Re: [Matplotlib-users] saving animations

[Benjamin R. <ben...@ou...>]

On Tue, Aug 24, 2010 at 10:28 PM, David Pine <dj...@gm...> wrote:

> I am trying to save the frames from a matplotlib animation and I have a
> question that is undoubtedly based on a profound lack of understanding about
> how matplotlib/python works, but I'm hoping someone can offer me an
> explanation.  The following code is excerpted from the animation examples
> "animation_blit_wx.py".  The example works fine unless I include the #*****
> line meant to record the animations in included.  If the line is included,
> the animated line is no longer drawn to the screen or to the sequence of
> plots I am trying to save.  That is, the background is displayed and saved,
> but not the animated line.  Why?
>
>
> def update_line(*args):
>    global blit_time
>
>    if update_line.background is None:
>        update_line.background = canvas.copy_from_bbox(ax.bbox)
>
>    # restore the clean slate background
>    canvas.restore_region(update_line.background)
>    # update the data
>    line.set_ydata(npy.sin(x+update_line.cnt/10.0))
>    # just draw the animated artist
>    ax.draw_artist(line)
>    # just redraw the axes rectangle
>
>    t = time.time()
>    canvas.blit(ax.bbox)
>    blit_time += time.time() - t
>    #***** plt.savefig('f' + str(update_line.cnt) + '.png') *****
>
>    if update_line.cnt == NBLITS:
>        # print the timing info and quit
>        frame_time = time.time() - tstart
>        print '%d frames: %.2f seconds' % (NBLITS, frame_time)
>        print '%d blits:  %.2f seconds' % (NBLITS, blit_time)
>        print
>        print 'FPS: %.2f' % (NBLITS/frame_time)
>        print 'BPS: %.2f' % (NBLITS/blit_time)
>        sys.exit()
>
>    update_line.cnt += 1
>    wx.WakeUpIdle()
>
>
>
Technically speaking, animation to the screen is completely different from
what you are trying to do here.  When showing an animation to the screen, a
bunch of tricks are needed to make it efficient and for looping.  However,
if you only wish to save the individual frames, I would suggest that you
just simply create your figures normally (none of this blitting and
update_lines stuff) and save each of them as you would normally. Be sure to
call clf() to prevent memory usage to grow out of control.

Persoanlly, I then use ImageMagick to merge the image files together into an
animated gif:

convert `ls -1 *.png | sort -d` -set delay 40 -set dispose none -loop 0
animation.gif

The experimental Animation module will have some preliminary support for
saving animations (that feature isn't cross-platform right now).

Ben Root