"Bob? Where does it say that "shadows" are black? Maybe it IS something more devious than that?"

Yes, my shadows will contain an alpha channel.
More troubling is there is indeed something very devious going wrong. I had thought there was actually nothing wrong with Vegas but further testing has shown otherwise sadly.

I upped my background to 50% grey. Now Vegas shows the shadow going from black to a lighter shade of grey and then back to 50%. That is very wrong. Upping the level of the background to 100% white and it looks correct.

In order to determine if maybe the comp went wrong when the shadow was created or when it is comped in the master project I tried the same AVI file in After Effects. The result is perfect. With a 50% grey background the shadow does nothing funky, it goes from black and gradually falls off into the background color.

I've always had a feeling something was a bit off with how Vegas handles the alpha channel, somethings never quite looked right but I was working with stock footage so I couldn't say really where the fault lie. Now it's hard to draw any conclusion other than the fault lies with Vegas. It created a perfect composition with a perfectly good alpha channel and it cannot composite that correctly whereas After Effects can.

Just to be 100% certain I changed compositing modes in Vegas to 32bit. Neither 2.22 or 1.00 gamma made any difference. I should also mention that in all modes the shadow also showed signs of banding when comped onto 50% grey.
This also happened in AE. Switching AE to 16bpc helped somewhat. 32bpc(float) produced no improvement over 16bpc.
Probably what Vegas lacks to combat banding problems with shadows in the ability to add noise to shadows i.e. dither.

Oddly enough doing the complete comp all at once in Vegas I see no problems. It seems to handle transparency perfectly internally. That's how I'll get around this issue for now. It just removes the convenience of leaving the choice of background until the final comp.

Bob.

It actually sounds like Vegas is doing the right thing here.

Things get complicated when going between Computer RGB (0-255) and Studio RGB (16-235) as the curve adjustment doesn't work just like a clamp or 16->0 and 235->255 expander, it affects the whole tonal range.

I'm well aware of how the curves will affect the whole tonal range.
That doesn't trouble me and agree Vegas is almost certainly doing the right thing.

I am concerned about how it's handling the alpha channel and this has been the subject of many posts. I invested some more time in this taking frame grabs of the same comp done in Vegas as AE and then measuring the outcomes in PS.

The comp done in AE shows the shadow values going from the backgound 128 evenly down to black at the edge of the object that created the shadow.

The comp done in Vegas shows the shadow values going up to 131 and then down to black. The increase in luma is what causes the appearance of a halo around the shadow. It is very similar to other artifacts I and others have seen trying to comp fire and explosions in Vegas.

The difference in outcomes could be because AE knows the comp is working in D1 and it does the correct thing. I'm no AE guru so that's just a guess.

Bob.

Hmmm, sounds like a transfer mode (compositing mode) issue.

Which mode did you use in Vegas and AE?

trying to grasp what you saying Bob,, but not sure I am understanding... I use stuff created in ae with alpha channels quite a bit,, have not had an issue... that I've seen. Have a simple AE project file that has the issue you can email me ? Yeah I;m a little dense sometimes

See my post above, tried them all in Vegas and AE.

Bob.

1) Do comp in Vegas. Render to uncomp AVI out of Vegas.

2) Take Vegas created AVI file and do comp with that in Vegas. Problem.
3) Take Vegas created AVI file and do comp with that in AE. No problem.

4) Go back to 1). Do same comp as final comp i.e. add background. No problem.

There is a difference between how Vegas handles tranparency internally and how it handles transparency from an alpha channel.

Yes it is subtle. You'd only notice it with things like shadows, smoke and flames.

I'll try to load some screen shots so all can see.
For this project I've very easily got around the problem.

Bob.

OK,
sample image showing the difference is here.

Bob.

I get similar results if I render the first element (with the shadow and alpha) with Broadcast safe filter on the final output.

If I do not turn the broadcast safe filter on during this render, then the video keys fine over the black. With the broadcast safe turned on, you get the white ghosting.

Dave T2

Thanks Dave,
I didn't have that filter or any filter for that matter but it is good to hear someone else has seen another manifestation of this problem.

Bob.