Mtg 10/26: Thu-06-Feb-2025

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Okay, I
so I said assignment one would be six marks
And then two and three would be
12 marks each.
This. I was wondering, well, let's not make any more of it. I
looks like the network is connected. There we go.
I we go. Oh. Now it's
undecided.
Look like You took the click I
so they would look at the Zoom transcript. No, it just came as
a lot of text.
So I have my phone recording here, so if the other ones are
back To the standard
the regular appearance of the transcript.
So Well, I guess I'll switch to my hot spot.
I don't need to do anything there. I
man, even the cell phone, Even
the cellular network is Slow. You
switch to i
I'm going to run it for my laptop,
See if That helps. If
Okay, so assignments appears on the web page, and if you're ever
able to access the web page, you'll see it here. So I'm going
to put some more details here, but I'm going to fill in the
rubric. So I have the file on Dropbox. It's also on the I
should put the link there the version four file format page.
So I want you to take that change the Image Size to 1280,
by Oh, type, Oh. 720, I,
So change the image size of 1280, by 720, and add and place
two more spheres in the scene, each with a different material.
So we run it with the specified integrator, and also use the
wavefront integrator and use different values of samples per
pixel.
Okay, so that will and then I want you to fully document your
pbrt file with comments in line that describe all the statements
used along with all parameters specified and default values for
those parameters not specified.
So lot of writing and lot of comments into the PBR file, pbrt
file.
So I propose that three marks for describing, documenting the
input file that's your you. Using, and then to run and
analyze the results
at a three marks, and that'll be due on February 25 so it's the
Tuesday back from the break You said
to me chairs on that Thursday.
Yes. Thank
Okay, do
Is it blue and black?
Blue and black, or one color looks the same as another under
certain conditions, under different completely different
dress? I'm somewhat familiar with that example.
How To summarize, I'm two distinct pairings of like
spectra that they're they're different, but they can look the
same depending on like when you convert them to like spectra,
like spectra to like color and stuff like that, they look
identical. It's kind of what I interpreted as pairs of spectra
was substantially different distributions that have very
similar X, Y and Z values, and thus the human observer appear
to be the same. Yeah,
so it's a white or gold or gold block, is the real question?
So I have to do more research.
I'm not sure that
the dress example
might be, it might be off his metamer. The idea is, so if we
look at the Wikipedia page for metamer, they say, Here's a
monochromatic light,
and then here's
an RGB. I
instead of me drawing
up the picture, I
so on the right in column one,
We have a yellow light,
and then we can also express the yellow light as a combination of
red, green and blue in column two. So the the images one at
the top of columns one and two are perceived to be the same,
but the luminance is has a very different spectral power
distribution.
So going down,
we have
so S, M and L and
anyone know what SM and l
are when you Don't cook your chicken enough you get sick?
Yes, I not
sound enough, isn't
it spectral? I
learning management
doesn't the stand for spectrum, no camera, stand for material,
no light. Getting cold. Short, reading cold. Short, medium
long, yeah, but
That's right. Should I Get
Good so
you Just
take a Picture. CS manager,
still. So if you use it,
so the
our eyes can perceive short, medium and long
wavelength. And
so there's a correspondence with red, green and blue. What
else did I ask? Do
what else do I
ask? Was A characteristic of A, C, E, s2, 064,
so that's a color space.
So it seems to extend past the visual light spectrum we can't
see like they're what was it
theoretical colors or imaginary uses primaries that correspond
To imaginary colors? Yeah, I
it. So the idea is that it allows All possible colors to be
represented. So I
so it's a benefit for archival
purposes that
we can
we're not limited to sRGB, for example, which is a standard for
the web. Does
that just mean it avoids data loss? If you just stored in RGB,
you technically are losing the data of those imaginary colors.
So is that why we use it for archival purposes? Do
to preserve it historically, in case the values of our GPS
change over time.
Yeah, I
I'm open to
doing your own research on this as well, but I feel, my feeling
is that, because we're not, I
the gamut
made available with picking primaries that are outside
the range of
visible Colors, so that we capture the whole horseshoe of
visible colors.
