11crazydiamond
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 alpha channels
are u able to explain to me fairly simply how alpha channels work? i've tried to look it up but they blast me with mathematics and that isn't my strong point. I'm trying to export a talking head shot with a transparent background
thanks
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25/10/07, 14:21
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SkiTroll
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Re: alpha channels
Alphas is basically the transparent background.... In short it's imbedded information within the quicktime (or targa or whatever export you are doing) telling any application where the actual footage is and where it needs to be transparent.... So on the alpha channel it makes your talking head as a white layer and the transparency as a black layer.... So if you take that footage and import it into fcp or avid as footage with alpha, it will treat it as such. Making anything white visible and and anything black transparent....
---
Merethe C. Rosvold
www.finalcutprogirl.com
London, UK
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26/10/07, 7:09
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cheesenightmare
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Re: alpha channels
Thanks MCR
Here's my take on it:
Video is the combination of a green, red and blue channel. Combining those three colours in different proportions can make any colour that exists.
But what about when part of a picture is not a colour but instead a degree of "see-through-ness"?
Where is this see-though-ness kept? Not in any of the coloured channels - but instead in a fourth channel. We call it the Alpha Channel.
Alpha Channel was always one of those terms which freaked me out...like "Differential" in a maths exam... "Calculate the differential between...." AH!! BRAIN SHUTDOWN!!
It just means see-though-ness.
Only certain types of Video can have see-through-ness.
DV-Pal can't. If there are any see-though bits, they just default to black.
Animation Codec can and it is widely used.
Export your talking head as ANIMATION codec and make sure to set it to millions of colours plus. The Plus is the see-through-ness (or the ALPHA CHANNEL)
When you put it on top of a another layer of video in FCP you will see the video layer underneath in the see through parts - "behind" the talking head.
[Green screening or chromakeying is the process of telling FCP to consider a particular colour (usually green) to be see-through instead of the colour it is .
In technical terms, it takes that particular shade and hue of green and assigns it to the alpha channnel.]
Does that kind of help?
---
Doug Suiter.
Freelance Director, Promos, Post & Creative.
LondonFCP/SydneyFCP Administrator.
Find Local FCP Operators (Sydney & London) at the FCP Talent Registry: www.FCPTalent.com
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26/10/07, 11:20
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11crazydiamond
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Re: alpha channels
cool thanks everyone
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26/10/07, 11:47
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jsm69
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Re: alpha channels
Hey the absence of all of the colour does not mean alpha it is black. An image or qt that has an alpha has 32 bits 24bits for Fill video and 8 bits defining the alpha, You must have software that can create an alpha then you must be able to deffine what that alpha is and export it as such.
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28/10/07, 6:41
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