Alpha Channel - Luminescent Premultiply

Cunhambebe wrote on 8/28/2008, 10:27 AM
Hi there, it's really been a long time. Here's me again to ask another interesting question on Vegas ;-)

I've just rendered a 3D scene showing a spaceship (UFO) and off course there are a lot of colorful lights around it. The thing is, I'd like to make a video compositing in Vegas, inserting a certain backgound, no matter what you may think of: space, beach, whaterever.

Unfortunately, I see that Vegas doesn't have as many treatments for the alpha channel as After Effects does, since as soon as I choose Properties> Premultiply or Premultiply (dirty), all reflections and light outside of the object are gone. They simply vanish.

Question: is threre any way to make a video compositing this way, treating the alpha channel as Luminescent Premultiply (such as one would do in After Effects)? Thanks in advance.

Cheers!
Mark

Comments

Cunhambebe wrote on 8/28/2008, 3:22 PM
Nobody? |-(
Rory Cooper wrote on 8/28/2008, 10:38 PM
Hi Mark

The closest would be hard light the fact that reflections are disappearing you said you were working in 3d so maybe you are out of your working area
“ pan and crop “ zoom out on that track and see if the image appears as you have to have a source for the channel to work on
So the source channel must match the size of your premultiply channel

How I understand it luminescent premultiply eg tiff or luminescent straight eg png should not make a difference each treats the 4 bit channels differently to get 16 bit

Cunhambebe wrote on 8/29/2008, 4:45 AM
Thanks for replying, but it's not a matter of using pan/crop. I'm using TARGA 32 files rendered in a 3D application (sequence files).

The lights (reflections, if we can call them that way) are there, when the alpha channel is set as Undefined. As soon as you change it to Premultiply, you can see the transparency (video underneath will show) but the reflections going outside of the object are gone :-(

Anyone?
Thanks in advance.
Cunhambebe wrote on 8/30/2008, 10:43 AM
No one?
farss wrote on 8/30/2008, 3:16 PM
If you scroll down to the bottom of the page here they talk about pretty much your problem. I suspect that Vegas does not support the compositing mode that you require as I've struck a similar problem trying to composite explosions in Vegas. I end up with the edge pixels turning black.
However reading that section of the Adobe article all might not be lost! They imply that this compositing mode is the same as doing two composites. You might get this to work in Vegas by doing the same trick. Add your foreground to another layer and invert it and set comp mode to Add.

If that doesn't work and you don't have a copy of AE I'd be quite happy for you to email me a couple of stills and I'll see if I can get it to work in AE for you.

Bob.
Rory Cooper wrote on 8/31/2008, 11:07 PM
Mail me the first 6 or 7 tga seq files and snap of the back I would like to have a look

I can sus the differences in Boris and AE and then Vegas also it would be a learning experience for me for future

Rory
Gary_G wrote on 8/31/2008, 11:39 PM
I haven't done a lot of compositing with Vegas, I tend to other apps for that. But have you considered altering the way you render the animation in your 3d app. The work flow I use is to render the 3d footage without premulitplication. It will look odd - like it is chunky around the edges - but that is good since you'll be using the alpha channel to do the composite and you won't be sucking in the background colour from your 3d render. I'd also do a number of render passes - different ones for shadows, glows, and reflections and comp them separately - that way you get much more control over the final look.

Regards
Gary
Cunhambebe wrote on 9/1/2008, 12:53 AM
Thanks to all of you (farss, xfx ans Gary G) who took time to respond.

farss:
They imply that this compositing mode is the same as doing two composites.
-I remember doing that once in Vegas, but was not quite sure on how to do the same process again. Thanks for your idea, farss - now I guess I remember how to do it - but it's not working. I'll try to e-mail you and send some sequences.

xfx:
-I'll try to send you a series of sequences as you've required, as soon as possible. Thanks for your help.

Gary G:
I'd also do a number of render passes - different ones for shadows, glows, and reflections and comp them separately - that way you get much more control over the final look.
-Thanks for the excellent idea. I've never considered rendering a sequence this way.