Drawing Metapost animations

Posted on July 15, 2018

The animation module provides a nice interface for using FieldStacks to create a JavaScript controlled animations in ConTeXt.¹ For example, the following code will show step-1.pdf, step-2.pdf, step-3.pdf, and step-4.pdf files one by one:

\usemodule[animation]

\startanimation[menu=yes]
  {\useexternalfigure[step-1.pdf]}
  {\useexternalfigure[step-2.pdf]}
  {\useexternalfigure[step-3.pdf]}
  {\useexternalfigure[step-4.pdf]}
\stopanimation

Sometimes, I use animation to give the illusion of motion. For example, to show a particle moving along a trajectory (drawn using Metapost). In principle, it is possible to use the animation module to draw such animations (e.g., see Wolfgang’s reply in on ntg-context mailing list), but the interface gets a bit cumbersome. In this post, I show a helper macro to simplify drawing such animations.

As an example, I’ll consider the use case of drawing a particle moving in a circle. With the helper macros given below, we can use the following code:

\startMPanimation{circle}{n=100}
    path p, q;

    p := fullcircle scaled 2cm;
    draw p withcolor 0.7white;

    n := \MPvar{n};
    t := \MPvar{t};

    drawdot point (t/n) along p
        withpen pencircle scaled 3bp withcolor red;

    setbounds currentpicture to fullsquare scaled 2.2cm;
\stopMPanimation


\starttext
\useMPanimation[menu=yes,framerate=50]{circle}
\stoptext

to generate this animation

Thus, an animation is defined using \startMPanimation ... \stopMPanimation. The macro takes one argument n=..., which specifies the total number of frames. To use the animation, use

\useMPanimation[menu=yes,framerate=50]{circle}

where the options (in square brackets) are options to the \startanimation macro from animation module. That’s it.

Behind the scenes, the \startMPanimation macro creates a useMPgraphic with the name animation:circle, which takes two arguments n (which is the same as before) and t (which specifies the current time or the current frame number). The body of the environment is the metapost code which draws a figure depending on the values of t and n. In the example above, I draw a circle and a point t/n along the circle.

The macros \startMPanimation and \useMPanimation are a wrapper around the code by Wolfgang:

\unprotect

\installnamespace{MPanimationvariables}

\unexpanded\def\startMPanimation
    {\dodoublegroupempty\meta_start_animation}

\def\meta_start_animation#1%
    {\normalexpanded{\meta_start_animation_indeed{#1}}}

\unexpanded\def\meta_start_animation_indeed#1#2#3\stopMPanimation
    {\doifsomething{#2}{\getparameters[\????MPanimationvariables#1:][#2]}%
     \setgvalue{\??mpgraphic animation:#1}{\meta_handle_use_graphic{#1}{n,t}{#3}}}

\let\stopMPanimation\relax

\unexpanded\def\useMPanimation%
    {\dosingleargument\use_meta_animation}

\def\use_meta_animation[#1]#2%
    {\edef\c_MP_animation_frames{\getvalue{\????MPanimationvariables#2:n}}%
     \startanimation[#1]
        \dorecurse{\c_MP_animation_frames}
            {\expanded{\frame{\useMPgraphic{animation:#2}{n=\c_MP_animation_frames,t=\recurselevel}}}}
     \stopanimation}

\protect

A test file and sample pdf output are below. The pdf file needs to be open in Adobe Acrobat to work:

Warning: Such animations only work with Adobe Acrobat. ↩︎

This entry was posted in Metapost and tagged metapost, animation.