Convert PNGs to webm video with transparency
<p>I would like to use <code>avconv</code> to convert a series of PNG images to a WebM video, preserving transparency.</p>

<p>I understand that the pixel format used in the output video must support transparency. So I tried:</p>

<p>$ avconv -framerate 25 -f image2 -i frames/%03d.png -pix_fmt yuva420p output.webm</p>

<p>Unfortunately, <code>avconv</code> complains:</p>

<p><code>Incompatible pixel format 'yuva420p' for codec 'libvpx-vp9', auto-selecting format 'yuv420p'</code></p>

<p>I am using <code>ffmpeg version 2.8.4-1+b1 Copyright (c) 2000-2015 the FFmpeg developers</code>.</p>