Mtg 25/26: Tue-08-Apr-2025

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Okay,
let's get started Here. I
so
from our quiz, what are two benefits of area lakes that I
wasn't really sure I put like
softer shadows and more wings,
deep lighting. Yes, that's what I was thinking of. Hello. I was
wondering if you're gonna join your backpack or not. I was
eventually,
the washer was closed, were closed. Had to go further. I
The squeaking on the whiteboard is new. I don't know if I like
it or not. Some some feedback that I'm doing stuff, I guess,
but
it's Not as bad as nails on
a chalkboard. Definitely. I
Is that supposed to be effects, more realistic lighting effects
looks
like an old from here
looks like old facts.
Thank you. You're welcome.
I'm not creating new coining new words in class here.
Okay? I just, I
so a comment he makes, or they make in chapter 12 here is that
the range of lights is limited by the Physically Based
Rendering paradigm they can't have lights that do non physical
things,
not physically accurate things. I
so that that could have, that has has been an effective mode
of operation in the past, but in the grand scheme of things, it's
better to use a physically based approach, because then Things
work together better and
so there are four different kinds of lights, four different
categories of lights,
point light sources,
directional light sources, area light sources and infinite
lights, so they don't have geometry associated with them,
But provide radiance to rays that escape the scene and
so it's a
when We have legs in The scene We Need I
so if we don't have lights, we're not going to be able to
see much of the scene.
If we have a case like this,
where there is an area late
so efficiencies can be gained. I
so if we focus samples, so
in that cone
that connects the point on the surface
to the sphere.
So those are the directions where
we'll find illumination.
Does that make sense? So if we just I,
yeah, if we just I to focus our samples and the directions from
which the light is coming, then we'll get a much quicker, more
Efficient rendering and less error In the image, less noise I
so here's a dragon with a point light source,
or lit by a point light source. What do you think about this one
hard shadowing?
Yeah, very hard shadows. The light
must be very close Because the shadows are very short. I
let's see what clues we have about where the light is. I
Okay, so this seems to be a shadow from here.
This is a
this is the shadow from here. And
I'd say if we were the dragon, it would be just over, like on
our left side, like right here. If we're the dragon,
let's go it's close.
If you're that close over here, I think this shadow would be
longer,
so maybe more to the
side, almost to the
side, yeah, maybe thinking like right here, because,
I guess, hard to say without duplicating it ourselves. Yeah,
I
so does it look impressive? This is a cool picture.
Depends on what you're trying to accomplish.
If I want to make a cool picture. Have I succeeded? I it.
Maybe this is a cool picture in the 90s. I
What do you think, cool or
not? Binous. Do you have a thought about this? No,
okay, I Okay. Astra full, what do you think about this image?
It's
different. I
so it's just
an expression of ambivalence. I
Okay, 12.4 I'm
so what do you notice about this? Do
so it seems like it's
it's not the same place as the first point light source is the
spotlight, definitely slightly different spot.
Yeah,
so it may or may not be the same position, But the difference is,
because It's a spotlight, so I
so we're specifying a cone.
So instead of having
the light emitted in all directions from that point,
we specify a cone, and that gives the
is area on the outside of the comb, like around the edges of
the comb are softer, where the shadow that's being reflected
from the dragon is still hard. Does
that make sense?
Yes, so there are two angles here.
So the total
the total width,
it's the whole angle. And then there's the theta i
Well, theta is in the i
Oh, so the two angles are fall off start and total width. So
anything within fall off, up to the angle of fall off start is
fully illuminated, and then
up to the total width, it drops off.
So we see the dragon in the in
the center here is fully illuminated. Then as the angle
drops, as the angle goes to the total width, it falls off to
zero.
Is this one a little more cool than the other? One
more interesting
to draw your attention, more stuff to analyze and look at on
it.
Okay, so here's steam runner with projection light using the
grid image.
So we have an image.
It's like a slide projector.
Anyone familiar with slide projectors?
I Okay, okay.
Using a projector image like that, you could put, like a
colored image in color
the light coming out.
Yes,
it's like if we had that exact same grid image. But the grid
image wasn't white background. It had like a orange. It would
project an orange hue over the Yeah,
so I, I wonder if there are, if
you just wanted one color, you could specify the spectra of the
light. But if you want to vary things, maybe that would be a
good way to do it,
to project the texture.
Go neophotometric diagram, legs and
do you remember
our Gonio photometer setup? We're measuring
the BRDF
for different materials by varying the light source and the
viewing position. So this is a similar idea. And
so the emitted intensity is defined in a fixed set of
directions on the unit sphere, and the intensity is found by
interpolating intensities of adjacent samples and
so the first one
mostly
eliminates in its up direction, only A small amount of
illumination in the down direction
so that's this is up and this is down.
Light B is a light that most illuminates in the down
direction. And
here's an example where they Have both up and down
contributions. And
I wonder if we can compare 12 point 12 with 12.3 we did You?
Did image diff. I
Let's try it out. I
Wow, that's a lot. I
No, it's not a banana.
So there's a lot of subtlety between those two Images. Ah,
more. Shadow Play around the outside.
