Why does the chromaticity diagram look like that?

That link is so slow where I live that I had difficulty getting the site to work, but as far as I could judge it gives a rather nice and understandable explanation of what is a rather complex matter. It's in considerable contrast to those sections of my textbooks on color theory, they're so dry as to make one yawn, they're full of algebra, the complex operator and matrices with precious little other explanation of what it all means.

Some of the comments have already covered most of what I'd have mentioned so I won't dwell on them now, although I'd add that I reckon GrantMoyer is on the mark with his point about the inappropriateness of displaying chromaticity on Cartesian coordinates.

It's worth noting that understanding the intricacies of chromaticity and color theory is difficult to the extent that its 'opaqueness' has been used to protect trade secrets (and likely still is for reasons I'll mention in a moment).

Commercial lab printers that print masked color negative (neg film with the orange mask) to positives—color photos and color film print stock—go to great lengths to protect their matrices (precision resistor banks) against copying. Similarly, companies like Kodak do not publish the 'film terms' for their various emulsions ('film terms' being the unique matrix information for each film emulsion).

The reason for this that to reverse-engineer the matrix with enough accuracy for a single film is a complex job let alone do so for a multitude of different films. Moreover, it's imperative the matrix be accurate if good color balance is to be achieved. Keeping this info secret provided a competitive edge, selling or licensing the info is worth money.

I'd add that the destructive orange mask used in color negative film is a brilliant concept for reasons I cannot cover here, however what's relevant here is that the mask makes reverse-engineering the negative's film terms that much more complicated.

I'm a bit out of touch these days but no doubt the same applies with inkjet printers and the like (matching coordinates to specific inks etc). So there's a modicum of truth to statements from Canon, Epson and HP when they say not to use third-party inks because the colors won't match properly (mind you, that's never stopped me at the exorbitant and outrageous prices they charge for inks).

My point is that if it were possible to unravel and make this chromaticity stuff simpler to understand then many of these expensive commercial decisions would disappear.

Ahh but alas, we're suck with it.

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BTW, for those who've used scanner software like SilverFast the manufacturer provides a list of film emulsions to select from before the film is scanned. Selecting the correct emulsion type ensures the proper 'film terms' are used for the scan, this in turn ensures the color balance is optimal.

I'm a bit cynical about SilverFast's approach to the problem, they've a limited range of film emulsions to select from (many of the old and important color negative types are missing). SilverFast's literature suggests that if one's color negative type is not listed then to select one that best suits. I am at a loss how one does that except to just make a guesstimate, so much for calibration. Also, one has to wonder why SilverFast has such a limited range given they've been in the business since many of said emulsions were still in production.

There are similar issues with Hamrick's VueScan software but I've not time to address them here.

Again, all these issues further illustrate the practical complexities surroundibg the chromaticity diagram.