On 2021-06-25 8:54 PM (-4), Bald Eagle wrote:
> 
> https://www.youtube.com/watch?v=uYbdx4I7STg

I noticed these problems early on when trying to render sea water.  For
reflective surfaces, RGB is usually adequate, but with absorbing media,
a primary color bias quickly becomes apparent.  It would be quite a
coincidence for deep ocean water to be *exactly* the same color as the
blue phosphor, and experiments with other colored transparent materials
confirmed my suspicions.

Enter spectral rendering.  Ive created a spectral rendering rig using
pure SDL, and clipka did an experimental patch for native POV-Ray
spectral rendering, although AFAIK he did not release that patch.

But I also started on a spectral materials module a few years ago that I
put off after running into some snags.  My idea was to create spectral
curves that resemble those of real-world materials, so as to reduce the
problems of metamerism that turn up under the 3-color model--or at least
make the quirks shown in this video reproducible with spectral
rendering.  I didn't see that clipka's patch would give the user this
level of control over spectral curves.

One problem the video did not touch on is out-of-gamut colors.  This is
especially a problem with greens and bluish greens.  We just don't
notice this much because natural greens in the real world tend to be on
the yellowish side and are far from color-saturated.  But if you look at
RGB renditions of, say, swatches of Pantone coated colors, you'll notice
that the bright greens look washed out compared to other hues.  And if
you look at their sRGB specs, you'll see that the red channel is zero!
Those pale looking greens are as bright as sRGB will allow!

While spectral rendering would not be able to display such greens in a
color system that cannot accommodate them, Ive's Yellow Magic thread and
my own experiments with my unfinished module show that spectral
rendering can actually give a brighter impression of those greens--and
of other colors as well--when rendered in context.  Compare the caustics
and dispersion of gemcuts.jpg (RGB render) with gemcuts-sr-q2.jpg
(rendered with Ive's rig).

> https://www.youtube.com/watch?v=wh4aWZRtTwU

I learned this as a child.  I used to paint a lot, and had always been
confused over where brown fit into the red-yellow-blue or CMY
subtractive system.  The mystery was resolved when I started reading
literature from Liquitex at age 13.

Essentially, brown is no different from other dark hues; we're only
confused by it because the English language happens to have a common
name for it.

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