How to produce hollow Greek letters (in math mode)

Question

I have a strange question regarding the notation of my thesis. My paper is on the topic of quantum algebra, so I have a plethora of symbols to sign to my definitions and defined functions. In a recent (yet brilliant) paper I found some very interesting symbols - hollow Greek letters. One of them looked like this:

And another one like this:

Or for example this beauty:

I don't know what packages were used to produce these symbols, but I started searching and soon found \usepackage[bbgreekl]{mathbbol} that produced the following result (code included):

\documentclass{article}

\usepackage{amsmath}
\usepackage{breqn}
\usepackage[bbgreekl]{mathbbol}
\usepackage{bbm}
%\usepackage[mtpbbi]{mtpro2}

\begin{document}

\[ \mathbb{A B \Gamma \Delta E Z H \Theta I K \Lambda M N \Xi O \Pi P \Sigma T \Upsilon \Phi X \Psi \Omega \alpha\beta\gamma\delta\epsilon\zeta\eta\theta\iota\kappa\lambda\mu\nu o \xi \pi\rho\sigma\tau\upsilon\phi\chi\psi\omega} \]

\end{document}

This font is nice, don't get me wrong, but it isn't what I wanted to produce. I'd like my output to look exactly like the three symbols above. So I searched on. Note that one of my lines of code is commented with % and for a very good reason. I found another package that would output the hollow Greek letters. I don't know what font they are written in as my code didn't compile after I added \usepackage[mtpbbi]{mtpro2}. The following error showed up:

! LaTeX Error: File `mtpro2.sty' not found.

I don't know what to do or if this package even offers what I'm searching for. I ask you (kind strangers) to show me a way to hollow the Greek letters so the three characters can be produced (alongside all other lower- and uppercase Greek letters, of course). Your help is much appreciated.

Answer 1

If pdf specials are an option then there is this approach. You can set the border color, fill color and border thickness (hat tip, Malipivo: TikZ: halo around text?).

\documentclass{article}
\usepackage{xcolor}
%%%
\makeatletter

%%% MALIPIVO's PDF SPECIAL TO CHANGE FONT OUTLINE THICKNESS/COLOR
% https://tex.stackexchange.com/questions/18472/tikz-halo-around-text/169549#169549
\input pdf-trans
\newbox\qbox
\def\usecolor#1{\csname\string\color@#1\endcsname\space}
\newcommand\bordercolor[1]{\colsplit{1}{#1}}
\newcommand\fillcolor[1]{\colsplit{0}{#1}}
\newcommand\outline[1]{\leavevmode%
  \def\maltext{#1}%
  \setbox\qbox=\hbox{\maltext}%
  \boxgs{Q q 2 Tr \thickness\space w \fillcol\space \bordercol\space}{}%
  \copy\qbox%
}
\makeatother
\newcommand\colsplit[2]{\colorlet{tmpcolor}{#2}\edef\tmp{\usecolor{tmpcolor}}%
  \def\tmpB{}\expandafter\colsplithelp\tmp\relax%
  \ifnum0=#1\relax\edef\fillcol{\tmpB}\else\edef\bordercol{\tmpC}\fi}
\def\colsplithelp#1#2 #3\relax{%
  \edef\tmpB{\tmpB#1#2 }%
  \ifnum `#1>`9\relax\def\tmpC{#3}\else\colsplithelp#3\relax\fi
}
\bordercolor{black}
\fillcolor{white}
\def\thickness{.3}
\begin{document}
\outline{This is a test $\eta \Lambda \nabla$}
\end{document}