L*a*b* question: why can't it be simulated in RGB?

[Chain]

Originally Posted by Zampano

Although the tone sometimes seems a little bit harsh, this thread is one of the most interesting ones here at RP...
Thanks a lot to all contributers for all the insights and especially for the example files...!

Yep, online discussion can easily spin out of control... But we're still all friends I think
The great thing here was that everyone had lots of useful information to add!

[Flashtones]

I think I speak for those who already know a lot more than me when I say: we all came out of it knowing more than we did coming in. And that's saying a lot considering the caliber of some of you.

[secretagents]

One good reason to use the Lab L channel over RGB applied in Lum mode, especially if using CPU hungry filters on large images, may be the gain in processing time which should be in Lab only a third of what it would be in RGB since only one channel is processed instead of three.

[Chain]

Originally Posted by secretagents

One good reason to use the Lab L channel over RGB applied in Lum mode, especially if using CPU hungry filters on large images, may be the gain in processing time which should be in Lab only a third of what it would be in RGB since only one channel is processed instead of three.

This is true.
However, I rarely run many heavy filters on only the brightness of the image tho. I guess it could make a difference if you're batching lots of stuff?

However the time same might not be 2/3 - it would depend a lot on what filter it is I think. I did a test with the "Reduce Noise"-filter (as an example of something hungry).
On a 5000x5000 16 bpp image running the filter on the RGB image, or a single LAB-channel reduced processing from 12 to 10 seconds (on a fairly slow computer, restarting Photoshop between each test).