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O
olivierr92135410
Participant
November 19, 2018
Question

Doesn't understand the logic behind ctrl-click for luminosity selection on the rgb channel

  • November 19, 2018
  • 1 reply
  • 2043 views

Hi,

I'm trying to understand the maths that are applied when I control click on a rgb channel? Apparently, it's 50% of the luminosity values (selection from 128 to 255), but when I intersect this selection by itself, (ctrl-alt-shift click), I don't get a 25% selection (0,5 x0,5 = 0,25 right?), instead I get a selection from around something like 170 to 255 (see screenshot in attachment), normally it should be 128 + 64 = 192 no?

Also when I create the luminosity masks using calculations on the grey channel in multiply mode, I get different results after repeating a few times the operation...

Thanks in advance !

Olivier

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1 reply

D Fosse
Community Expert
D FosseCommunity Expert
Community Expert
November 19, 2018

No, you misunderstand what a luminosity selection is. A pixel can be partially selected. It's not a threshold, it's not either/or.

The marching ants just mark the 50% boundary, but the selection itself is a full-range grayscale image.

If I ctrl-click RGB on this image -

- I get a selection that in reality looks like this. Dark is less selected, light is more selected:

JJMack
Community Expert
JJMackCommunity Expert
Community Expert
November 19, 2018

Its very easy to see this is correct:

JJMack
O
olivierr92135410Author
Participant
November 19, 2018

Thank you for your answer,

so the marquee doesn't indicate the true luminosity selected range, but why then, when I create luminosity masks with ctrl-click and intersection, I don't get the same alpha channels that those created by the calculations method (multiply) ? (the 5 first alpha channels here under were created by  the calculations method, the following 5 by the ctrl-alt-shift click method) Also, in the alpha channels created by  the ctrl-alt-shift- click method some banding appears, altough I'm viewing them @100% zoom level and working in a 16 bits file ?

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