Mtg 17/26: Tue-11-Mar-2025

Outline for Today

Midterm review

Administration

Today

Going over the midterm exam

For Next Meeting

Read Chapter 8.1
Take quiz before the start of our next meeting

Wiki

Link to the UR Courses wiki page for this meeting

Media

Transcript

Audio Transcript

So Did everyone see The grade scope? Email? I
this is what it looks like. So it'll have a link to view your
graded work, and it'll Be from no reply@gradescope.com
so what Is sRGB?
Stand it up in I
Five in the 1990s I
so it was defined in the 90s
so that
we'd Have a standard representation of colors on the
web and
this is a color space.
So what about a gannet? I
What is? What do we mean? We're talking about a color gamut for
so gamut is just means, like a range. So when we see out of
gamut, it just means something's outside the range. So in our
case, the color space, so if it's not represented in the
color space, it would be out of gamut. Yeah, I
so how do we define the gamut for an RGB color space
would that just be considered the zero to one or the zero to
255, depending on like, which version you're doing or and then
anything above those numbers would be considered, I Guess,
out of gamut, or they get truncated normally.
So the zero to 255
talk about
zero to full
color? Yeah, I
so typically, we're defining a gamut,
so we have the tri stimulus theory of colors, so we can
represent colors based on red, green and Blue, and the gamut is
defined by the particular chromaticity of those three
colors.
So we can make
all the point. All the colors inside the triangle can be
represented by the sRGB color space.
So how does the gamut for sRGB compare to other gamuts we
looked at and that are available now,
bigger or smaller?
Bigger?
No, I remember the diagram. I just don't remember which was
labeled what? There's just one that was bigger than all the
rest, though. Yeah.
SR should be as the smallest gamut in That picture you
Does That makes sense. You.
No. Sorry about That. I
so here's the section of the book
from which this question came.
So in pbrt, we assume that
the geometric, geometric optics is adequate model for late
scattering. So we have linear, linearity, okay. Linear
linearity, combined effect of two inputs an optical system is
always equal to the sum of the effects of each input
individually. Energy Conservation, when light
scatters from a surface or participating media, the
scattering event can never produce more energy than it
started With no polarization,
no fluorescence or phosphorescence and steady
state. So the environment is assumed to have reached
equilibrium, so the distribution of radiance is not changing over
time. So realistic situations, as happens very, very quickly.
So we lose a geometric up with a geometric optics model,
diffraction and interference effects and
does that make sense?
And just to confirm the thing, steady state, that's like, if
you were taking, let's say, a picture of the stars at night,
and then all of a sudden, a street line turned on beside
you. It's assuming nothing like that street mount will appear.
You're capturing the light as if it was all steady, so no sudden
light turns on.
No cars are driving by, therefore causing new lights to
appear for just moments and things like that. Consistent
luminance, right? I
I would say even when you have new lights appearing,
their contribution is
reaches a steady state almost instantaneously.
So it's not
that you're avoiding people. You're disallowing people from
turning on lights or street lights or
driving by a scene,
but the way we model it is that
we use exposure settings right where there's like a long
shutter time or whatever. So in that time you're capturing light
as though it was steady or consistent. So if you've had
suddenly something turn on midway through, would that would
interfere with an actual photography so I assume it has a
thing like we're assuming that can happen. That's one of our
assumptions with the steady state.
I think the steady state, it means that we're not, we're not
dealing with a period of time when the light comes On,
and seeing the effect of the light coming on
over the period of our exposure,
that there's A difference in
the inner energy from the light. Rather,
if the light appears,
then we're just we just accept that it's
In the steady state, there's a new steady state, And
that might be Interesting to To Explore I
so what is literate programming, and Do you have examples aside
from pbrt?
How about Writing Code. I'm
I think that's a Good explanation of literary
programming. I
anyone. What do you mean by the narrative?
So describing how code is designed or written? Do
you want to create an exposition of the design of the code,
and so You're writing it a way that supports the narrative So
does anyone have examples of literary programming systems
I've done pbrt Do thanks.
Yeah, It's pronounced tech. I
What do you think about Jupiter Notebooks? Does anyone use those
before?
So just to clarify, literate programming is not like C Plus
Plus, where you have to declare the function at the very top,
and then later you can do the function, but you have to do
this here and stuff like that, like a very ordered and
structured thing. What exactly is literate programming, as far
as Like how does it differ?
So the documentation I
and Then there are Two pieces and
so in Python with These Jupiter Notebooks, you can have
documentation, and then you have code that runs. So it's not
necessarily the same, but it's an
so first of all, how many people are familiar with tech and late
tech.
Cool, so,
yes, with a computer notebook example, it's more like, it's a
coding environment that
also kind of, like allows you to work with the building blocks.
It allows you to build up various blocks until you reach
the very end. Because, like, that is, I mean, that's kind of
nice. Mean,
I think with these notebooks, you can intermingle descriptions
and and code, and so they're a little bit separate. So you
you're including the code in the document, but the document and
the code are different, so it's about having a single source
that can be parsed into these two different streams. I
so it's an interesting discussion. Okay, who's who
here. Is familiar with Emacs? Does anyone know what emacs is,
what kind of application it is?
So editor, I
so we can have a continued discussion about this. I'll put
some resources online, but it won't be on the final exam. The
other things will be on the Final Exam. I
it rasterization versus ray tracing. So we talked a little
bit about, very little bit about Z buffer and being a
rasterization algorithm. We're keeping track of the closer
pixel that's closer. We're testing against a Z buffer to
see whether the new picture pixel will be computed is closer
