The following terminology might not be generally accepted but the distinctions need to be made in order to talk about “unbounded” RGB image editing. So if someone knows of “official terminology”, please let me know! But for purposes of answering @afre’s question, here are some definitions and proposed terminology:
Given a specific RGB working color space ICC profile such as the sRGB ICC profile:
- “In display range”: all RGB channel values are between 0.0f and 1.0f inclusively. These colors are “in display range”: (0.5, 0.75, 0.01), (0.5, 0.0, 0.0), (0.5, 1.0, 0.0), (1.0, 1.0, 1.0), (0.0, 0.0, 0.0).
- “Out of display range”: At least one RGB channel value is either greater than 1.0 or less than 0.0. So these colors are out of display range: (0.5, 0.75, -0.01), (0.5, 1.01, 0.2), (0.5, 1.01, -0.2), (1.2, 1.2, 1.2), (-1.0, -5.0, 15).
- “In gamut”: All RGB channel values are >=0.0. So (0.0, 0.0, 0.0), (0.5, 0.75, 0.01), and (25.0, 50.0, 3.75) are all “in gamut” colors, but the last color is “out of display range”.
- “Out of gamut”: At least one RGB channel value is less than 0.0f. One or two channel values might also be greater than 1.0, but the color is not “out of gamut” unless at least one channel value is less than 0.0. So these colors are “out of gamut”: (0.5, 0.75, -0.01), (0.5. 1.01, -0.2), (-1.0, -5.0, 15). But these colors are not “out of gamut” though they are “out of display range”: (1.2, 1.2, 1.2), (0.0, 0.0, 25.0).
- HDR colors that are also “in gamut”: At least one RGB channel values is greater than 1.0 and none of the channel values are less than 0.0. Please note that the “Y” value of an HDR color can easily be less than 1.0. For example the sRGB color (0.0, 0.0, 5.0) is an HDR color, but the Y value is only 0.30304 on a scale from 0.0 to 1.0 (30.304 on a scale from 0.0 to 100.0 as ArgyllCMS xicclu uses)
xicclu -ir -pX sRGB-elle-V2-g10.icc
1 1 1
1.000000 1.000000 1.000000 [RGB] → MatrixFwd → 96.420288 100.000000 82.490540 [XYZ]
0 0 1
0.000000 0.000000 1.000000 [RGB] → MatrixFwd → 14.305115 6.060791 71.392822 [XYZ]
0 0 5
0.000000 0.000000 5.000000 [RGB] → MatrixFwd → 71.525574 30.303955 356.964111 [XYZ]
- Well, that leaves one last category of colors, which are HDR colors that are also “out of gamut” with respect to the specified RGB working space. Such colors require having at least one RGB channel value that’s <0.0 and also at least one RGB channel value that’s >1.0.
As @Carmelo_DrRaw notes, when converted from the specified RGB color space to XYZ, if the “Y” value is greater than 1.0, the color is by definition HDR. But I’ve suggested that the only requirement for in-gamut HDR colors is that at least one channel value be greater than 1.0, which means an HDR color might easily have a Y value that’s less than 1.0, in which case sometimes (I’m not sure about always) the color won’t be HDR with respect to some other RGB color space. However, I think it makes sense to say that the sRGB color (0.0, 0.0, 5.0) is a high dynamic range color with respect to the sRGB color space, even though the Y value for this color is less than 1.0.
In the xyY color space it’s really easy to tell if a color is out of gamut with respect to a given RGB matrix color space: on the xy plane of the xyY color space, draw a triangle connecting the xy values of the color space’s primaries. All colors that fall outside the triangle are out of gamut wrt to the specified color space . Here are some pictures to illustrate:
Notice that in the above image, the white point is D50 even for the sRGB color space, because we are dealing with ICC profiles, not the color spaces specified by the color space specs.
All ProPhotoRGB colors can be encoded in the sRGB color space, but doing so for the colors outside the sRGB xy triangle requires using RGB channel values that are <0.0 (and hence out of gamut wrt to sRGB).
Notice that in the above image of course ProPhotoRGB reddest red is out of gamut with respect to the sRGB color space (it’s outside the sRGB “triangle”) and also HDR with respect to the sRGB color space (it’s brighter than sRGB’s reddest red). I’m assuming it really does make sense to talk about HDR colors with Y values less than 1.0. I think it does, in HDR editing an RGB channel value of 1.0 is just another channel value on the way from 0 to whatever, and Luminance values and/or RGB channel values that are greater than or less than 1.0 in parts of the image, doesn’t make parts of an HDR image “HDR” and other parts “not HDR”.
In any given RGB color space, “in gamut” just means the xy values are within or on the triangle defined by the xy values for the color space primaries. “In display range” means the colors are not only “in gamut” (no negative RGB channel values) but also all RGB channel values are <=1.0.
The above images are from this article: https://ninedegreesbelow.com/photography/display-referred-scene-referred.html .