Transparency, Additive Blend, & Nested Projects

Andy_L wrote on 8/23/2014, 1:40 PM
I've got a cool logo that gets filled with snowfall as a nested project clip that I use at the start of my videos.

Deconstructing it, there is a png image (the logo), and then a video clip of actual snowfall over a creek, which acts as a black background.

Leaving out the masking step, what I've got is two components: logo png, and then snowfall with a black background. Combined like this as tracks on the timeline:

SNOW_CLIP (additive mode)
LOGO_PNG (source alpha)
MAIN VIDEO CLIP (source alpha)

I get exactly what I want, which is the snow-filled logo appearing transparently over the main video clip track. The "snow" appears to fall over the scene.

However, if I combine the top two clips -- snow + logo -- into a single project and then add that new project like so:

COMBINED_VEG_FILE (any mode)
MAIN VIDEO CLIP

Transparency is ruined. Now, the black background of the creek becomes a black blob obliterating the lower video track. It doesn't seem to matter whether I choose source alpha or additive blend modes. The veg file won't behave the same as the two separate tracks added directly to the main video clip.

I use this clip a lot, and I'd like at a minimum to be able to create a shortcut VEG file to use, and better yet a rendered file or image sequence that preserves transparency and doesn't waste render time.

Any ideas on why nested projects aren't preserving transparency in the way I'm hoping for?

Thx!

Comments

videoITguy wrote on 8/23/2014, 2:54 PM
I don't totally understand your workflow or your expectations... but keep this in mind - if you are creating a .veg where your expectations is to have transparency in some of the outcome and especially in a blend to other projects - then you must do this.

Create your first project with the images you want and make sure there is a given color that you expect transparency - usually white area. Then render this project to an outcome digital intermediary which must be either a .png image sequence or an .avi uncompressed with alpha channel support. Bring the digital intermediate into any other projects as a higher level track for compositing.
Andy_L wrote on 8/23/2014, 6:44 PM
I guess, on further experimentation, what I'm really trying to preserve is selective additive blending. I want one of the tracks in the nested project to blend as additive, and one as source, with that behavior preserved in the resulting veg file.

When I create the veg file, it either blends both embedded tracks as source, or both as additive, depending on what you select in track compositing mode.

for easier visualization, what's happening here is a poor man's particle generator. I've effectively got a clip of snow falling shot against a black background. The black is supposed to act as transparent, so the snow clip can be overlaid over other scenes at will.

Instead, the black renders as solid, which obviously isn't what I want. I don't know any way within Vegas to make black act transparent. ??
Rory Cooper wrote on 8/25/2014, 2:46 AM
Ok if I visualise the composite the additive composite in the nested veg is for the lumens = the snow on your background correct

So you don’t actually have a transparency at all. = the add composite is not creating a transparency
So in the nested veg duplicate the snow track = parent and make the top one a multiplied mask track add lumens mask fx and the parent composit =add
Save and in your final composite it will be as you want it
Jedman wrote on 8/25/2014, 2:55 AM
Can you post the veg and the source pics?
Andy_L wrote on 8/25/2014, 12:14 PM
Rory that's an interesting approach. I was playing around with the chroma keyer to make the black background transparent (that works, too).

Is there any real difference in these two ways of doing it?
Chienworks wrote on 8/25/2014, 12:40 PM
The difference is that black is not transparent.
Rory Cooper wrote on 8/26/2014, 2:13 AM
Yes as Chienworks says = when you composite add this does not create an alpha channel this does not make the black transparent so you need to create an alpha channel.

Creating a mask from lumens = white bits. is one way. This is when you create a matt for compositing

Now don’t want to start a war on the forum so this is directed at you. Duck while you read this, LOL.
If you want to use lumens and other colours retained then use channel blend fx
At the bottom row it says alpha. on the extreme right column it says A+ = black drop this down this will reduce the black
On the R.G.B column on the alpha row increasing them will increase the lumens and chroma/colour together so up these to what you desire
On the left column of the alpha row this will reduce the entire opacity of the whole row if you want

so you can use this method to create an alpha channel on an image sequence as in your case you want to retain some blue in the snow with the white bits.
Have a go it’s not that difficult.

You can use this also to choke an existing image with an alpha channel.
Some compositing NLE have a matt fx creator or make alpha