Background doesn't render transparently despite checking Film-->Transparent; RGBA settings (cycles, v2.83)

Question

I'm trying to render an object with a transparent background.

As per the instructions I've found online, I have "Transparent" checked in Render Properties, RBGA/PNG selected in Output Properties.

Both Render Preview and the rendered image have the checkered background. But, when I save the PNG and import into Illustrator, PS, Word, etc., I see a semi-transparent gray background (top right object).

As a control, I opened a new blender file and simple dummy figure (glowing cube + plane with yellow emission), rendered it with the same settings and that worked.

So, I'm not sure if there's some other setting here that I'm overlooking. To test that, I made a blank blender file, imported all the objects and lights from the model I was first trying to render, matched the world color/PNG transparency settings, and found that that worked...but I lost a lot of vibrancy despite carrying over the lights and everything (bottom left object).

Clamps, light paths, samples are all the same.

Any tips would be immensely helpful.

Blender file that isn't rendering fully transparent:

Answer 1

You are using an Alpha Over node and mixing something over the image messing up the alpha as well. Just plug the correct alpha you want to the output to fix this:

If you need glow, you should add it instead of mixing it. Using alpha will not give you the correct results in other application if you need glow. Glow is made of light and it should be added to the image, not mixed with it, so you should make a separate glow image to add it in whatever software you are using for compositing over the background and the object placed on the background.