Today’s Question: Could you provide some details about how the color RGB data is factored to arrive at the areas shown in the gray for the histogram in Lightroom Classic?
Tim’s Quick Answer: The area of the histogram in Lightroom Classic that appears gray is simply the area where the individual histograms for all three color channels (red, green, and blue) overlap each other.
More Detail: As noted in an Ask Tim Grey post last week, the histogram for an image reflects the relative distribution of tonal values in the image. However, when the histogram is presented as an RGB histogram, meaning that the individual color channels for red, green, and blue are represented, the shape of the overall histogram becomes a bit more complicated.
Specifically, instead of simply displaying a single histogram chart based on the luminance (brightness) values for pixels in the image, individual histograms are calculated for each of the color channels (red, green, and blue). Those three histogram charts are then overlaid together to provide a visual representation of the distribution of tonal values for each color channel that is used to create the overall image.
In Lightroom Classic, the area where all three color channels overlap completely is shown in light gray. Taken by itself, this gray area would be completely meaningless, because it only reflects the areas where all three color channel histograms overlap. However, it is helpful to have that gray area displayed as a way to provide context for the relative shapes of the individual histogram displays for the red, green, and blue channels.
For example, by having this gray area shown, it is easier to see which colors are more dominant in the shadows versus the highlights. Those colors would still stand out reasonably well without the gray indication of channel overlap, but having the gray color representing the overlap makes the color channels that stand out a bit in terms of pixel value distribution also stand out a bit better in terms of visibility on the histogram display.
