Ever since my first post about my practice of “grayscaling” or “desaturating” images of paintings to study value relationships, many have written in to ask about my particular manner of removing color. Well…this is a great question that I hope I can answer adequately.
As many of you know—while it is extremely effective for artists to navigate the dimensions of color as independent variables (Hue, Value and Chroma)—biological vision has a inconvenient habit of inextricably conflating them. As such, equiluminant colors may appear lighter or darker due to the influence of hue or chroma.
It is important to keep this in mind when choosing what manner of BW conversion will provide the most useful information about value from color images. Two of the most common methods for BW conversion are—“Remove Color” or “Desaturation” (often a manual slider in the Hue/Saturation region of PS/E) and “Convert to BW or “Grayscale.”
If you are converting an image to BW to better grasp objective luminance levels then you may want to consider the former as the function diminishes saturation evenly across the board. However, you should know that this process will diminish much of the contrast that may be apparent in the color image due to mechanisms of color perception (e.g., sensitivity variations among photoreceptors, saturation as an aspect of luminance (Helmholtz-Kohlrausch), etc.)
Some state that Photoshop’s “Desaturate” or “Remove Color” works by taking the averaging the highest and lowest of the three RGB figures (ignoring the middle guy), regardless of what colors they represent.
However, If you wish to convert an color image to BW and “compensate” for the perceptual contrast that is being lost you can use the latter. “Convert to BW” (at least in my version of PS) offers a real time preview of the preset conversions and sliders to adjust the “weight” of R, G, B, as well as overall “contrast”.
I have read this “Grayscale” function works in part by a routine that assigns to gray a value equal to 30% of the original’s Red, 59% of its Green, and 11% of its Blue channel. These “weights” are meant to preserve more of the contrast apparent in the original color image.
Sharad Mangalick, the Senior Product Manager of Digital Imaging at Adobe had this to say about the two methods: “When you desaturate the image, you are toning down the color. The color information is still there though. Clicking on the black & white button (or using the B&W portion of the HSL panel) converts the image to grayscale. Converting to grayscale allows you to tweak the B&W mix, which is not something that can do when you desaturate the image."
In the image I have shared here, the two cubes register the same brightness values via PS. This can be measured/verified in PS and far more noticeable when the image is “desaturated” (right:top). When the image is “grayscaled”, the resulting values are added specific weights to compensate for lost perceptual contrasts (right:bottom). The color swatches shown below also register the same brightness values in PS (again, obvious when desaturated as opposed to “grayscaled.”
I hope that this helps all who were confused about this.
Happy Painting!