Keyer vs comp

Can someone explain why I get these different results, they are both difference matted with the original scan, the only difference should be that I’ve added embers to a fire.
The top image is the result of using the MK, the bottom is using the resulting matte from the MK, but with a comp node.
What extra processing is going on in the MK node?

Does clamping do any difference? Somehow it looks like the MK differences between the unclamped result.

No difference when clamped.

Care to share a screenshot of the batch schematic?

If you gamma up your screen can you see any noise in the second option?

No, both images are with the screen gamma’d up a huge amount.

Hi Sinan, I can’t really share the clips involved, but it’s a straight luma key, with a couple of gmasks.
The only difference really is the way its comped over, comp node vs MK result node.

Is ti maybe that MK divides the input in order to un-mult it and that’s pushing some rogue pixels through?

Fred? Can you shed some light on this?