

  (0,0.5) -- (0,-0.45) -- (0.4,-0.45);

  path (0.6,0);}},

  M/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] (0,-0.45) -- (0,0.45) -- 

  (0.3,0.25) -- (0.6,0.45) -- (0.6,-0.45);

  path (0.8,0);}},

  N/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] (0,-0.45) -- (0,0.45) -- (0.6,-0.4) --

  (0.6,0.45);

  path (0.8,0);}},

  O/.style={code={draw[line width=5] (0.3,0) circle(0.3 and 0.48);

  path (0.8,0);}},

  P/.style={code={draw[line width=5,{ink[bend]}-] (0,-0.45) -- (0,0.45) 

  to[out=0,in=0,looseness=2.5]  (0,0);

  path (0.6,0);}},

  Q/.style={code={draw[line width=5] 

  (0.3,0) circle(0.3 and 0.48);

  draw[line width=5,{ink[bend]}-{ink[bend]}](0.35,-0.25) -- (0.6,-0.45);

  path (0.8,0);}},

  R/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] 

  (0,-0.45) -- (0,0.45) 

  to[out=0,in=0,looseness=2.5]  (0.05,0) -- (0.4,-0.45);

  path (0.6,0);}},

  S/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] (0.5,0.4)  

  to[out=160,in=165,looseness=2]  (0.3,0)  

  to[out=-15,in=-20,looseness=2] (0.1,-0.4);

  path (0.65,0);}},

  T/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] (0.35,-0.45) -- (0.35,0.45) (0,0.45) -- (0.7,0.45);

  path (0.85,0);}},

  U/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] (0,0.5) -- (0,0) to[out=-90,in=-90,looseness=2.5]

  (0.6,0) -- (0.6,0.5);

  path (0.8,0);}},

  V/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] (0,0.5) -- (0.3,-0.4) -- (0.6,0.5);

  path (0.8,0);}},

  W/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] (0,0.45) -- (0.3,-0.4) -- (0.45,-0.1)

  -- (0.6,-0.4) -- (0.9,0.45);

  path (1.1,0);}},

  X/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] 

  (0,0.45) -- (0.6,-0.45);

   draw[line width=5,{ink[bend]}-{ink[bend]}] (0.6,0.45)

  -- (0,-0.45);

  path (0.8,0);}},

  Y/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] 

  (0,0.45) -- (0.3,0);  

  draw[line width=5,{ink[bend]}-{ink[bend]}] (0.6,0.45)

  -- (0,-0.45);

  path (0.8,0);}},

  Z/.style={code={draw[line width=5,{ink[bend]}-{ink[bend]}] (0,0.45) --(0.6,0.45) -- (0,-0.45)  

  -- (0.6,-0.45);

  path (0.8,0);}},

  space/.style={code={path (0,0) (0.2,0);}},

}

begin{tikzpicture}[blue!80!black]

pic[local bounding box=box1,scale=2] at (0,0) {A};

foreach X [count=Y,evaluate=Y as Z using {int(Y+1)}] in {B,...,Z}

{edeftemp{noexpandpic[right=0mm of boxY,local bounding box=boxZ,scale=2]

{X};}

temp}

end{tikzpicture}

end{document}

(Yes, obviously these are not at all as beautiful as dedicated fonts. However, one can subject them to nonlinear transformation, such as those of this impressive master piece. I spare this application for another day. Also because arrow tips and nonlinear transformations require some extra work.)