than the existing one. If it is, we update the pixel. If it's not
We discard it. You
writing a little bit too small there, I apologize. So
rasterization says for each object in the scene, then for
each pixel i
that is possibly associated with the object as it gets scanned
converted. Is it closer than what's stored in the set buffer?
Okay, so what does ray tracing do? I
to do the same loop, To organize the same way I
so who says it's the same loop as rasterization for each object
done for each pixel? Anyone?
My brain's not working today, but that's how I interpret it.
Okay?
So actually, we're starting from the pixel,
yeah, it comes, it comes towards that Ray, right?
The ray comes out of the pixel? Yeah, so the eye, which is the
camera, we trace a ray through the Pixel and into the scene.
So for each pixel, then
so what's as we Go through the scene, what's the closest and
then how is the value of the pixel determined? So we're
tracing rates. If it's a reflective surface that we
bounce the reflection rate, retrace rate in light source,
see if it's in shadow and so forth. We're
Does That make sense? I
so the depth of field is what, what happens
in the space? Sorry,
I'm really bad headed right now my brain can the best I can give
you is This pretty much a second of
that. So what's in focus compared to what's out of focus?
I
so when it's not in focus, the lines don't converge to a point
on The film. They convert it comes towards a desk. I
boundary of the disc.
So as to focus, the larger this disc is the film, the more out
of focus the image will be without the image will be or
that part that's out of focus,
so what are the different court assistance we talked About in
pbrt. So
the world, the object, the camera, then I also put texture
coordinates which I didn't know if it was a Click of
one, yeah. World, object, Camera,
camera, world, i
i also seen a normalized device coordinates discussed in Chapter
Five about the projections.
So if we need to move between them, we use matrices. So why,
when we're doing projections, do we have four by four matrices
discusses
the classes? Because when we're doing translation, stuff like
that, I so we
Got homogeneous coordinates and
so for registers allow rotation, scaling and translation to be
expressed in the same matrix.
That's why we
a four by four matrix for three dimensional objects is a useful
thing. Take A
Picture. I
it. Somebody asked, where this was mentioned? Does it be the
four that we talked
about? So we have a square,
and then we have
an arc, one quarter, one quadrant in a circle. So just
imagine that's that's part of a circle. So when we're doing
Monte Carlo and
and number
of the it's basically trying to find The ratio of points that
are inside the circle compared to inside the Square.
I'll put to the link to that administration and anything
clarificational to that as well. Does that make sense? I
spherical geometry and
why is it important for rendering? I
So why can we are we supposed to specifically worried about, or
concerned about spherical coordinates vertical Geometry
and ray tracing.
The so we're dealing with all in coming and outgoing directions.
Look at that in terms of these directions are expressed in
spheres or hemispheres, depending on the case.
We got to translate all of that to be a two dimensional screen
at the end, because it's got to be viewable.
Yes,
you're taking in like every single direction you could
possibly go up your day to get all that information.
But also, if you did every single direction, you would be
there for more than a day. I think,
yeah, we do it every day.
It just works in nature.
It's a simulation that's the hard part, and we're getting
more fidelity with our simulation
so
and if we have
a bright light source
that makes it easier for having dimly lit space, then it's hard
to or to Find the light in the scene.
But anyway, so I
so what are the three parameterizations for spherical
coordinates,
octahedral encoding, equal area, mapping spherical coordinates, I
so this spherical coordinates gives us an easy representation,
easy To convert between this and Cartesian coordinates.
Octahedral encoding Is space saving. I
so I wrote down E coli mapping some reasonable equilibrium
mapping is important, because if you're sampling
with spherical coordinates, you might get, we're not going to
get a uniform distribution, or we get extra stuff in the poles
and some distortion there as well.
Okay, Does that make sense? I'm
so how do we use bone and glasses to accelerate re
intersection tests? You
can use it to truncate rays that aren't hitting your object. And
rather than checking like the very fine shape of the object,
you just search a general area. If you don't hit that general
area, well, you're not going to hit the shape. And also, when
the ray leaves that general area, if it doesn't hit the
shape, well, it's not it's never going to hit
the shape unless it bounces. I analysis.
So imagine
these are two axis aligned bounding boxes.
What can I what other improvement can I do in CS
having a collection of bounding boxes at the same level, so very
shallow tree can
have multiple different like levels of it. So you have one
boundary box that maybe contains both of those ones. If you don't
hit that one, you're not gonna hit any of the ones inside of
it, either, right? And you can just keep going outward.
So hierarchical, I
realize we're in a time here. So Ken,
the chapter seven, kind of was about that when you told us
to read, yeah, that's making the the data structures for
for the hierarchy hierarchical bounding structures. So there's
a list of things that are common to Ray tracing systems,
cameras and lights and visibility and light scattering
and so forth.
I'll post A link to that. I
so question 11 about the importance of choosing an
integrator in the sampler
that's from the user guide.
That's a guide for the answer anyway, But I
tried to be open. I
so bi directional distribution functions and
the bi directional reflectance function, bi directional
transmit distribution function, bi directional surface
scattering, reflectance, distribution function. These are
the three we've talked about. So why are they bi directional? You
equation, incoming direction and not going direction. So we have
those pair.
Okay, so it seems like we're just at a time since I'm
learned about my next meeting.
So that makes sense. Okay, so I was generous with marking. So
this is an opportunity, so 15 marks for the midterm won't sink
you,
but if so, be clear about this stuff for the final.
Okay. Thank you for today. Now the winters back, stay warm and
take care, and we'll see you on Thursday. Oh, have a look at
chapter 8.1, section one of chapter eight. Okay, thanks
again.
again.

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