aspect.sty

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{aspect}[2019/07/23]
\RequirePackage{kvoptions}      % option processing
\RequirePackage{geometry}       % for \geometry
\RequirePackage{etoolbox}       % for \ifdefstring, \ifdefvoid, etc.
\RequirePackage{xfp}            % for \fpeval

%% Options
\DeclareStringOption[4/3]{aspect}
\DeclareStringOption[6/7]{hscale}
% vscale=6/7 is very large. 7/8, 8/9 good. 9/10 small but acceptable.
\DeclareStringOption[9/10]{vscale}
\DeclareStringOption[345pt]{width}
\DeclareStringOption{paperheight}
\DeclareStringOption[1]{scaled}

\ProcessLocalKeyvalOptions*

%% Styling.
\pagestyle{empty}


%% ---------- Calculating paper & text size ----------
\edef\aspect@aspect{\fpeval{\aspect@aspect}}
\edef\aspect@hscale{\fpeval{\aspect@hscale}}
\edef\aspect@vscale{\fpeval{\aspect@vscale}}

\newlength\aspect@tw \newlength\aspect@th
\newlength\aspect@pw \newlength\aspect@ph
\setlength\aspect@tw{\aspect@width}

%% Calculate paper size.
\ifdefvoid{\aspect@paperheight}{
  %% No paper height given, make up a reasonable default using hscale &
  %% aspectratio.
  \setlength\aspect@pw{\fpeval{1/\aspect@hscale}\aspect@tw}
  \setlength\aspect@ph{\aspect@aspect\aspect@pw}
}{
  %% Paper height supplied, use it.
  \setlength\aspect@ph{\aspect@paperheight}
  \setlength\aspect@pw{\fpeval{1/\aspect@aspect}\aspect@ph}
}

%% Calculate text height.
\setlength\aspect@th{\aspect@vscale\aspect@ph}


%% Scale everything by 1/\aspect@scaled.
\edef\aspect@scaled{\fpeval{1/(\aspect@scaled)}}
\setlength\aspect@th{\aspect@scaled\aspect@th}
\setlength\aspect@tw{\aspect@scaled\aspect@tw}
\setlength\aspect@ph{\aspect@scaled\aspect@ph}
\setlength\aspect@pw{\aspect@scaled\aspect@pw}

\geometry{
  papersize={\aspect@pw,\aspect@ph},
  total={\aspect@tw,\aspect@th},
  vratio={1:1}
}