Alpha vs Luma

[Mylenium]

Hi all
I have to admit. I been using AE for a little while and I think it's great. I'm learning a couple of tutorials a day but here's what makes me wonder everytime I encounter it.
What exactly is Luma?
I guess I know what Alpha is , correct me if Im wrong, but is it what defines the transparency of the 3 channnels of an image?
I find the track matte thing very useful but before I tell people that I dabble in AE, I thought I should know thorough definition of Alpha and Luma
If someone can refer me to a link or tell me yourself, either would be great
Thanks!

Alpha defines the opacity/ blend amount of an images' pixels over the background. A simpler, though technically incorrect term would be transparency.

Luminance (or Luma) is the normalized brightness of the sum of the RGB channels expressed as a greyscale value. For color systems such as YUV/ YIC that are used in video systems it refers to the base brightness of a given pixel, on top of which the color difference info is modulated. In that case it's amere matter of signal processing.

Both items are not related to each other, their only commonality being the greyscale values which can be used for mattes or to modulate effect parameters.

Mylenium

[beenyweenies]

Mylenium's description is correct, but in the interest of simplicity you can think of it like this:

- Alpha = Transparency
- Luma = Brightness

[TastyKellogg]

Thanks
I get it now haha
But I guess it would take some time to get used to it but i get the concept. Thanks!

[sielgaudis]

Mylenium's description is correct, but in the interest of simplicity you can think of it like this:

- Alpha = Transparency
- Luma = Brightness

Very good explanation! I will remember it.

[beenyweenies]

Thanks
I get it now haha
But I guess it would take some time to get used to it but i get the concept. Thanks!

The easiest way to remember everything is to consider the root meaning of these words. Luma is basically shorthand for Luminance, aka Illumination. Think of Luma as ilLUMAnation and it will stick better that this represents brightness values.

"Alpha Channel" isn't as easy. The name refers to a formula that uses the greek letter "a" (or alpha in math-speak). This formula describes the interpolation between two values, which is really all the alpha channel does - tells a program how to blend the footage pixels with the pixels from footage beneath it.

[rhodeder]

Yeah I think you have the question pretty much answered maybe he had some confusion along the way where a luma map was used as a alpha matte for a flame or smoke or some glowing text or something.

[TastyKellogg]

Haha thanks for the name tip. I'll just remember it Luma and the other guy
Oh yeah I always always was confused when my teacher said "here do alpha matte" and i'm like why?
But i get it now. Thanks again !

[rhodeder]

Haha thanks for the name tip. I'll just remember it Luma and the other guy
Oh yeah I always always was confused when my teacher said "here do alpha matte" and i'm like why?
But i get it now. Thanks again !
you know how to apply a alpha matte?

[TastyKellogg]

Umm
I meant the track matte thing? and you choose alpha there?
Sory I don't know what exactly its called. I just started learning about 3 months ago. haha

[rhodeder]

yeah, you got it!

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