That gives us
more flexibility in
terms of
keeping a reference to
and as we Get
as technology improves, we have wider Color gamuts and
are you Looking for your last question? I I
just want to get back to my teacher
today. So what's the relationship between x y z
coordinates and visible colors?
Not all values of x y z coordinates correspond to the
sizeable spectrum,
realizable spectrum, yeah. So we can have values of x, y and z
that don't correspond to realizable colors. But on the
other hand, all the realizable colors I have coordinates in x,
y and z. So that was a true and false question. Not true and
false, True or False. I
so pbrt is dealing with spectral rendering, So we have to
be able to do
so we have to deal with
spectral power distributions and colors and
So this is physics, physical.
This perception, perceptual.
There are two issues. I
So we want to I
want to specify the spectrum, maybe By piece wise. But then We
also need to, I
so we need to work with RGB in some cases, And then we also
this is part of the other
so the idea is, first of all, we have to do input, whether we
specify
spectra to
begin with, that's input to the renderer, or whether we specify
things in terms of RGB, then we need to convert the spectra to
do the processing. And then we also need to think about for
output, converting the spectra to RGB
for storage you
it. So this is this diagram. Is the horseshoe that I started
with or ended with last day. So we could see the wavelengths of
visible light around the edge,
so blue is 380
and red is at 700 and
so the chromaticity is independent
of illumination and
so we can talk about i
So, X, Y, Y and
has anyone heard of y, u, v as a color space?
Does it sound for ultraviolet? By any chance? No,
u and v are the coordinates. Perhaps. Parameterization.
This is what it was called when I was familiar, when I was
learning these things that For the first time, I
it so now, more generally, We
talk about why CR, CB,
so illuminance, of
course. Corresponds roughly to the green. And then we can
specify Chroma differences in
red and blue and
So has anyone watched a black and white TV lately?
No, there are some movies that were made in black and white
that I kept watching color.
Yes, except for Ted Turner's wonderful the
show called one division that was in black and white for the
first half of it. Yeah,
it's a wonderful Street.
Yeah? Chris, the
old black and white don't convert well to color in today's
world.
Yeah. Well, this a little bit different.
Let's see. I
i blame Ted Turner for this. Is that justified?
So the original black and white with Alistair sim is that called
A Christmas Carol, and it gets called Scrooge, I can verify
that changed the name. So,
Miracle on 34th Street. It's a Wonderful Life. It's very
knowledgeable.
Yeah, so I'm
the point I was getting or losing towards was,
well, okay, talking about one division and different. It's a
Wonderful Life and so on.
Unless we
so the source material is black and white, Unless we mess with
it. I
so TV signals. I
So the black And white y'all,
so
it's like, where should we have the black and white inside the
color transmission? So we're just using part of the data to
create the black and white images. I or the black and white
TV. If it doesn't have Chroma, it's just using the
luminance to display the image.
Does that make sense?
Yeah. Did the original like cameras that were used to film
these things, were they able to perceive the red and the blue?
But we just didn't have a way of like capturing them, like, like
putting them on screen. Like, for instance, if we went to an
old black and white thing and we had the original tape, could we
then convert that to IE, to use Chroma, for instance. That's
part of what they did with the colorizing. Is trying to do
that.
I mean, a lot of colorizing just algorithms to just
read like do it. But
if you had the original tape, would you be able to just have
it? I don't believe that the systems were able to record that
data
at the time. That's the question.
Yeah. So there was no tape, it was film,
and that was stuck in black and white. Yeah. I think the
machines that it was recorded on, I don't believe that they
had capabilities
for life. Yeah, I will look into the history.
There was color film at the cameras. Like, if you put
colored film in those cameras, they would still, like, accept
it. It's just that the film itself, it was way too expensive
to actually do any kind of color, so black and white, just
black and white was very Cheap. I
If anyone's Interested in Very cones.
I So Talking about gametes. This is the range of colors that we
can represent. So we have the blue, green and red primaries.
