How can I select specific a value of grey throughout an image based on grey value only?

[johns9381837]

I am trying to do some Color Gamut correction for some CMYK images. To do this I need to be able to select specific values of grey in each CMYK channel so that I can adjust them.

Problem I run into is that even though the grey value is the same, if the RGB values are off at all it, it won't include those areas. Since the values I am looking for are very specific I'd need the tolerance to be at 0.

Is there a way to make it so that the magic wand tool makes selections soley based on a pixel's grey value versus both grey scale and the RGB values?

Thanks!

[Bojan Živković]

What method you are using to select pixels? Magic Wand in channel view mode? What is set from Sample Size drop down in the Options bar? Point Sample or something else?

Can you post some screenshot to explain visually what are you doing?

[johns9381837]

Sure thing!

So I've set up a basic example image using pure CMYK + white colors to demonstrate the issue.

My magic want  doesn't have a  point sample, is set to non-contiguous, with tolerance set to 0.

What I am trying to do is select all areas in the image of 8% grey on the cyan channel. Which it looks like it has done however if you look closer you can see that it has selected area of 8% grey that also have the same RGB values. I want to be able to select all areas of 8% grey regardless of the RGB values. The main idea is to select all areas of 1-4% of grey and make it 0% and areas of 5-9% brought up to 10%.

What I want to do is be able to quickly select all the necessary specific grey values in each of the CMYK channels and adjust as necessary.

Hopefully that all makes sense and thanks for trying to help me out!

[johns9381837]

Wanted to follow and mention that it appears as though the magic wand tool in only looking at the RGB values not the K value which is what i want it to do.

[davescm]

"I want to be able to select all areas of 8% grey regardless of the RGB values. The main idea is to select all areas of 1-4% of grey and make it 0% and areas of 5-9% brought up to 10%."

Why not do that with a curve? You adjust the curves for individual channels with a curves adjustment layer.

Dave

[Stephen_A_Marsh]

@davescm 

Agreed, however, I think this is for flexographic tone correction, so using the curve/pencil/arbitrary map option would likely be required so as not to adversly affect other tones or to lock down the curve with many extra points.

It has been a long time since I have done any flexo production, so no promises on how well these will work and you may need to create your own as not all tonal values will are covered:

https://www.dropbox.com/s/5e28ha300yuihwh/FlexoCurves.zip?dl=0

FlexoCurves Presets (there are also actions):

• Min 3% CMYK Dot Curve.acv
• Min 5% CMYK Dot Curve.acv
• Clip 1% CMYK Tones.amp
• Clip 2% CMYK Tones.amp
• Clip 3% CMYK Tones.amp Clip 4% CMYK Tones.amp
• Clip 5% CMYK Tones.amp
• Min 3% CMYK Dot ArbMap.amp
• Min 5% CMYK Dot ArbMap.amp

P.S. You may be interested in the following:

https://www.retouchxpress.eu/

[johns9381837]

Thank you for this.

I am specificalll planning on using this for screen printing CMYK images.

Anything under 10%, doesn't print so I am trying to figure out a way to easily take 1-4% down to 0% and 5-9% up to 10% without effecting any of the other grey values in the CMYK channels.

I was hoping there was a simple option in the magic wand tool, that I was unaware of, that would allow selections based specifically on the % of K instead of values of RGB. Regardless you've given me alot to look over and research!

Thank you again!