Why do `ftcap` and `caption` produce different vertical spacing?

Question

I usually load the caption package to fix the spacing between tables and their captions in the article class. Since I don't use any other features of the caption package, I decided to try using the dedicated package ftcap to fix it instead. However, when I try each package, I get slightly different amounts of vertical space on the page. With the following MWE, when I use ftcap, all of the text moves slightly higher up on the page compared to the output when I use caption. As far as I can tell, the space between the table and caption is identical between the two (presumably 10.0pt).

MWE:

\documentclass{article}

\usepackage{caption}  % a bit less than 1mm lower
%\usepackage{ftcap}  % a bit less than 1mm higher

\begin{document}
    
This is a paragraph.

\begin{table}
    \caption{This is a caption.}
    \begin{tabular}{c}
        This is a table.
    \end{tabular}
\end{table}

\end{document}

The following code can be used to see the difference more clearly:

\documentclass{article}

\usepackage{fgruler}
\textwidth=\paperwidth
\oddsidemargin=-1in
\parindent=0pt

\usepackage{caption}  % a bit less than 1mm lower
%\usepackage{ftcap}  % a bit less than 1mm higher

\begin{document}
    
\rule{2cm}{0.1pt}
This is a paragraph.

\begin{table}
    \caption{This is a caption.}
    \begin{tabular}{c}
        \rule{2cm}{0.1pt}
        This is a table.
    \end{tabular}
\end{table}

\end{document}

I've tried going through the documentation and implementation of both packages. caption is quite a bit more complex, so I haven't been able to make as much sense of it.

Answer 1

The implementation of caption and ftcap are different.

As far as I can see, the difference is caused by caption adding a strut in the caption, which is not done by ftcap that doesn't modify the \caption command.

Let's do three examples. The top thin rule is added by the showframe option to geometry, showing the top edge of the type block.

ftcap with no \strut

\documentclass{article}
\usepackage[pass,showframe]{geometry}

\usepackage{ftcap}  % a bit less than 1mm higher

\begin{document}

\begin{table}[htp]
\centering
    \caption{This is a caption.}
Something
\end{table}

\end{document}

ftcap with \strut

\documentclass{article}
\usepackage[pass,showframe]{geometry}

\usepackage{ftcap}  % a bit less than 1mm higher

\begin{document}

\begin{table}[htp]
\centering
    \caption{\strut This is a caption.}
Something
\end{table}

\end{document}

caption

\documentclass{article}
\usepackage[pass,showframe]{geometry}

\usepackage{caption}  % a bit less than 1mm lower

\begin{document}

\begin{table}[htp]
\centering
    \caption{This is a caption.}
Something
\end{table}

\end{document}

There is also a tiny difference in the spacing between caption and the text.

Conclusion

Use either package, you can't hope that different implementations (in particular complex one such as caption's) produce the same output.