Welcome for computer graphics.

And we are right now speaking about ray tracing.

So I introduced ray tracing on Tuesday.

Ray tracing is just an alternative technique

for generating images.

So it's an alternative for rasterization.

And it has fundamental differences.

So the idea of ray tracing is that instead

of iterating over all the objects

and projecting these objects and rendering these objects

onto the screen, we go the other way around.

We iterate over all pixels.

And for each of these pixels, we find out

which object is visible.

And in order to do that, we need to be

able to cast a ray into the scene.

That means for every pixel in our scene,

so if this is our image plane, this is our eye.

And I told you that in this example,

we can really imagine that the image plane lives in 3D.

It's a grid of pixels in 3D just in front of my eye.

And now to find out what is visible in that pixel,

I have to cast a ray through that pixel into the scene

and find out which object this ray intersects.

So here's one that gets a hit point.

And while this is just another way to generate images,

if we only apply that for eye rays in general,

we will be slower than with rasterization.

But the advantage now is that once we

have this interface casting rays,

we can also use that, for instance,

to compute reflections, refractions, and stuff

like that, and also can easily generate shadows using

this ray interface.

So this is something that is not that simple to achieve

using rasterization.

So that's a big advantage of ray tracing.

Nevertheless, ray tracing is typically much slower

than rasterization.

And in particular, it's very, very slow

if this ray casting operation here is implemented or not

implemented very well, or if no optimizations are

made for that part.

And this is what we want to speak about now in this chapter.

How can we do this efficiently?

How can we perform this ray casting operator efficiently

on very complex scenes with millions of triangles

and stuff like that?

And in fact, there are two things

that we have to consider.