After looking through the info that I report below I still wonder: 🤌 but in Photoshop what is the correct (or best) method to display only the Luminance of the colors while maintaining their Brightness?

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Lab. Period. Stop. Tell me that the following points can be ignored. Adobe says about the Lab color method: The CIE L * a * b * color model (Lab) is based on the human perception of color. The numeric values ​​in Lab describe all the colors that a person with normal vision sees. Because Lab describes how a color looks rather than how much of a particular colorant is needed for a device.

Adobe has a more basic explanation on Grayscale, or rather it does not explain the perceptual principles of this color method, but there is some interesting information elsewhere. Is this info compatible with what happens in Photoshop? If so, how would "Intensity" translate? Where could we learn more about it? The Importance Of Grayscale

Grayscale is a well-known technology, but it is not commonly used to display images. Because grayscale images measure intensity rather than color variation, they are commonly used in SEM / AFM and other non-visible light microscopes. Scientists and technicians can measure the color shade between products and the samples or pieces in production with grayscale images, allowing them to more accurately evaluate color differences between samples and finished products. https://dwphotoshop.com/how-to-convert-a-photo-to-grayscale-in-photoshop/


There are a number of commonly used methods to convert an RGB image to a grayscale image, such as average method and weighted method.

Average method: Grayscale = R / 3 + G / 3 + B / 3

The average method is simple but doesn't work as well as expected. The reason is that human eyeballs react differently to RGB. Eyes are most sensitive to green light, less sensitive to red light, and the least sensitive to blue light. Therefore, the three colors should have different weights in the distribution.

The weighted method, also called the luminosity method, weighs red, green, and blue according to their wavelengths.

Grayscale = 0.299R + 0.587G + 0.114B



According to point 2 which Grayscale conversion method is used by Photoshop? Is the aspect described below also involved? "Reducing a color image to monochrome grayscale is useful in many situations and can be accomplished in a variety of ways in addition to the obvious methods of selecting the R, G, B H, S, I, L, a, or b channel, or applying a selected color filter. If obtaining the maximum grayscale contrast between all of the structures present in the image is desired to facilitate grayscale image thresholding and measurement, then a unique function can be calculated for each image that fits a line through the points representing all of the pixels' color coordinates in color space. This least-square-fit principal component line gives the greatest separation of the various pixel color values ​​and the position of each pixel's coordinates as projected onto the line can be used as a grayscale value that gives the optimum contrast (Russ, 1995d). The Image Processing Handbook, Seventh Edition

🤯 Extra questions 🤯

Do the interactions that can be achieved with the Hue / Saturation / Lightness options require transformations from the RGB color space to the HSL / HSV color spaces and then back to RGB? Is this why the luminance of the tints does not remain uniform with this method? "The HSI spaces are useful for image processing because they separate the color information in ways that correspond to the human visual system's response and also because the axes correspond to many physical characteristics of specimens. One example of this is the staining of biological tissue. As a useful approximation, hue represents the stain color, saturation represents the amount of stain, and intensity represents the specimen density. Similarly, in remote sensing images the hue often identifies land use (crops, water, urbanization) while the intensity variation arises from the local sun angle and the shadowing by clouds. But these spaces are awkward ones mathematically: not only does the hue value cycle through the angles from 0 to 360 degrees and then wrap around, but also changing the intensity or luminance alters the saturation ". The Image Processing Handbook, Seventh Edition


About the formula of the desaturation command in Photoshop they say this about Stackoverflow. Which desaturation method would this formula refer to? 👉 "The real Photoshop desaturated formula is average of minimum RGB and maximum RGB components". 👉 "I believe HSL operations in Photoshop are run in min-max-hue space, so this formula is chosen for speed."


Side notes

A. Here the originals of the image in the post and how the different desaturation methods behave in Photoshop https://www.capturemonkey.com/many-ways-to-desaturate-in-photoshop/#comment-123223


B. In the Capture Monkey site the desaturation method is missing, with which an achromatic color is used with the "Color" blending method. But in comparison with the Grayscale and Lab method (only the luminance channel) even with this approach the luminances of the hues are altered, but after all this round which is the correct one to use?

Adobe says: Color - Creates a result color with the luminance of the base color and the hue and saturation of the blend color. This preserves the gray levels in the image and is useful for coloring monochrome images and for tinting color images. https://helpx.adobe.com/photoshop/using/blending-modes.html


C. Some nomenclatures in Photoshop do not help to fully understand some mechanisms of what is indicated in the previous points (hold on tight): https://www.facebook.com/groups/colorcorrectioncampus/permalink/5261591957235817

  • Do you perhaps want a grayscale image which has in every pixel (x,y) the luminosity taken from pixel (x,y) of a given color image? I guess your best approach is to put below the layer containing the color image a new layer, fill it with grey and let the color image layer have blending mode = luminosity. An example i.stack.imgur.com/uHQOH.jpg The result is true at least as a calculation in RGB numbers by using Adobe's secret formula for luminosity. Adobe has not published its blending mode formulas, but I guess they are the same as in the PDF specification.
    – user82991
    Oct 6, 2022 at 13:02
  • (continued) PDF blending mode formulas are printed here printtechnologies.org/wp-content/uploads/2020/03/…
    – user82991
    Oct 6, 2022 at 13:05
  • @user287001 thank you for your reply. The intent is to obtain the most accurate representation of just Luminance from colors on a given picture based on human visual perception: tints Brightness (perceptual Luminance) changes with different tints that have the same Luminance. I know that visual artists use dot gain 20% that is pretty similar if not the same to Grayscale color mode. I would know if that is the correct approach and possibly why. Thank you again.
    Oct 6, 2022 at 13:14
  • I'm sorry but I'm voting to close this question. This is about color science - not graphic design.
    – Wolff
    Oct 9, 2022 at 12:01
  • I answer this (at least tangentially) in stackoverflow.com/questions/596216/… and discuss a lot of other color theory over there at stackoverflow. See also: stackoverflow.com/questions/56198778/… and then stackoverflow.com/questions/56198778/…
    – Myndex
    Oct 30, 2022 at 2:21


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