When is ProPhoto RGB gamut bigger than Adobe RGB?

Started Dec 31, 2010 | Discussions thread
ForumParentFirstPrevious
Flat view
Joofa Senior Member • Posts: 2,791
When is ProPhoto RGB gamut bigger than Adobe RGB?

Hi,

Many 2D projections of color space gamuts show ProPhoto RGB wider than Adobe RGB. For e.g., see the image below from Wikipedia:

However, such renderings rely on a 2D projection. Ignoring the 3rd dimension of a 3D color space may have its consequences as seen below.

A little bit on terminology before going forward: Normally the white point is defined as that color which is obtained by adding unit stimulus of the 3 primaries (say RGB) after normalization. Hence, a "gamut" can be constructed for all colors that fall within this normalized set of primaries. A color can be outside this gamut if either (1) at least one of the primaries is used with a negative coefficient, which is not the case we are considering here, or (2) it needs more than the unit stimulus of at least one primary. For the sake of this experiment the unit stimulus of all primaries can be scaled by a constant factor and the state of affairs will remain the same as we are more interested in the overlap of Adobe RGB and ProPhoto RGB. It must be noted that unit stimulus magnitude is not the same among primaries, in general. With this understanding we shall deem a color to be clipped if it requires more than a unit stimulus magnitude primary. I think this is referred to as saturation clipping.

In the image below I plotted the normalized unit stimulus for each of the six primaries, 3 of Adobe RGB (D65 white point) and 3 of ProPhoto RGB (D50 white point). The magnitude of unit Adobe blue primary stimulus turns out to be longer than ProPhoto unit blue stimulus. It appears that when the third dimension is taken into account it is seen that the area near the saturated unit stimulus blue region of Adobe RGB requires more than unity stimulus from blue ProPhoto RGB primary, i.e., there might be colors that clip for unit stimulus blue primary of ProPhoto RGB but are within the unit stimulus of Adobe RGB.

If the direction of primaries are kept the same (x,y chromacity coordinates are the same), but the white points are changed, which means that the magnitude of unit stimulus changes, then the following situation emerges:

Fraction of unit stimulus blue ProPhoto RGB primary needed to match unit stimulus blue Adobe RGB primary:

(1) Adobe RGB white point=D65, PropPhoto RGB white point=D50, Fraction needed=1.2

(2) Adobe RGB white point=D65, PropPhoto RGB white point=D65, Fraction needed=0.91

(3) Adobe RGB white point=D50, PropPhoto RGB white point=D50, Fraction needed=0.88

(4) Adobe RGB white point=D50, PropPhoto RGB white point=D65, Fraction needed=0.67

I have drawn case (1) in the image shown above. So, except (1) other modes offer a scenario where no clipping needs to happen. I.e., for (2),(3), and (4) the gamut of Adobe RGB seems to be contained within ProPhoto RGB, but for (1) the gamut of Adobe RGB around saturated blues gets clipped in ProPhoto RGB. Incidently, (1) is the standard mode of specification of white points for both Adobe and ProPhoto RGB. So, is standard specification of ProPhoto RGB saturation clipped near Adobe RGB blue primary?

Comments?

Joofa
--
Dj Joofa

ForumParentFirstPrevious
Flat view
Post (hide subjects) Posted by
cpw
cpw
ForumParentFirstPrevious
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow