Hi list,

IS has some videos where the description says an alpha channel is included. I am preparing some rendered animations, using png with transparency as output. How can I produce a video which includes the alpha channel? One of those videos on IS uses the Motion JPEG A codec, but it was my understanding that this doesn't support alpha, so I am a bit confused. How is this done?

The few ones I've seen on IS include the plain footage followed by the alpha channel. So the video file is twice longer than the footage time.
The buyer then "selects" the second half of the footage and uses it as an alpha channel in video editing software.

Right. It's not technically an alpha channel. Rather a luma keyable version added to the end of the clip. A true alpha channel can be created by the buyer using this, or they may just use the luma key to composite the clip.

Here's a link:
iStock "What is the alpha matte?"

Pond5 accepts files encoded using the ANIMATION codec which supports a true alpha channel.  Less work.