[Bf-committers] Colour Managment

Troy Sobotka troy.sobotka at gmail.com
Sun Apr 20 19:03:43 CEST 2014

On Apr 20, 2014 8:28 AM, "Tom Wilshaw" <tom.wilshaw at btinternet.com> wrote:

> I see what you mean now about the colour wheel. Is there a projected time
for fixing this issue?


> "The issue is that with scene referred data it is deadly tricky to
> the out of gamut values. "
> Could you elaborate on this? Is it because sRGB or REC.709 assumes things
are normalised?

If you down gamut a scene referred (0..infinity) image to sRGB / 709
primaries from a wider gamut, it isn't terribly easy to spot the out of
gamut channels on the positive side without checking the transformed RGB
values against max() and 0.

My suggestion of normalizing the values is nothing more than an easier way
to spot them; anything less than zero or greater than one.

> "So is XYZ. If you look at ACES primaries sources dumped raw to sRGB,
it looks equally wonk."
> It does, but code values are still rather more intuitive in ACES by
virtue of it being RGB based. For example, a mid grey card, an inportant
reference in visual effects, should read 0.18, 0.18, 0.18 in ACES. In XYZ,
it reads 0.1715, 0.1800, 0.1816.

0.18 is a convention of course, and any RGB set could theoretically align
in XYZ as you described (Warning: Guess math alert!) by normalizing your
RGB white point to xy 0.333333 0.333333 via a Bradford.

In a scene referred image of course, many grey cards may land at random
value levels depending on whatever convention you implement. Value 103.1 is
equally valid.

Again, a single exposure data value of 0.18 for a grey card is a pure

> A move to XYZ would fulfill our own needs well enough for working with
the cameras that we do, however it doesn't seem like the best long term
decision for Blender itself. ACES is an open standard which is set to
become increasingly common. Blender might do better to be ahead of this
trend, rather than effect a move to XYZ only to have to move to ACES at a
later date in order to satisfy users in these fields.

The issue is that ACES _currently_ does not easily fulfill the broad range
of needs.

Deviating from the specification would effectively negate the upside of the

All we can do is wait until either ACES evolves further. Until that time,
in my estimation, it is a sub-optimal decision.

XYZ on the other hand, is going nowhere and is a universal standard as
well, so _if_ there is a debate about internal representations, it is not
entirely without merit. It is perhaps the first de facto open standard in

> Do you think it would be worth trying to gather the opinions of other
users who use Blender for visual effects or postproduction (or even
animation)? We might start a Blender Artists thread and see what a few
other people think? Many artists don't follow the mailing list.

My experience is that the color theory discussions on BA have resulted in
underwhelming participation.

Color still confuses the heck out of folks, and can be an intimidating

It certainly is for me.

With respect,

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