How can I uncover the fraction line in beamer?

Question

I would like to uncover elements of an equation one at a time. This is easy enough usually, but in this case I'd like to progressively reveal a fraction. The problem with the way I'm doing it is that the fraction line is visible throughout, and for obvious aesthetic reasons I'd only like it to show up on the last uncover. Here's how I'm doing it now :

\documentclass[8pt,xcolor=dvipsnames,compress]{beamer}
\begin{document}
\begin{frame}
$\frac{\uncover<2->{\int} \phi(\lambda) \uncover<2->{T(\lambda) \lambda d \lambda}}{\uncover<3->{\int \phi_{S}(\lambda) T(\lambda) \lambda d \lambda}}$
\end{frame}
\end{document}

This reveals the elements I'd like to reveal when I'd like to reveal them, but as I've said keeps the fraction line throughout. Any ideas on how to only make it appear on step 3 ?

Answer 1

Inspired by some of the tweaking found here, I've come up with the following solution :

\documentclass[8pt,xcolor=dvipsnames,compress]{beamer}
\begin{document}
\begin{frame}
$\uncover<2->{\int} \phi(\lambda) \uncover<2->{T(\lambda) \lambda d \lambda} \uncover<3->{\above 0.4pt} \uncover<3->{\int \phi_{S}(\lambda) T(\lambda) \lambda d \lambda}$
\end{frame}
\end{document}

Basically you just avoid the \frac command entirely by doing what it does yourself using \above, allowing you to use \uncover on the \above command.

Answer 2

Here's a pretty fun non-complete solution. I've taken the \binom command and removed the parentheses. The elements are not completely unmoving though. We can probably tweak the command some more. I will look into it more later.

---

After @LoopSpace's kind encouragement I made a new attempt and solved it. The good thing about \binom is as @LoopSpace was discussing that it works the same way as \frac but without the fraction bar. I added a phantom of the denominator to get the correct alignment of the fraction.

However, even with the newly defined command, I couldn't get the correct horizontal alignment if the \int was within a \uncover environment, to solve this, I put the \int in the full fraction within an \uncover environment as well.

\documentclass[8pt,xcolor=dvipsnames,compress]{beamer}
\newcommand{\hidefrac}[2]{\genfrac{}{}{0pt}{0}{#1}{#2}} %new command, 0 in the fourth argument means displaystyle (change to 1 for textsize)
%taken from definition of \binom in the amsmath package

\begin{document}
\begin{frame}
$\only<1-2>{\hidefrac{\uncover<2->{\int}\phi(\lambda)\uncover<2->{T(\lambda)\lambda d\lambda}}{\phantom{\int\phi_{S}(\lambda)T(\lambda)\lambda d\lambda}}}$%
$\only<3->{\displaystyle\frac{\uncover<3->{\int}\phi(\lambda)T(\lambda)\lambda d\lambda}{\int\phi_{S}(\lambda)T(\lambda)\lambda d \lambda}}$
\end{frame}
\end{document}

And be careful with spaces. Try removing the % in the end of the first \only line to see what I mean.

Answer 3

Just use \pause:

$\frac{\uncover<2->{\int} \phi(\lambda) \uncover<2->{T(\lambda) \lambda d \lambda}\pause[3]}{\int \phi_{S}(\lambda) T(\lambda) \lambda d \lambda}$