The L channels of LCH and LAB are the same.
GreenKoopa wrote:I thought C and H were simply the polar version of a and b.
I've often seen statements like that, but I don't know precisely what it means.
The "H" channel of LCH (aka LCHab) isn't exactly what we normally think of as Hue.
The "C" channel is Chroma, but the neutral point is at 50%. If we want to decrease saturation, we should push this value towards 50%. If we simply halve the value, then C=100% will become C=50% (removing colour entirely), but 50% will become 25% (giving it colour), 40% will become 20% (increasing saturation), 60% will become 30% (which is the other side of 50% so the colour will change) and so on.
Are we confused? I am. But wait, it gets worse.
LCH has a different gamut to sRGB. I think that all colours expressible in sRGB have one (or two!) equivalents in LCH [see *EDIT]. But the converse isn't true.
For example, LCH=(50%,50%,10%) is a shade of gray. Fair enough.
LCH=(50%,25%,10%) translates to sRGB (-267%,56%,71%). The negative number will often be clamped to zero.
And LCH=(50%%,10%%,10%%) translates to sRGB (-563%,56%,71%).
*EDIT: I was wrong; some sRGB colours are not expressible in LCH. sRGB pure blue #00f has C=102%, so is out-of-gamut for LCH.