-
Alpha Channels – choosing
Hi all,
I’m new to this wonderful forum but not new to Vegas. I have been using it for music and video for several years but I’ve not dwelled into the depths of the video side apart from simple editing, colour correction type operations. Now I want to do some basic compositing.
Alpha channels in Vegas ? I’ve used compositing packages in the past and I’m now faced with doing similar things in Vegas which I know is not ideal but I want to try it here first and avoid other programs if I can.
Something simple. I want to overlay video 1 ontop of video 2 using an alpha created from video 3. Video 3’s alpha channel is being created using the channel blend effect. Simply creating an alpha channel from the red channel. Simple grey scale.
For the life of me I can’t see how I can do simple alpha channel operations beyong one video layer and also how I apply the alpha channel to video 1 so as to allow video 2 to show through ?
I also thought of doing it in two stages. Firstly creating a video 1 with the alpha derived from video 3 and saving it off as an AVI with an alpha channel. I couldn’t see how this would work.
Some help please would be greatfully appreciated.
Steve R.
Amongst Projects
——— Sound, Visual and Electronic Content
projects.amongstmyselves.com -
7 Replies
-
Steve, might I ask just what it is you are trying to achieve. Meaning, the “Look”! What is it you are wanting to see ON the screen? A man walking on the Sea but he is coloured RED? Or are you wanting to see a Red Rose against a background of Grey or Black and White?
What is the “Look” you are after? – I do better with a sample . . .
Grazie
-
Howdy Grazie,
Well it’s more of a case of seeing what I get !
One shot I am thinking of doing would come out something like this. You would start by seeing a timelapsed dolly shot of an outback Australian landscape then clouds would start to appear from the left of screen and build up and eventually cover the whole frame to white. But what you see in the white part of the cloud is the next shot which may be a timelapsed sunrise over an island.
Essentially it’s a motion wipe. I hope that makes sense.
Like I said in the inital email I can do something like this in AE quite easily but I don’t have it. It also seems such a simple idea.
Whether the shot actually works visually is another thing.
Cheers,
Steve R.
Amongst Projects
——— Sound, Visual and Electronic Content
projects.amongstmyselves.com -
> Video 3’s alpha channel is being created using the channel blend effect. Simply creating an alpha channel from the red channel. Simple grey scale.
You haven’t created an alpha channel at all. You have a gray scale mask. To use it, place it above the track you want to mask and change the track compositing mode to Multiply (Mask).
~jr
http://www.johnrofrano.com
http://www.vasst.com -
Hi John,
Thanks for the reply.
I’ve tried what you’ve suggested and it doesn’t solve my problem.
The issue with the method you’ve suggested is that it’s relying on the the video layer with the mask to also have the video that’s being overlayed. All I want from the mask video is it’s mask and using that mask to overlay another (3rd) video.
I’m starting to think this is not possible with Vegas – well not obviously easy anyway. I might try AviSynth and do some simple colour channel manipulations. Not idea as I will essentially need to have the exact segments pre edited which is a little time consuming.
Thanks again. I’ll keep playing !
Cheers,
Steve R.
Amongst Projects
——— Sound, Visual and Electronic Content
projects.amongstmyselves.com -
> The issue with the method you’ve suggested is that it’s relying on the the video layer with the mask to also have the video that’s being overlayed.
Not at all. What I said was to place the mask track above the video track that you want to mask and use the mask compositing mode. The video below the mask track should show through the mask.
~jr
http://www.johnrofrano.com
http://www.vasst.com -
Hi John,
Thanks for putting me right on that one. But it’s confusing me.
The following paragraph should be read with the image of me confused and not shouting, which is the way I read it 🙂
I may not have been clear before when I talked about creating an alpha channel from the greyscale. You mentioned in your first reply that “You haven’t created an alpha channel at all. You have a gray scale mask.”. I find it a little confusing the talk of masks and alpha channels. Photoshop’s use of the two words for similar things also confuses me :-). On that confused note, I have created an alpha channel by using the Channel Blend plugin which allows me to assign the values of the R, G or B channels to the Alpha channel. Unless I am mistaken that video now has an alpha channel. I look at the alpha channel on the video preview screen and there it is. When I place this over the top of another video I see the lower layer of video and black where the white parts of the alpha channel exists. This is what I would expect. The question is how do I then replace those black parts with another video sequence. The problem now is that the top layer doesn’t have an alpha channel that reflects the inversion of the lower layers alpha channel which is how I would imagine I then over lay the top layer of video.
I understand the reasons you talk about masks now as you can’t really use alpha channels the way I would expect. Video editors work on a top down stacking system (that’s pretty obvious) whereas my previous working method is the node based one.
In Node based compositing I would just assign the alpha channel to that top layer of video and the jobs done.
So what I have now is vision where the clouds (masking vision) dark parts have blocked out the vision of the lower video layer. How do I get the final layer of vision to show up in the inverse areas of the final vision ? That’s were I’m stumped via this method. I can’t see how I can do it ?
Sorry for the rant and thanks again for the help.
Steve R.
Amongst Projects
——— Sound, Visual and Electronic Content
projects.amongstmyselves.com -
Steve, I agree that terminology is confusing because it’s use inconsistently. An alpha channel only affects the media that you create it in. The alpha becomes transparent and the non-alpha shows the media. A black and white mask has the ability to show one track for the white and another for the black. I think this is what you want (but maybe I still don’t understand).
What you would do is add a Mask Generator to the track with the mask. Set that track’s composite mode to Mask and the track below will show through everything that is white. Finally, make the track below the mask track a Compositing Child and the track which is two tracks below will show through the black areas. You will be left with Track 1 as the mask, Track 2 shows through white, Track 3 shows through black.
I don’t know any way of achieving this with an alpha channel.
~jr
http://www.johnrofrano.com
http://www.vasst.com
Reply to this Discussion! Login or Sign Up