Thanks Thomas !
I think it has been brought on Slack that a proper definition of the Notorious 6 would be interesting for this group. So I have tried to come up with a definition and some examples.
Let’s see if we can start with the definition given by Troy and elaborate from there :
The “six” are the inherent skew poles all digital RGB encoding systems skew to; pure red, green, blue, and their complementaries of red + blue magenta, green + blue cyan, and red + green yellow. As seen with camera clipping at the high end to the compliments, and to the primaries on the low end. So as emission increases, all mixtures skew to the gamut volume of the device, but towards compliments of whatever the working / device range primaries are. As emissions decrease and exceed the floor, they skew toward pure primaries.
The best example I have seen so far (not only on this forum but also in the whole wide world) is this video by Jed Smith.
Some may call it mind-blowing, some others may not… But I personally think it is simply the best way to show the Notorious 6.
You start with a whole range of colors/mixtures :
And you end up on the path to white with only 6 of them (aka The Notorious 6) :
You can clearly see 6 spikes on the path to white in the image above : red, green, blue, cyan, magenta, yellow. If I understood correctly, any curve that asymptotes at 1 will have this unfortunate behavior.
The following plot is also quite useful to visualize them (ACEScg sweep of values on their path to white through the P3D65 Output Transform) :
On the path to white/achromatic axis, we can clearly see a trend towards 6 mixtures : three primaries and three complementaries. This is why by only doing these sweeps we can really appreciate the issue.
A render that shows clearly one of the notorious 6 (magenta) is the following one :
We start with a sweep from an ACEScg blue primary to a magenta complementary. And on the path to white, we end up with one mixture : magenta ! (Well, actually two, but you get my point.)
What this render should look like in my opinion ? Possibly this :
On their path to white, hues are respected/preserved rather than going towards a single colour/mixture. This is actually a big deal because per-channel actually prevents us from reaching certain chromaticities at certain levels of exposure and forces us into one of the notorious 6.
Which is put much more nicely by Troy this way :
As in as we move toward the vast majority of unique mixtures, per channel makes it virtually more and more impossible to hit them.
I also think it would be important to add (from Troy again) :
Anyways, primaries and compliments are the worst demo. Because the skews are for all other mixtures. As in the least distortion happens along those axes. The most heavy distortions come from all other mixtures.
Just for fun, I have plot the same sweeps with sRGB primaries under different DRTs :
Nuke_default : sRGB eotf
spi-anim OCIO config : Film (sRGB)
ACES 1.1 OCIO config : Rec.709 (ACES)
TCAM v2 OCIO config : Rec.1886: 2.4 Gamma - Rec.709
I hope these several examples and definition clarify a bit what are the Notorious 6. And if we want to dive a bit deeper in the topic, two of them already have nicknames :
- Cyan hell ® (typical of overexposed skies for instance)
- Rat piss yellow ® (typical of lamp shades shot at night)
In summary, the notorious 6 are the values hit at display on their path to white by any system using per-channel (or any curve that asymptotes at 100% display) and are a direct consequence of the hue shifts.
I am barely an image maker trying to point out stuff/issues that could/should be improved. Sharing and learning is at the core of this community. So if anyone could reply to this thread explaining what is a Jones diagram, how do you read it and why it is important to this OT VWG, that’d de much appreciated. @Alexander_Forsythe maybe, since you were the one who brought up this on Slack.