creating alpha channels

[getho]

this is something I've muddled through with since the get-go. Now I'm trying to get my head round it properly:

when add a luma key to a clip on a timeline, does that create an alpha channel for that clip?

I come across this when I'm doing a track matte - I usually end up using luma, but sometimes I want to generate a proper alpha channel on a clip.

Is there a way to view the alpha in premiere?

[Ann Bens]

You can view the alpha by selecting Alpha in the wrench option of the Program monitor.

[text edited]

[Ann Bens]

From the help.

Luma Key effect

(32-bit) The Luma Key effect keys out all the regions of a layer with a specified luminance or brightness.

Use this effect if the object from which you want to create a matte has a greatly different luminance value than its background. For example, if you want to create a matte for musical notes on a white background, you can key out the brighter values; the dark musical notes become the only opaque areas.

White background of original (top and left) is removed using the Luma Key effect and composited over underlying layer (right).

[getho]

thanks ann,

I think part of my confusion was that applying the effect on a clip doesn't seem to make it available to another clip in the same sequence - you have to nest it for that.  (So I was applying keys and trying to se what was going on with the alpha and getting nothing).

[Ann Bens]

I am trying to imagine what you are trying to achive?

You dont have to nest to use the Luma.

[getho]

using a 3rd party filter - it tracks the alpha channel of a specific video layer.  If I set it to say video #3, and apply the luma key to the clip on that track, it doesn't work.. If I nest it it does.  I've found this before as well doing complex masking, and track matte.