luminance channel in AE
by kirk tabalotny
on
Apr 21, 2008 at 12:28:24 pm
Is there any way of separating out a luminance channel for video assets in AE - similar to 'lab colour' mode that allows creation of a luminance-based layer mask in photoshop?
Re: luminance channel in AE by Darby Edelen on Apr 21, 2008 at 3:27:03 pm
If you only want the luminance values for the layer you can apply Color Correction > Tint with the default values (black & white) and that should do it.
Otherwise, if you want the luminance and color difference channels you can use Channel > Channel Combiner and set it to RGB > YUV. Then the R channel becomes the Y (luminance) channel and the G & B channels become the U & V (color difference) channels.
Also, if you like the Tint method, but want the RGB values to be weighted differently, you can use Color Correction > Channel Mixer, enable 'Monochrome' and play with those values.
Darby Edelen Lead Designer Left Coast Digital Santa Cruz, CA
Re: luminance channel in AE by david bogie on Apr 21, 2008 at 3:29:12 pm
You can shift any channel to the alpha. Luminance is just a grayscale, you can pick any of the RG or B luma information. Then you would extract a simple luma key from the alpha.
The comparison of tool and methods with Photoshop is not always accurate. AE uses objects in layers, PS uses pixels in layers.
bogiesan
This is my standard sigfile so do not take it personally: "For crying out loud, read the freakin' manual."
Re: luminance channel in AE by kirk tabalotny on Apr 22, 2008 at 2:01:46 am
thanks David & Darby - judging by the times on your replies there seems to have been a race to post... This turned out to be one of those times when usin' da manual was stymied by not knowing which search term/s to look under. My bed-time reading lately is about colour theory - is my life out of balance or what?
Re: luminance channel in AE by Darby Edelen on Apr 22, 2008 at 4:26:11 am
[kirk tabalotny]"My bed-time reading lately is about colour theory - is my life out of balance or what?"
Glad I'm not the only one =)
You should already know, then, that luminance is based on human perception which is more sensitive to green than red, and more sensitive to red than blue.
There are several ways to preserve this model of sensitivity in AE, one of which is the default Color Correction > Tint effect. Alternatively, you can place a Solid color layer at the top of a composition with 0% saturation (pure white, black or gray works) and set it to the 'Color' blend mode. Also, the same results can be achieved with the Color Correction > Channel Mixer effect as long as the weighting is modeled after human perception...
Just to complicate things, video doesn't generally use luminance, but rather luma which consists of gamma encoded luminance... yikes! Different color spaces (SDTV, HDTV, etc.) can apply different gamma curves to the values and apply different weightings to the luminosity values. The former generally doesn't pose a problem, since the inverse gamma curve should be applied farther down the pipeline (e.g. at display)... The latter, I swear, can be useful information if you want to use Channel Mixer.
HDTV for example uses these coefficients: 0.2126 R' + 0.7152 G' + 0.0722 B'
So you can emulate the default Color Correction > Tint effect with values of 21.26 Red 71.52 Green and 7.22 Blue in the HDTV color profile or come up with your own similar or dissimilar values (because lets face it... human perception can be so boring).
Darby Edelen Lead Designer Left Coast Digital Santa Cruz, CA
Re: luminance channel in AE by david bogie on Apr 22, 2008 at 2:40:30 pm
If Darby's in the race, he wins. No matter when he crosses the line, he wins. A total wonk, he lives and breathes this stuff, thinks in Java and dreams in layers linked by expressions.
bogiesan
This is my standard sigfile so do not take it personally: "For crying out loud, read the freakin' manual."
Re: luminance channel in AE by kirk tabalotny on Apr 24, 2008 at 7:43:06 am
ooooow... my brain hurts. And then I read a bunch of articles about luma/gamma correction/colour space standards. Tylenol won't fix this one! Thanks again guuys - what an amazing forum eh!
