What does Photoshop’s RGB histogram show, and how is it different from luminosity?
Asked 7/16/2016
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In Photoshop, the Histogram panel can show Red, Green, Blue, RGB, Luminosity, and Colors. I understand the per-channel Red/Green/Blue histograms, since each channel is already on a 0–255 scale. What I’m confused about is the single combined RGB histogram.
A color image contains three channel values per pixel, so how does Photoshop turn that into one histogram with a horizontal axis of 0–255? Is it computing luminance from the RGB values, or is it doing something else?
I’m specifically asking about Photoshop’s “RGB” histogram, not the “Luminosity” histogram.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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I have found the following link explaining the RGB histogram. Note that it talks about the Adobe Photoshop RGB histogram.
It basically states that the RGB histogram is just the addition of the R, G and B histograms, and so as it is, a very misleading histogram. It is not a histogram of luminosity or similar quantity.
Originally by user24785. Source · Licensed CC BY-SA 4.0
user24785
10y ago
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Generated from our catalog & community — verify before relying on it.
In Photoshop, the combined RGB histogram is generally just the sum of the red, green, and blue channel histograms.
So it is not reducing full 24-bit color to a single 8-bit “tone” value, and it is not the same as luminosity/luminance.
How to think of it:
- Each channel already has values from 0–255.
- Photoshop counts how many pixels have each value in Red, Green, and Blue.
- For the RGB view, it adds those three counts together for each bin from 0 to 255.
That means the combined RGB histogram is really showing the distribution of channel values, not a single perceived brightness value per pixel. Because of that, it can be misleading if you interpret it as brightness.
By contrast, a Luminosity histogram is based on a weighted brightness calculation from the three channels, such as: Y = 0.2126R + 0.7152G + 0.0722B (or a similar formula depending on color space).
So:
- RGB histogram = added R + G + B channel histograms
- Luminosity histogram = one brightness value computed per pixel
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