Better LAB support

[IPv6]

AP has some problems with LAB color space. So this is a list of possible improvements in this area

Intro: Both AF and Photoshop can work in LAB mode, but LAB support is different in some areas - and PH way feels better in some cases. The most notable case - HSV correction layer. In Photoshop, its meaning is different for Lab documents, it work in some "LAB Logic", where "Hue" is angle between vector (A,B) and arbitrary axis on imaginable color wheel, "Saturation" is length of (A,B) vector and Lightness - is pure Lab lightness, after all. Difference easy to notice: if you disable all channels except L in Photoshop and try to shift Hue - you will see NO difference. Hue completely separated from lightness. And you will see huge changes in AP (with Lab-converted document and A/B channels off). So in AP hue still contribute to L-channel a lot, even in Lab color model. And to sum this up - PH gives better visual results for some artistic styles (anime style, for example).

Another problem - some AP tools with LAB support wrongly scale A/B channel

So, suggestions:
1) Add LAB support into "HSV" correction layer. It already has a hsv-checkbox, which change the way values are evaluated, so another interpretation for LAB would fit perfectly. With the same scheme/meaning as in Photoshop - where "Hue" is angle between vector (A,B) and arbitrary axis on imaginable color wheel, "Saturation" is length of (A,B) vector and Lightness - is LAB lightness, L-channel

2) "Black and white" correction layer could use a similar switch for "Ligtness meaning" - and give "L" channel as another alternative to color tones mixing.

3) Currently "Channel Mixer"  correction layer treat LAB for A/B channels wrongly... It seems to "center A/B channels" not around zero value, but around value shifted by 1. Because of this user is not able to desaturate image by settings A/B to 50% - instead of pure desaturation "Channel Mixer" shifts hue. which is totally wrong for LAB space, or at least unexpected/not usable

4) Same problem with wrong treatment of A/B channel exist in "Apply image" filter, when using LAB in equations. see this answer for details -