When in the Image Editor, left clicking on a pixel makes a small row of numbers appear at the bottom.
I know what some of these mean, but others are a mystery to me...
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From left to right: X & Y are the coordinates you clicked on.
R, G, and B are the RGB values for that pixel.
A is Alpha.
These values are in Scene Referred values in a infinite linear scale,
The numbers after "CM" are the Color-Managed values. Based on how you have set the Color Management section of your scene, it's telling you what the effective colors will be after applying the settings.
R, G, and B are adjusted for your color management, and the little square next to them is showing a visual representation of the adjusted color.
Then H, S, and V are Hue, Saturation, and Value, which is simply a different way of representing the R, G, B values.
HSV is closer to how humans perceive color, whereas RGB is based on colors of light, so some people prefer to work in that color space.
The L is Luma, which is a gamma-corrected sum of R, G, and B. Basically if you converted a color to grayscale, how bright it would be.
Since yours already is grayscale, the L value is the same as the RGB and V values.
Here's way too much detail: https://en.wikipedia.org/wiki/HSL_and_HSV
The RGB on the left are "scene-referred" values, meaning they are the actual values in the scene.
Only formats such as .hdr or .exr can hold this kind of information.
If you were to display this information without a color transform by the color management, the colors would look wrong, too dark and contrasty, and all values beyond 1 would be ignored or clipped in the standard sRGB display transform (the clip point is at 16.19 when using filmic).
The numbers to the right of the "CM" indicator are "display-referred" values, meaning they are adjusted to show on a monitor based on your CM settings.
These are my scene settings, for example. If you change them you'll see the "CM" numbers will be different.
If you're not familiar with color management, I suggest watching this video: https://www.blenderguru.com/tutorials/secret-ingredient-photorealism
Read also:
Render with a wider dynamic range in cycles to produce photorealistic looking images
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$\begingroup$ I'm afraid this only answers a very small portion of my question. Could you please edit to elaborate? $\endgroup$– LegomanNov 25 '18 at 2:00
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$\begingroup$ added more detail; let me know if that helps or if you need more info on any of it! $\endgroup$ Nov 25 '18 at 2:55
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$\begingroup$ R, G and B are the scene referred pixel values. Scene referred values have no maximum, and are not some sort of representation of a 0-255 range. They can be infinitely high. The CM values are the resulting values after Look and View Transform, both found in the Color Management section, are applied. The CM values have to be within a 0-1 range, and are sent like this to your monitor. $\endgroup$ Nov 25 '18 at 5:08
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$\begingroup$ ah yes, you are correct. I wasn't thinking of images like .hdr or .exr that can have a large range of values. I'll edit my answer. $\endgroup$ Nov 25 '18 at 5:57
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$\begingroup$ Better, but please note that there are no conversions of 0 - 255 range happening anywhere. That terminology comes from Applications like Photoshop, which made that choice rather from a UI perspective, not from the data which was meant to be represented. In Blender we are dealing with a rendering context. Rendering is theoretically unlimited in terms of R,G,B values it can produce (practically limited by hardware). I'm pointing this out because your answer refers to .hdr and .exr imagery, but does not mention the content of a renderlayer before it is saved to an image at all. $\endgroup$ Nov 25 '18 at 6:52