Footnote citation in beamer with disappearing image

Question

I am preparing the presentation for my master's thesis dissertation using beamer.

In a frame, I have a list of items, with an image I want to hide in the second slide using \only<1>. However, the first item contains a citation I am inserting with \footcite. At first, I was surrounding everything with an overlayarea, but the resulting footnote was not aligned with the others and the footnote mark was not consistent.

Following this SE question, I moved it to a top aligned frame and the result looks better. Anyway, the footnote is not shown in the first slide (when the image is visualized) even though the mark is present, and I do not get why.

Does anybody know how may I fix it, showing the footnote in the first slide as well? Thanks in advance!

---

Here is the code I am using:

\begin{frame}[t]{Method}
    \framesubtitle{Adapting the Feature Pyramid Network}
    
    \begin{itemize}
        \item<1-> \alert{FPN}\footfullcite{lin2017feature} is widely used for object detection:
        \begin{itemize}
            \item Extract feature maps at multiple levels from a single image
            \item Sequence of convolutional layers, size divided by two at consecutive stages, with top-down connections
            \item Increases detection performances, but adversarial domain alignment is non-trivial
        \end{itemize}
        \only<1>{
            \begin{center}
                \item[]
                \fbox{\includegraphics[width=.5\textwidth]{imgs/fpn.png}}
            \end{center}
        }
        \item<2-> Tested approaches:
        \begin{itemize}
            \item Align levels of extracted feature pyramid
            \item Align layers of the \alert{ResNet} feature extractor, input for FPN
        \end{itemize}
        \item<2-> Extracted representation is meaningful and independent of input domain
    \end{itemize}
\end{frame}

I am using pdflatex and biblatex:

\usepackage[backend=biber]{biblatex}
...
\addbibresource{my_biblio.bib}

and the citation is contained in my_biblio.bib:

@misc{lin2017feature,
      title={Feature Pyramid Networks for Object Detection}, 
      author={Tsung-Yi Lin and Piotr Dollár and Ross Girshick and Kaiming He and Bharath Hariharan and Serge Belongie},
      year={2017}
}

Answer 1

For lack of a MWE and of the local image fpn.png that you are importing, I could not reproduce your output exactly, but the following homemade MWE appears to replicate your issue:

\begin{filecontents}{mybib.bib}
@misc{lin2017feature,
      title={Feature Pyramid Networks for Object Detection}, 
      author={Tsung-Yi Lin and Piotr Dollár and Ross Girshick and Kaiming He and Bharath Hariharan and Serge Belongie},
      year={2017}
}
\end{filecontents}

\documentclass{beamer}
\usepackage[backend=biber]{biblatex}
\addbibresource{mybib.bib}

\begin{document}
\begin{frame}[t]{Method}
    \framesubtitle{Adapting the Feature Pyramid Network}
    \begin{itemize}
        \item<1-> \alert{FPN}\footfullcite{lin2017feature} is widely used for object detection:
        \begin{itemize}
            \item Extract feature maps at multiple levels from a single image
            \item Sequence of convolutional layers, size divided by two at consecutive stages, with top-down connections
            \item Increases detection performances, but adversarial domain alignment is non-trivial
        \end{itemize}
        \only<1>{
            \begin{center}
                \item[]
                \fbox{\rule{.5\textwidth}{.3\textwidth}}% a box in the approx size of your image
            \end{center}
        }
        \item<2-> Tested approaches:
        \begin{itemize}
            \item Align levels of extracted feature pyramid
            \item Align layers of the \alert{ResNet} feature extractor, input for FPN
        \end{itemize}
        \item<2-> Extracted representation is meaningful and independent of input domain
    \end{itemize}
\end{frame}
\end{document}

The issue appears to be that the center environment that contains the image takes up too much vertical space. Thus, the footnote overflows beyond the bottom page margin. It's there, but it's invisible. For comparison, a "shorter" image allows the footnote to be printed:

\fbox{\rule{.5\textwidth}{.1\textwidth}}

In the pics you posted, he problem comes not from the actual picture, but from the inordinate amount of whitespace below it. Some of it can be sacrificed without penalty to readability or aesthetic.

Hence, I'd recommend the following quick and dirty fix: Subtract some vertical space at the end of the center environment, to leave enough space for the page footer. The footnote pops up where it is expected.

\only<1>{
            \begin{center}
                \item[]
                \fbox{\rule{.5\textwidth}{.3\textwidth}}
            \end{center}
            \vspace{-2.5\baselineskip}
        }

Choose a value of this negative vspace that allows for the footnote without messing with the rest of the layout, remembering that \baselineskip = the distance between the baselines of two consecutive lines of text.