4 Replies Latest reply on Mar 4, 2008 3:57 PM by Soulbot

# Compositing - Channel Mixer Help

I've been using AE now for a few years, (since v5.5) and mostly use it for motion graphics. Using CS3 now. But right now, I'm beginning to work through Steve Wright's book, <a href="http://www.amazon.com/Digital-Compositing-Film-Video-Second/dp/024080760X">Digital Compositing for Film and Video</a> to focus more on true compositing.

The author's aim is to keep the lessons application-independent and present concepts in a way that would be applicable to most software packages. (However, considering the author's frequent references to "nodes" and his well-known status as a Shake user & <a href="http://movielibrary.lynda.com/html/modPage.asp?ID=408">Shake instructor</a>, it's apparent that using Shake, while reading this book, would lead to the least bumps in the road, when attempting to follow along.)

But I'm not using Shake, I'm using AE and have encountered a small bump in the road. Regarding luma keys he mentions how luma values are represented in the individual colors R,G,B. To cut to the chase, he says, "One standard equation that mixes the right proportions of RGB values to create a luminance image on a color monitor is:
Luminance = 0.30R + 0.59G + 0.11B"

Great. But I can't figure exactly how to mix the RGB values with the above proportions in AE. It's quite easy & straight-forward in Photoshop, using the Channel Mixer. <a href="http://www.geocities.com/kevincbrock/channel_mixer.png">Example here</a>. But AE's Channel Mixer does not seem to work quite the same way as it's counterpart in PS. There are values there, for instance, "Red-Red", but they seem to run in values from -200 to 200, not percentages, like PS. So my question is, how do I achieve the same channel mixing results in AE that I can in PS?

I'll add this: I was able to achieve the desired result in AE, but by using a round-a-bout sort of tactic. Instead of using the "Channel Mixer", I applied the "Set Channels" effect to my sample shot. By changing the "Set Red to Source 1's LUMINANCE", then doing the same for Green and Blue, setting them all to LUMINANCE, the resulting image is identical to the sample shot that was tweaked in Photoshop. Again, it worked, but I'd like to know how to change the proportions of RGB to other values than those represented in the formula above.

Hope this all makes sense.

Oh, and while I'm here.....I'll throw this in: In AE, when you apply the "Curves" effect, can you input precise values for input / output. Again, this is simple in Photoshop, but unless I don't have some display items turned on in the Curves editor in AE, I don't see a way to enter precise values. Is this possible?

Thanks in advance for any input.

EDIT: for some reason, the links I've posted aren't showing up properly. So if your'e interested in following any of them you'll have to copy/paste on your own. Don't know why......
• ###### 1. Re: Compositing - Channel Mixer Help
>Great. But I can't figure exactly how to mix the RGB values with the
>above proportions in AE.

You could use expressions... You create sliders for each input, then feed their output to the actual effects. Something like

//----begin expression
linear(value,0,100,-200,200)
//----end expression

should do in most cases. Still, be careful. Not everything written in that book directly applies to AE. It does not figure in any specifics of color management nor different Gammas which could massively skew results because the source footage will already be pre-adjusted and the comp will add another level of adjustments.

>Oh, and while I'm here.....I'll throw this in: In AE, when you apply >the "Curves" effect, can you input precise values for input / output.

Nope. AE's Curve effect will adapt to the project bit depth as a lot of other effects, presenting you with the problem of which values to actually use. This would only make sense if the effect itself would be independent from AE and process in float all the time and only the inputs were presented as different normalized ranges. There was/ should have been an alternative Curves effect from Frischluft, but at this point development has reached a dead stop, so it's doubtful it will ever be unleashed upon the world.

>Don't know why......

HTML-Code functionality had to be disabled thanks to spammers. :-|

Mylenium
• ###### 2. Re: Compositing - Channel Mixer Help
Thanks much for the response. Regarding the RGB proportions, 'll give the sliders concept a go and see how that works. The curves thing makes more sense now after your explanation. Figured something had to have changed regarding the HTML functionality on the management side. Thanks again!
• ###### 3. Re: Compositing - Channel Mixer Help
> for some reason, the links I've posted aren't showing up properly. So if your'e interested in following any of them you'll have to copy/paste on your own. Don't know why......

You can still post a direct link on a single line and the Forum engine will hyperlink it - not so tidy, but does the job.