Yeah, the heart shadow underneath looks like it's
staying in the same spot. I
off so
this shell, Shadow
of the dragon, it seems like light is from
behind the dragon, and
this part
is very subtle. It's hard to detect in the original image. I
I see we're sure.
Really look, you can see it's there, the 12, three.
It's hard to pick out.
It's there. It's actually, if you look, it's got like a little
bit of a red hue where that is actually
coming through. So it's like it's reflecting off the dragon
and going Down on the ground.
It's It's
a
bit there. I
So we're Interested in the emitted
power of distant light, and an approximation of that is to Find
the disk
that can be inscribed in the sphere that bounds the scene
elements and oriented towards the direction of the distant
light
that gives enough precision for what they need to do in you.
It's not a very it's not an accurate estimate of the emitted
power, but
I won't underestimate it.
So when we get
to area lengths, we have a geometry associated with the
emitter. And this is comparison of a small disk and a large
disk. Do
12 point 15. Is this becoming more cool or not?
Yeah, I think that's I like Being
I like be overly, yeah, I
so in this case, we're Taking area lights and putting them at
infinity.
So environment lighting, so taking an image as
specifying the lighting for the scene in an
environment map.
So the first one is illuminated by a few area lights, and the
second one is illuminated with mid day sky
light from an environment map. And C is using a sunset
environment map. So this is area lakes. This is mid day.
This is sunset. And I
prefer infinite area lengths because I find it illuminates
the objects from all sides better and is more realistic.
So here's the mid day environment map.
Here's The Sunset And
so there's lots of
environment maps available On the web And
so this is environment
map illuminating the dragon Model,
morning skylight. Environment Map through two samples per
pixel. A is rendered with uniform sampling distribution. B
rendered with distributed samples distributed according to
the environment map image pixels and C, rendered using the
compensated distribution that skips sampling unimportant parts
of the image, all images took essentially the same amount of
time to render, though B has over 38,000 times lower MSE than
A and C further improves MSE by a factor of 1.52
what's MSE mean
squared error? Yeah,
that's quite an improvement. What
that's impressive.
Very Clean that last one I
so in this case, the portal is the window. So we're trying to
focus our samples. Our goal is to focus the samples out the
window and
see that looks a bit better.
Here's the idea Of the Four Pole and
any thoughts about that?
So I thought I would go through some of the questions that were
suggested for the final Going to
not show you. I
names of people i
What is the Nyquist rate? The
and what's the answer? I
two times, yes, remember it
was the two times something. I
so choices are ambidextrous, left handed, right handed. Nd,
indeed, there is no default coordinate system. I love
You. Kevin, yeah, I
What is that word of the minimal coordinate system is, is it
supposed to be used
in pbrt? Yes?
What?
One what needs to be accounted for to prevent textures from
being too blurry. You choices are camera integrators, angles,
bounding boxes or lights you
which one is not a member of the exposure triangle. Aperture ISO,
shutter speed
or angle and
what is the primary purpose of a bi directional scattering
distribution function, to define the geometry of a surface, for
array intersections, to model how light is transported through
participating media, to describe how light is reflected and
transmitted at a surface to simulate the spectral response
to the camera sensors, yeah,
describe how light is reflected and transmitted at A Surface i
What does the rendering equation primarily describe how light
travels through a vacuum, how surfaces reflect Light, how
textures are mapped, how pixels are colored.
So the suggested answer is B, how services reflect plane from
I'm wondering if that's
the Answer is more nuanced than that. I will
think about that a bit more.
Which of the following statements
about spectral rendering is true, but PBR TV four performs
rendering computation computations using only RGB
color values. Spectral rendering in PBR TV four involves
converting all spectral data to gray scale before processing PB.
PBR TV four performs rendering computation using Point Sample
spectra, limiting the use of RGB color to scene description
inputs and final image output. PBR TV four system does not
support spectral rendering. I
so use Point Sample spectra,
so we can specify things in RGB, but internally.
So that was the third answer. That was the third one, yeah. So
I was thinking it was, it
wasn't 100% Sure.
I What Is blends, Law I
as technology advances, rendering time remains constant
and
what are the three Main physical processes that infect The
distribution of radiance? What
I explain the importance of Monte Carlo integration In the
rendering algorithms described in the book.
Why is it commonly used in realistic image synthesis? I
A so Monte Carlo methods provide a way, a straightforward way, to
get a numerical solution to
integrals. That would be difficult otherwise I'm
you're still in class. We're still in class every
what is a texel And how is it different From a pixel? I
Yeah, I
so I will post
the questions after a bit of editing, I
any questions or concerns. So we'll do a review on Thursday,
and then the final will be
the following Thursday on my birthday, yes,
and which time I will be 60 years old. So
cupcake. I didn't think about that. You finish the
exam, you get a cupcake. We're supposed to bring them.
Actually, we're supposed to bring them. That's
right, yeah, cupcakes aren't we're thinking about doing
cupcakes for the 50th
celebration, but they were like, three or $4 a cupcake, so it was
more cost effective to get a slab cake. Anyway. Gathering
on the green in the summer.
Yes,
thank you for today.
See you on Thursday. Take care.

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