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Consider the following minimal example meant to illustrate a single figure with a subfigure using the memoir documentclass:

\newsubfloat{figure}% Allow for subfloats (subfigures) within figure environment
  \caption{This is a caption}

The caption of the subfigure is empty so that it only displays the subfigure counter - '(a)' in this case:

Figure with subfigure in memoir class

However, the subfigure label is not perfectly centered the way I had hoped with an empty caption. Changing the subfigure definition (using some negative spacing via $\!\!$) to

\newsubfloat{figure}% Allow for subfloats (subfigures) within figure environment
  \caption{This is a caption}

instead produces a more acceptable empty subfigure caption, however minuscule the difference:

Figure with subfigure in memoir class

Is this a known bug, or at least fixable?

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that is the space between (a) and the caption. Seems like a bug because memoir already tests if the contents of [] is empty. –  Herbert Jul 14 '11 at 7:51
@Herbert Actually it doesn't test for emptyness, but it tests whether the contents is \@empty, which happens if there is no optional argument. –  egreg Jul 14 '11 at 18:06

1 Answer 1

up vote 5 down vote accepted

The label leaves a space before the subcaption; it's necessary to kill it if there's no subcaption.

  \@ifnextchar [%
    {\@memsubfloat{sub\@captype}[\@empty #1][#1\unskip]}}

Some explanations

The complete syntax of \subbottom is

\subbottom[A][B]{<figure code>}

where A is the entry for the list of figures (tables) and B is the subcaption (they may be different just like captions for figures and tables). When B is not expressed, it's assumed to be the same as A. If also A is missing, no subcaption is set, not even the letter.

The control is actually passed to some other macros and eventually \@memsubfig is called when A is present (it will be substituted for #1 in the code). This macro checks if there is another optional argument. If there is, then


will be evaluated, otherwise LaTeX will be confronted with

\@memsubfloat{subfigure}[\@empty A][A]

(Note: we assume to be dealing with a figure; for a table, \@captype would be table.) This in the original definition. With my definition we'll have

\@memsubfloat{subfigure}[\@empty A][A\unskip]

The \@empty is there for technical reasons, it's not important since that is what will be written in the auxiliary file. The task of \@memsubfloat is also to typeset the subcaption and it basically does

(a)\space A\unskip

where "a" is the "number" (the \unskip isn't there with memoir's definition); if now A is empty, as in your case, the \unskip will kill the \space, otherwise it will do nothing. Bingo!

Of course, if you have captionless subfigures, you won't put subcaptions in the list of figures.

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If and when you have the time and patience, could you annotate that? I appreciate among experts it's probably obvious, but for us noobs an annotation would be a useful learning resource. If you don't have the time, of course I understand. –  Brent.Longborough Jul 14 '11 at 21:42
Thank you. Excellent explanation. –  Brent.Longborough Jul 15 '11 at 11:53
suggested fix will be added in the next release –  daleif Jul 21 '11 at 8:52

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