Alpha Channels

Alan Mills · #1 (**permalink**) 02-22-2007, 08:04 PM

I learned how to create an image with a seperate alpha channel in Photoshop and use it in Premiere Pro tonight. Yay me!

irishmark · #2 (**permalink**) 02-22-2007, 10:16 PM

WOOT WOOT - Yay You Alan, its always cool when you crack something!

coommark · #3 (**permalink**) 02-23-2007, 08:45 AM

hey hey hey big brother! let us into it! whaz it this time? sure i know its something really cool and killer! what did you do? how did you do it? what did it achieve for you? what else did you figure it could achieve?

Alan Mills · #4 (**permalink**) 02-23-2007, 11:22 AM

I started to look at some freebie examples of virtual sets on a recent cover disk I have.

The 'set' I was looking at had a complete image for the set and also to images in a 'mattes' directory. I opened all three in Photoshop CS2 and they were all identical. I was stumped so I linked to the site of the company offering the freebies and they had a 'chat online' link so did that. Spoke (typed actually) with a very helpful guy (considering I'm not a paying customer you understand) who talked me through how to use these things.

It turnes out that the images in the 'mattes' directory (all tga files) are not the same. indeed they are the same image but they also contain a fourth channel which is only visible from the 'Channels pallet' in Photoshop (er... Der..). This fourth, alpha channel simply cotains black mask over certain areas of the image.

The magic is in Premiere (haven't tried After Effects yet with them yet but will very soon because chroma keying is tons better there). Import one of these matte images in Premiere and you do not see the entire picture at all. The fourth channel takes over and all you see is this mask applied to the main picture. Works like a charm. I even tried creating my own so's I know how to do it. Worked first time. Not sure if the name of the fourth channel is significant though. Might be.

Given that I would have done things differently to (try to) achieve the same end this a great workflow saving for me.

Needless to say I'm having a play in this area lately. No virtual set involved here but this is my very first chroma keying. I seem to have caught reasonably satisfactory results here but further reading has shown I did things in not the best way so I'll do better next time. http://www.alandmills.co.uk/Chroma_Key_First_Test.wmv (1.3Mb). All done in my brand new conservatory. Now if only I had designed it a yard longer it would have made a perfect studio...

If you can lip read you'll have a VERY big clue as to what this is all leading up to.

Anyway, that's what I learned last night. Glad to share.

Alan

irishmark · #5 (**permalink**) 02-23-2007, 11:35 AM

the dancing made me weep

Alan Mills · #6 (**permalink**) 02-23-2007, 11:50 AM

Originally Posted by irishmark

the dancing made me weep

note to self... move Mark to top of hit list!

coommark · #7 (**permalink**) 02-23-2007, 11:56 AM

that is tight man, with lotsa geek-speak, but really cool! thanks for sharing da knowledge!

irishmark · #8 (**permalink**) 02-23-2007, 12:00 PM

I've used alpha chanells alot with After effects for DVD menus and stuff. Its all very easy really once u get used to it. - there be a tick box under the edncoding profile when u've adding the comp to the render queue