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Question

Adjustments/Posterize is making orange steps on gray ramps in Lab color mode.

Forum|Forum|1 year ago
June 7, 2024
6 replies
626 views

Photoshop version: 25.9.0
Mac OS 12.7.5

Issue: When I apply Image/adjusments/posterise to a gray ramp in a Lab color mode document, even values of posterize levels colors the image steps orange instead of gray.

However, odd values of posterize levles color the image in gray steps as expected.

Steps
1: create new document in 16 bit lab color mode.
2: with the gradient tool make a "classic" gradient from black to white.
3: choose application menu 'image/Adjustments/Posterize'
4: Choose an odd number for 'Levels'

Expected result. : posterized image should be steps of gray
Actual result: posterized image steps are orange.

macOS

This topic has been closed for replies.

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peterm34359157Author

Known Participant

@D Fosse We should probably just consider posterize an RGB tool, not applicable in Lab.

It works fine on the lab images I tried*. Looking just as I would expect, but only for odd number posterize levels. It's even numbers posterize levels it gets wrong. So a fix should not be hard.

(* Roman16 test images in Lab color mode)

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peterm34359157Author

Known Participant

This report has been moved out of the photoshop "bugs" section and into "discussions".

I can only presume the list manager had some doubts about this behaviour actually being a bug.

Please could this report be moved back into the "bugs" section.

For list users reading this, if you see the same behaviour, would you please consider 'up-voting' this. And leave a comment that you don't consider this normal behaviour. Hopefully we can get this treated as a bug and fixed.

Thanks list subscribers for your help.

D Fosse

Community Expert

BTW it's everything in Lab, not just gradients. It does this to images too, whether monochrome or color.

My guess is that the algorithm "expects" full range a and b channels. We should probably just consider posterize an RGB tool, not applicable in Lab.

D Fosse

Community Expert

@Stephen Marsh

I'm guessing it's been there forever.

Yes, probably. Not something you would come across often. I could probably have a long and prosperous careeer without ever seeing this 😉

Stephen Marsh

Community Expert

From a practical perspective, target/select the L channel before you run Posterize (or uncheck the A and B channels in advanced blending).

Lab mode is the only mode that truly separates colour from tone. Use that accordingly.

As the A and B channels contain mid-grey for a neutral image, they in *theory* shouldn't be affected by posterize even if they are selected. So much for theory. :]

Compare the result of posterize on an alpha channel copy of the A or B channel.

It's most likely a bug, I'm guessing it's been there forever.

D Fosse

Community Expert

Yes, happens here too (on Windows).

Something wonky here. Sometimes it's orange, sometimes it's green. Here's what I get with a gradient fill layer, and then a posterize adjustment layer over that. This is a standard black to white gradient:

Only in Lab. RGB is fine.

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peterm34359157Author

Known Participant

opps, step 4 should read...

4: Choose an EVEN number for levels.

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