"Channel mixer" in LAB mode is broken

[IPv6]

I believe current behaviour of "Channel mixer" in LAB mode is broken.

Steps to reproduce:
- add "Channel mixer" to colored image and

- switch Channel mixer to LAB and set "AOpponent" to 50% and "BOpponent" to 50% also.

You should (by definition of LAB A/B channels) get image with 50% of saturation - since both color channels was halfed
But your image will became even more saturated! Which is completely wrong, making AOpponent/BOpponent simply completely unusable //

You can try the same in RGB (Setting R->50% on R page, G->50% on G page, B->50% on B page) - And you will see that in RGB this work as expected - image will become more "grayish". In LAB this is obviously should be similar

[hanshab]

My experience is as follows in LAB.  When you reduce say Ao to 50% then what happens is that only the magenta is reduced to 50% but not the green it stays at 100%.  When you reduce the Bo ( after resetting the Ao to 100%) what happens is only the yellow gets reduced to 50%, the blue does not get reduced.   I believe that is what IPv6 may mean. BUT in LAB the AO and Bo sliders each work on two colors which is not the case in RGB . However, if in Ao you reduce both Ao and Bo to 50% then the colors are true.  Same is true in Bo  .  Is this the way it should be?  Thanks

[IPv6]

Not clear what you mean... i`m talking about changing AOpponent value on AOpponent page and BOpponent value on BOpponent page. Without inter-influence.

Just checked the most simple case that prove Channel mixer is wrong in LAB mode. Just set all values on AOpponent and BOpponent pages to zero. So all values on all pages are zero except Lightness on Lightness page (leave default 100) . You should get completely gray picture, in that case, since both color channels == 0. But you will get very bluish image instead //

[hanshab]

Actually I am agreeing with you in my statement above   I got the blue image as well when I set Ao and Bo both to zero... Even when fixed, the channel mixer will work differently in LAB than in RGB. The Ao and Bo sliders don't really work right now and I dont know what algorithm was used.  I would not even speculate...  

[Gabe]

Hi both,

Sorry for the delayed reply. 

You would need to offset both A and B channels by 50% if you wish a gray image. In LAB, you have gray in the middle of "A" and "B" not at 0%, therefore you would need to offset them by 50%. So, by setting the sliders to 0 for both A and B you have a combination of Blue and Green, which is expected.

 

Have a look here: 

 

Thanks,

Gabe.