So based on the particular quantities of those primaries
that defines the range of colors that we can represent the
So here are some
other gamuts. So SRGB, that's the web Standard, and D 65 and
So the White Point The
so it's a perceptual quantity, but
so the white point is when, if we're Talking about red, green
and blue,
that we have
full full red, Full green, full blue.
So here's the Plot of the spectral distribution for D 65
So the nanometers are along the bottom.
Pardon, me, I said daylight, but noontime daylight at European
latitudes. I
so F is F series illuminance, and that corresponds to
fluorescent lights. And it.
So earlier in the text had mentioned
about a lemon skin,
not just a skin, but a lemon with its skin on, and the
behavior of that under lighting conditions. And so this is the
spectra for reflection from a lip from a lemon
skin.
Seems funny to say, lemon skin, right?
That feels like you're talking about the inside of the skin,
Anyway,
so we can have different kinds of spectra and
we can have a piecewise Linear spectrum, defining a
distribution Using a set of values for
And then we can sample and
so the tri stimulus theory of color,
I'll do a dramatic reading here. Of this paragraph
says that all visible spectral distributions can be accurately
represented for human observers using three scalar values. Its
basis is that there are three types of photoreceptive cone
cells in the eye, each sensitive to different wavelengths of
light. This theory, which has been tested in numerous
experiments since its introduction in the 1800s has
led the development of spectral matching functions, which are
functions of wavelength that can be used to compute a tri
stimulus representation of a spectral distribution I
so these are the color matching curves for X, Y and Z, a given
spectral distribution can be converted to x, y and z by
multiplying each of the three matching by multiplying it by
each of the three matching curves, and integrating the
result To compute the values of x, y and z so,
here's a representation that I hadn't seen before. Plot of x,
y, z, color coefficients for the wavelengths of light in the
visible range. Curve is shaded with the RGB color associated
with each wavelength, so,
blue here, red
here, Green and
how did I do? I
so when we're studying, we don't need to worry about, like their
process of how they're doing it in the program. We just need to
worry about the confidence, like when
we're reading the textbook and stuff. I
formulas or anything like that,
in the textbook, yeah, I would say I
the details of the mathematics,
really going to focus on that, because we're not focusing on
that in our discussions
it's about, How do we get
how are we
able to
experiment or access these physical simulations in pbrt,
so
the details of the integrals and so forth
won't be our focus.
Maybe, if you think about the figures in
the text.
That might be a good way to think about
does that seem helpful?
Thanks that formula is pretty, pretty easy and useful. It's
really just a percentage like and then the last one, even even
show at z is equal to one minus x minus y, which is just saying
to all the one in reality, dividing one wavelength by the
like, the total of the
other three, the three of them combined,
just what percentage of your lights, x, what percentage of y,
what percentage Z? Well,
then just saying, the kids in a lot of the choppers and stuff,
they go into details, show the lines of code and from different
functions and stuff. Do we need to going into that much detail
about that stuff, or is it more mainly about the concepts that
they're
they're dealing with?
If we want to change from a gold sphere to lose fear. What do we
need to do with the
code? So it's about using pbrt So
the implementation details might be helpful to look At,
or maybe not. I
I don't
find these controls particularly helpful because
it's jumping around.
But anyway, yeah, I
the kinds of questions maybe
that may come up would be and
you can give your feedback about this. Why do we use a certain i
Why do we have Three different parameterizations of spherical
coordinates used in pbrt? Do
just to confirm we're only going up to Chapter Five for the New
Jersey, chapter six,
yes,
Sorry. I'm throwing all these random questions at you. I
I'm sorry. Don't have to be sorry. These are good questions.
I
CS. So here's the overlap of the gametes for the different color
spaces that pbrt supports. The smallest one is the sRGB
and DCI, p3,
rec, 2020,
that's for
Ultra High Definition, Standard i
So here's The color gamut and
for the rec 2020, color space, and
then we See the 65 there. I
so for next day,
look at 5.1
and 5.2 and
then we'll cover three or 5.3 and 5.4 next Thursday.
Okay. I
on Chapter Six will be after the break.
Yes, maybe we can
Yeah, well after a break, I think,
thanks for today. Have a good weekend. Stay warm. See you on
Tuesday.

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