Why when using \ref {} for equations work, but when using to reference images i get a (??) as result ?

Question

i'm using the {fei} class im my document and my preamble is pretty simple as follows

\documentclass[rascunho]{fei}
\usepackage[utf8]{inputenc}
\usepackage{gensymb}
\usepackage{cancel}

when using the \ref{} command to call an equation as in the example below, the resut is pretty much the expected:

\begin{equation}
\nu_{max} \propto T \Leftrightarrow \nu_{max}=Constante_{2}.T
\label{eq4}
\end{equation}
onde, para um corpo negro, a equação (\ref{eq3})

But when using the exact same command to call in a figure i get an error, the code go as following:

\begin{figure}[h!]
\center
\includegraphics[scale=0.9]{fig15.jpg}
\label{fig15}
\end{figure}

Any equation that i reference works perfectly fine, but every single figure i try use the same method shows this error, i read that compiling 2 or more times might be able to fix the issue, but i tried several times and it persists.

i'm pretty sure it must be something really simple and easy to fix, but cant find what no matter how much i try....

Answer 1

You can use \label after LaTeX has generated a number; a figure environment, by itself, doesn't generate a number: it's \caption that does.

\documentclass[rascunho]{fei}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{siunitx}
\usepackage{graphicx}

\sisetup{output-decimal-marker={,}}

\begin{document}

\begin{equation}
\nu_{\mathrm{max}} \propto T \Leftrightarrow \nu_{\mathrm{max}}=
\mathrm{Constante}_{2}\cdot T
\label{eq:nu-max}
\end{equation}
onde, para um corpo negro, a equação~(\ref{eq:nu-max})
vale \SI{2,898e-3}{\meter\kelvin}.

\begin{figure}[htp]
\centering

\includegraphics[width=3cm]{example-image}

\caption{Esta é uma legenda}
\label{fig:example}

\end{figure}

O gráfico (A) de figura~\ref{fig:example} mostra a distribuição de Boltzmann.

\end{document}

Now the reference can work.

Some other points. The (low) period is never used to denote multiplication. Textual inserts in a math formula should be upright. For the constant and in other similar instances, it's best to use siunitx that guarantees uniform appearance across the document (for instance, the symbol for kelvin is an uppercase K). If you prefer to have explicit multiplication symbols between units, change the \sisetup command in the preamble into

\sisetup{
  output-decimal-marker={,},
  inter-unit-product=\ensuremath{{}\cdot{}},
}

and the output would become

with no change in the document code.

Use a tie between the word “equação” as well as “figura” and the reference number.

It's incorrect to use \center; rather use \centering or nothing at all, as the class sets left aligned captions, so probably left aligned images are better suited.