Weird artifacts when blurring video w/Alpha

I'm having an odd problem with Resolve (15.2.2 Studio). I've got a DNxHR 444 12-bit (with Alpha exported from Resolve, .mov container) that imports fine, interprets the alpha channel fine (premultiplied, same as export settings), and drops onto the timeline fine as well. However whenever I try to apply any kind of blur effect (both OFX and the Blur/Sharpen tab), I get these strange artifacts around the edges. If I right click the clip and go to Clip Attributes and turn off the Alpha Channel Setting ("None"), the artifacts completely disappear. However obviously this is unusable since it brings back the black background.

Edit Tab:


Color Tab:


Edit Tab (Alpha Channel Set To 'None'):


Any Ideas what's causing this? I have no other color correction or filters applied.

Pre-multiplied alpha is intended to blend at 100% and includes blending with the background used during the 3d object render. I would try using a straight alpha version of your clip.

I tried exporting the clip using Alpha as Straight, and my clip renders with a red background and severe noise/dithering around the edges...

Ok, so disregard the last post, the red background was ignored when I Imported the clip into Resolve, however once I apply a blur to the clip it still has the odd artifacts on the edges. I'll post a closer pic for detail:

Also, the blur does not extend past the edges of the keyed matte. I'm an old time After Effects user, so in this case I would attempt to pre-compose the clip and apply the blur to the pre-composition so that it doesn't interfere with the key. However I'm not sure how to do this in Resolve/Fusion.

The artefacts in your rendered clip is the main issue, so solve that problem first.
Always render your clip with a similar background to the final composite to avoid edge issues.
Use a blur effect that has alpha support, or create a node tree that can process the alpha channel.
Compound clips are unnecessary.

I'm not rendering my clip with any background. Ok, maybe I should layout my steps I've taken so far:

1. Was provided with a ProRes 422 (.mov container) clip containing the pre-animated 3D object over a solid chromakey green background.
2. Imported it into resolve, created a new timeline from clip, went to fusion and applied DeltaKeyer, which provided a very clean key with default settings (after setting the background color of course)
3. Exported the timeline as DNxHR 444 12-bit (one Premultiplied and one Straight Alpha)
4. Reimported the clips, put one on timeline above video clips, and tried to apply all kinds of blurs (built-in blue, Gaussian blur, Lens Blur) all of which produce this odd effect.

For reference, here's the original clip I used to key:

Any reason you are not doing the complete composite in Fusion? You will have way more control.
I can't see any benefit in exporting the chromakeyed clip and then re-importing.

Also, can the supplied clip be re-supplied with it's matching matte or embedded alpha? It's very unusual to use chromakey for this kind of rendered animation.

PS As soon as you blur the alpha, pixels beyond the edge of the 3d model become visible. You will need to use a matte choker to control this. It's common for the 3d animator to create the correct amount of alpha blur for the final composite.

If you can post a link to the original clip I can test for you.

Sure, thing, that'd be much appreciated! Uploading now I'll DM you a link

https://drive.google.com/drive/folders/1wes59hpBggwCyshczuyEpVrwAheE86vY?usp=sharing

Using the 3D keyer in the qualifier works fine here. No edge issues. No need to output the clip with alpha.

My node tree...

Screen Shot 2019-01-16 at 7.53.58 pm.png (157.65 KiB) Viewed 5017 times

Thanks for the help! I ended up doing the whole thing in Fusion. Turns out it was just my lack of depth when it comes to Fusion (aka user error )