Spare channel from K channel

[FrenskeP]

I try yo create a luminosity mask from the K channel in color mode CMYK.

I convert the document to CMYK.

In Channels right click on Composite Black and choose "Create spare channel".

The result is a sort of inverted K channel.

See the attached files. The first picture is the real K-channel. The second the created spare channel.

In other color modes the same procedure gives me a perfect copy of the desired channels.

What went wrong? That's the question.

 

Edited May 11, 2021 by FrenskeP

[sfriedberg]

Ahah!  So black is 100%K, right.  When you look at the K channel as a monochrome channel, 100% is going to be displayed as white (full brightness) and 0% is going to be displayed as black (no brightness).  That's why you get the apparent inversion, as a display issue.  Properly, a monochrome version of the C, M and Y channels would be presented the same way.  White for 100%, black for 0%.  You can chide Serif for inconsistency if they don't.

This is not the way C, M, Y, or K plates printed on white paper look, where 0% is white, and 100% is solid ink color.

[FrenskeP]

Thank you for sharing your ideas about this. Sounds like a reasonable explanation.