Can't render out alpha in simple video file

[Alphafool]

I have a read node in Nuke with a .mov file which definitely has an alpha channel (an explosion).

The simple question is how do I export this from Nuke while maintaining the alpha channel?

I want to do this directly, in other words I have nothing else plugged into it, just want to render it out with alpha as a test.

It's to then be opened in FCP, and I'm using Apple ProRes 4444 which is supposed to support alpha channels. I have tried PNG too which is the format the clip is originally in and that hasn't been a success either. In FCP I make sure to change clip to 'straight alpha' in 'item properties>format'.

^Despite this, I have no alpha channel after the render from Nuke.

I've tried many different settings of 'premultiplied'/'auto alpha' etc. and I get no joy.

I know/think it's going to be something simple, but I just can't see it!

If this test works, then I am hoping to key some footage with Primatte and export the footage with alpha for compositing in FCP, so this was meant to be a 'simple' test. Is it possible to do that - remove green screen and export the footage with alpha channel for direct overlaying in FCP?

Thanks for any help.

[mreithoffer]

if you reimport your render does it have an alpha channel in nuke ?
if not then you probably only render RGB and not RGBA (in the write node, set channels to RGBA)

[Alphafool]

Thank for the reply!

Nope, there's not alpha when I bring the footage back in.

I've also checked the read and write nodes before render and the alpha channel is ok.

I've attached a pic of my read/write nodes if you could have a look please?

[theta]

I think there's an issue with ProRes 4444 and alpha channels. I think I've had problems with this codec and alpha before in the past.

Maybe try rendering out in the animation codec, which supports alpha and is lossless.

[Alphafool]

I'll give that a shot, thanks!

The original explosion file was rendered as PNG files.

[Alphafool]

Well, whatta you know, it works!

Thanks a million! Now I can key all my footage

Damn Apple and their not living up to promises codec