Desaturation differences on Photoshop...

[Neiluj_____]

Hello,

I need explanations about the differences in values obtained when performing various desaturation methods in Photoshop.

Let me explain :

I need to occasionally switch my color digital paintings (RGB) in black&white to check the efficiency of my contrasts which can be distorted by the color.

So, I have explored several desaturation paths of my colors, but the grayscales come to change according to the method used...

The gray values appear to be similar when making adjustments :

-Picture / Adjustments / Desaturate

-Image / Settings / Hues-Saturation / Slider of Saturation brought all left

And they are identical on these settings, but different from those above :

-Image / Mode / Grayscale

-creating a clipping mask layer above my image / filling it in black / switching to color or saturate mode

Why ? And what values do I trust ?

I need a desaturation tool that shows the actual gray values of my colors, not a software automation that changes the gray values for better rendering.

Thank you !

[Mylenium]

I need a desaturation tool that shows the actual gray values of my colors,

There is no such thing and there can't be. It's all about perception, Gamma and a few other things. I don't even see how it would be relevant for traditionally oriented painting. I mean daVinci couldn't desaturate the Mona Lisa just to check. Perhaps you need to train your eye more. Otherwise you might want to read up on what Gamma means and how and why images on a computer screen are not a linear representation of natural colors, icluding their underlying luminance/ grey value. In fact if your images were linear with a Gamma of 1.0, your greys would look even more wrong, so in the end the issue remains: The perceived luminace and color has little to do with the technical rendering of colors on screen in realtion to the color spaces and things such as color profiles and your monitor's color settings, gamut and what have you.

Mylenium

[c.pfaffenbichler]

Setting the Info Panel’s Second Colour Readout to Grayscale and using View > Proof Colors (after using Proof Setup > Custom to create an appropriate grayscale psf) should allow to provide numerical and optical representations, but what good they ultimately are seems somewhat unclear.

[Neiluj_____]

Mylenium, you understood that I have very little knowledge... (and a very bad level of English...) and I thought that as we can choose a percentage of grey and saturation for a given shade to get a unique color in the color selector, we had to be able to get the exact grey value of a color just by removing its hue and saturation...

daVinci couldn't desaturate the Mona Lisa just to check, yes, but, he did not have the constraints of our time.

Let's say that I am looking for strong contrasts in my illustration.

I use these first two colors. They contrast sharply, and when I pass them in grayscale, the values also dissociate well.

I made a good choice ! (in english : *contrast colors/*grayscale conversion)

I continue my image, and decides to use two other contrasting colors. They also work ! But look what happens when I pass them in Grayscale ...

Imagine that I apply these colors on a logo that will be declined on a document in grayscale... it will be just illegible...

This is why I believe it is crucial to insert a highly reliable desaturation tool that does not change the values of my colors by changing them to Grayscale in my workflow.

c.pfaffenbichler, creating a grayscale preview of an RGB document without having to apply it is very interesting, in fact, but in my case, the result is the same as a grayscale conversion.

Here are the results, in Grayscale, for two different actions

-Image / Mode / Grayscale

and

-Image / Adjustments / Desaturate

(in english : *grayscale conversion/*desaturation)

Notice the differences about my Green according to the method used...

[abharat_in]

Hi Neiluj...,

As I could understand, your workflow requires you to preview the RGB tint in grayscale while working & before actually converting it. So in this case instead of relying on "desaturate" you can use following option to preview the results in grayscale itself while you are working -

01. Keep your RGB image open & then open a new window for your image from 'Window>Arrange>New Window' option.

02. Place your image windows side by side.

03. For the new window, enable grayscale soft proofing from 'View>Proof Setup' option.

04. Now keep working on your image in original window where image renders in color & whatever colors you are creating will give you a run-time preview in grayscale on the second image window.

Hope this helps.

Regards,

Ajay

[Neiluj_____]

D fosse, currently I work for the Web, and my workspace is sRGB IEC61966-2.1, Gray Gamma 2.2, Dot gain 15%.

So if I look at this information, how can I use it to find out what results from the many grayscale/desaturation conversions can serve as my reference ?

abharat_in, yes, it's a good method, but as the L layer of the Lab mode give me other gray values than those of the grayscale mode, I still do not know which one to rely on...

[D Fosse]

All numerical readouts are relative to color space. This includes grayscale, which will give you different numbers depending on tone response curve (gamma curve or dot gain) - for the same on-screen gray tone.

The readout in the Info panel refers to the working spaces, except for open files, where the embedded profile overrides the working space.

An RGB desaturation just averages the three channels. This gives the odd effect that bright yellows become dark, and deep blues become lighter. Converting to grayscale doesn't do this - it's first converted to Lab and then the L channel is converted to grayscale (discarding the chroma channels). The difference between RGB and Lab is illustrated here:

[Neiluj_____]

Okay, I understand with difficulty the informations...

But, do you mean that for my case, trying to refer to shades of gray is illusory, and that it is better that I refer to the scales of luminance ?

[D Fosse]

Any numerical gray value depends on what gray space, or RGB/CMYK color space the file is encoded in. If you include that information, the value is unambiguous. Gray gamma 2.2 is different from dot gain 15%, and sRGB is different from Adobe RGB.

The simplest way is to give Lab values, which are always specific and can be used as reference.