Simple chromakey: black -> alpha and then overlay

[boghi...@gmail.com]

Hi!
I am very new to Natron (and video processing in general). I have a short (2 sec) video, which is on a complete black background and I would just like to convert the bg to alpha, so that I could then overlay it on another video. Is this possible? I couldn't really find a guide on the internet. Help would be much appreciated (with chromakeying and overlaying as well) :)
Thanks,
Bogdan

[soz...@gmail.com]

you don't really need to use chromakeying in this case, with some channels operations it should be alright ...

the first step is to get a good mask for your alpha, just use a grade node to contrast your image.

then ,you can use a shuffle node to set one channel (R,G or B) to be the alpha channel of your original shot.

maybe after that you have to put a premult node before overlaying on the other video.

good luck if you're beginning with natron, it can be a bit complicated at first but with a few experiments you'll get along very soon...

[frederic.devernay]

You can use the keyer node, in luminance keying mode. If you have black parts in your foreground, use the roto tool to put them and in the fg mask

[boghi...@gmail.com]

Where is this grade node? And how could I set the channel to alpha if there are other colors in the clip (or is it only applied to the mask). Some more info: this 2 sec clip is basically a margin animation (golden margins), that are on a black bg. I just want to be able to apply this clip wherever it's needed on an actual (3+ min) video, so that it would get those golden margins. I don't need anything really fancy for this, I only need to remove the bg (which is a completely solid black background, it was created in software, not filmed)

[Andy Witkowski]

Try using a Merge Node with the Operation set to "screen"

"A" input is your foreground layer and "B" input is your background.

It should eliminate the black from your image and overlay on top of your background

- andy w.

[boghi...@gmail.com]

But how can I change the time of the overlay, it also happens multiple times.

[Alkesh Nanavaty]

I have only downloaded/installed Natron.. and have not played with it yet. Awaiting 3D workspace, hopefully coming soon!

I use Fusion. Is there a bitmap mask node in Natron? If so, you can take the output from Shuffle, put in bitmap mask node and change it from "alpha" to the appropriate color channel. then use that node as mask on the image itself. That's how I do in Fusion.  

On Wednesday, April 29, 2015 at 2:13:30 PM UTC-4, boghi...@gmail.com wrote: