Why does animate distort shapes?

Question

I recently played around with the animate package. For a different question I created an animation of bouncing balls like this one.

\documentclass{article}

\usepackage{pgfmath}
\usepackage{animate}

\def\bounce#1#2{%
  \pgfmathparse{ 3ex * abs(cos( (#2) / (#1) * 180 )) }%
  \raisebox{\pgfmathresult pt}{\(\circ\)}%
}

\begin{document}

Here are some bouncing circles:
\begin{animateinline}[loop]{24}%
  \multiframe{144}{r=0+0.0417}{% We need 6*24 = 144 frames here.
    \bounce{1}\r%
    \bounce{2}\r%
    \bounce{3}\r%
  }%
\end{animateinline}

\end{document}

To my surprise, the balls (i.e. circles; the glyph created by \(\circ\)) are being distorted at different points during the animation. Here is a screenshot of one frame. This happens both in Adobe Acrobat Reader DC and PDF XChange Editor.

Why does this happen? How can it be prevented?

This must be the air resistance. ;-) – Mar 05 '20 at 22:28 — , Mar 05 '20 at 22:28

score 6 · Accepted Answer · edited Mar 10 '20 at 11:55

You have to make sure that all animation frames have the same vertical and horizontal dimensions. While viewing the animation, the PDF viewer scales the frames to fit into the animation widget which has the first frame's dimensions.

Use the optional arguments for height and depth to ensure that all TeX boxes that are output by \raisebox have the same vertical size

\raisebox{<raiselen>}[<height>][<depth>]{...}

as in the following example:

\documentclass{standalone}

\usepackage{pgfmath} 
\usepackage{animate} 

\def\bounce#1#2{% 
  \pgfmathparse{ 3ex * abs(cos( (#2) / (#1) * 180 )) }%
  \raisebox{\pgfmathresult pt}[4.2ex][0ex]{\(\circ\)}% 
} 

\begin{document} 

\begin{animateinline}[loop,controls,scale=10]{24}% 
  \multiframe{144}{r=0+0.0417}{% We need 6*24 = 144 frames here. 
    \bounce{1}\r% 
    \bounce{2}\r% 
    \bounce{3}\r% 
  }% 
\end{animateinline} 

\end{document}

Why does animate distort shapes?

1 Answers1