computations caught on camera



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Film computations




You will need ffmpeg and imagemagick installed to use Reel. This package supersedes ComposeVideo.jl


Reel exports the all-important function roll which you can use to roll the camera on various arguments:

using Reel
using Gadfly

function render(t, dt)
    # t is the time into the sequence
    # dt is the time to advance for the next frame

    # any expression that results in an object which can be
    # rendered as png or jpg
    plot([x -> sin(x+t*π), x -> cos(x+t*π)], 0, 6)

film = roll(render, fps=30, duration=2)

write("output.gif", film) # Write to a gif file

# Or
write("file.webm", film) # Write to a webm video

# Or
write("file.mp4", film)  # An mp4 formatted video

The same thing can be concisely written with Julia's do syntax:

film = roll(fps=30, duration=2) do t, dt
    plot([x -> sin(x+t*π), x -> cos(x+t*π)], 0, 6)

write("output.gif", film)

Note that the fps you specify to roll will be enforced no matter how long the computation actually takes: you set the speed at which you want to watch it.

Reel can also render an abstract array of objects:

using Compose

Compose.set_default_graphic_size(3inch, 3inch) # Square

# draw a regular n-gon
ngon(n) = compose(context(units=UnitBox(-1, -1, 2, 2)), fill("lightblue"),
                polygon([(cos(x), sin(x)) for x in π/2:2π/n:3.5π]))

roll(map(ngon, vcat(3:10, 9:-1:3)), fps=5)

Reel exports Frames, a type for a collection of frames.

Frames constructor takes a MIME type of the frames, and the fps at which the animation should be rendered. e.g.

You can push a new frames to a Frames object using push!.

# A Glider gun from Conway's game of Life

using Reel
frames = Frames(MIME("image/png"), fps=2)

g = glider_gun # initial board state
for i=1:31
    push!(frames, draw(g))
    g = decidefate(g)


You can render a Frames object as usual:

write("output.webm", frames)

In IJulia

A call to roll returns a Frames object. A method writemime(::IO, ::MIME{symbol("text/html"),::Frames) orchestrates the rendering of a Frames object in IJulia. Any cell which results in a Frames results in the animation being rendered. By default, a webm video is generated. You can change this behavior:

Reel.set_output_type("gif") # or "mp4"

They see me rollin', they hatin'

# particles in a box.

using Color, Compose

box(x) = let i = floor(x)
    i%2==0 ? x-i : 1+i-x

colors = distinguishable_colors(10, lchoices=[82.])

dots(points) = [(context(p[1], p[2], .03, .03), fill(colors[i%10+1]), circle())
    for (i, p) in enumerate(points)]

velocities = [(rand(), rand()) for i in 1:40]

roll(fps=30, duration=10.0) do t, dt

            dots([map(v -> box(v*t + 0.5), (vx, vy)) for (vx, vy) in velocities])...)

# Solution of Helmholtz equation with Dirichlet BC using ApproxFun

using ApproxFun


plots = [ApproxFun.contour(pdesolve([B,Δ+k*I],ones(4),150))
            for k=vcat([1:1.0:100], [99:-1.0:1])]

roll(plots, fps=24)

# Warning: this gif is 8 mb!


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