All language subtitles for 09_Glows,the_limits_of_diffraction,and_the_airy_disk

in this lesson

we're gonna learn about the limits of light diffraction

which causes glows around bright areas

so here's where we left off

when we have our flares in place

now these ultra bright areas

in the real world would be causing some kind of glow

now let's take a look at the actual physics behind this

because it's actually rather interesting

you recall earlier

we talked about diffraction

in that when light hits a closed or a tight opening

it begins to spread out like waves in a pond

so the result of this is that light is not coming in

as a uh almost a dot

what it comes in as is an airy disc

and this is what it looks like

so you can see here we have a very bright center

and then we have these waves around it

now you might be thinking

why don't we see this at all times

and the answer is

the edge rings of the airy disc are very very dark

so in other words

the amount of energy in these discs is very low

so most of the time

they are overpowered by the surrounding light however

if the original energy in the light is strong enough

we're gonna see these discs

and that is what causes the glowing effect

and we can see this easily

if we take a look at a photograph right here

so we can see the edge of this is very sharp

but wherever we have an incredible amount of light

the airy disc is actually causing the light to spread

and cover everything around it

and this is what causes

the glows we see from very bright lights

now I found this image from a popular Hollywood movie

and you can clearly see

the airy disc right here on this uh light

and it's actually quite amazing

you can actually see the various rings around this

so how are we going to create this inside of nuke

well there's quite a few different ways

the easiest way and the fastest way is just add

uh some glows

so let's uh add in a few glow notes here

and the glow node

uh is a rough approximation of the airy disc

so we can see here

that our entire image becomes much softer

and so we probably wanna increase the tolerance a

bit on this so that we're only glowing

the brightest areas of our image

so I'm gonna increase the tolerance to about

point nine four

now this is an okay glow

but again I like to layer on these effects

so I'm gonna turn this mix down to point three three

and then copy this three more times

and on each of these different copies

let's go in and let's change the

size of our glow a little bit

so maybe on glow 2 we reduce the size

and on glow 3 we increase the size

so we've got a normal glow

and then we have a smaller glow

and finally a large glow

and you can see that gives us a pretty nice effect

now if you're not too worried about this

we can stop here

but if we wanna actually

uh mimic the physics in a much more accurate way

we can do that inside of nuke using something called

uh convolution

now we have a node to do this called the convalve node

but it runs rather slow

and the point of the convalve node is to take an image

and then essentially

use that image as a template or a base

for every single pixel inside of our image

so let's just quickly look at what

this would do if we just added in a simple radial

so I'm gonna add in a simple radial

and let's give this a pretty small size

uh I'm gonna go and give it a width of 1 hundred oops

a width of 100 and a height of 100

so we can see our radial there

and let's convolve this

to our original image

so I'm gonna plug our B pipe into

our original comp and the a pipe into this radial

and if we take a look at this

you can see it is now copying or using that radial

for every single pixel in our image

now it's gonna be incredibly blurry

and this is one

possible consequence of using the convolved

notice we have to be careful where

we're applying it now

as I said before

this is a little bit slow

so uh the new community is actually created a

gizmo that we can use to make this much faster

and they've also created some wonderful

uh convolution sources for us to use

so I'm gonna jump back to my browser

and it's currently on Wikipedia dot com

you can see the link here

or you can search for this in

uh your favourite

uh webman or search engine of choice

and what this is now doing is

it's using something called a fast Ferrier transform

which really I'm not going to get into

but it's a slightly faster way of using these

images to replace the pixels

in our comp

now uh uh this gentleman

Bob Roastler

has been nice enough to actually process some very

small convolution images

and has put them up for download

so if you have an account here at Wikipedia

which is free

you can go to the footage

and then down here

we have some lens kernels

so I've already gone in

and downloaded these three kernels

as well as downloaded

this nuke gizmo

so let's take a look at how we can use this now

to create a much more realistic

diffraction effect

alright so we have our convolve

and we have our normal glow

and I'm just gonna move these over here to the left

and what we can do is we can go to file and we

can import a new script

and I've downloaded those to my local system

and we're not gonna be able to redistribute these

but again you can download these

for free on Wikipedia dot com

so I'm gonna

import this gizmo

let's open this up

oopsie Daisy

let's go back to file

import script

there we go

so now we have our lens kernel FFT

now before we use this

we have to connect in our various kernels

so I'm gonna go over here and uh again

I've downloaded these three kernels that he's uh

been so generous enough to include

so we have our 1.4

our 2.8 and are 5.6

and those are

uh just different kernels at different focal lengths

so that they have slightly different profiles

now this is actually quite interesting

and it kind of shows us the airy disc

uh in the real world

so let's take a look at one of these uh files

and we can see here that it has nothing in it

now that is actually not true

if you hover over this

you'll notice there are some very

very small values

if we totally increase our viewport here

we can begin to see this airy disc

and you can see it has a rather unique profile

slightly different than

the one that physics is telling us we're going to get

and this is the great thing about the fraction is it's

very lens specific

so we can see here

we have this very bright inner center here

and then we have these wonderful rays

of surrounding it

so once we add this in

we're gonna create a very subtle

but very nice effect okay

so with our lens kernel FFT

let's plug this into our merge node

and take a look at this now

so if we come over here

we can see we are now

uh convolving or convoluting our image with this um

a real life captured airy disc

and we can see the difference is

uh very subtle

but it is quite pronounced

so with our glows

we get this

it's kind of soft

kind of fake

uh but with our lens kernel

you can see that we're getting this very nice glow that

even adds in a little bit of chromatic aberration

it begins smoothing everything out very nicely

now one problem with using this is it's going to soften

the entire image

so we can see here

we're losing quite a bit of definition

by doing this

so instead of directly applying it to the entire image

what we recommend is applying it only to the

bright areas of the image

so again we can add in a Kia here

so let's add in a Kia

and let's plug this in

and maybe let's give this a slightly softer key

so uh maybe around there

looks good so we're just convolving the

brightest areas of our shot

and then we just need to add it on

back to our original image

so let's take our lens kernel and our merge

and hit the m key to merge those together

and let's take a look at this

let's just give ourselves a little

bit more room to work

and clean up this flow a little bit

there we go

and let's take a look at this merge and add it in

now one thing we're gonna notice is that the

uh overall energy or the

uh brightness of our entire image has increased

by doing this

so we wanna be careful with that

so what we can do is

uh first we need to pre multiply

the luminance key so that we are only

modifying the brightest areas

there we go so again

our original and then our convolution added back on

but we're still uh having it be much brighter

so let's actually remove or subtract out

the pre multiplied key

before we add back on this lens kernel

so I'm gonna hit m to drop in another merge node

and connect the a pipe to my premult

and then change the operation to from

and what that is gonna do is it's going to subtract out

the pre multiplied information

so it's gonna subtract out

the color that we're going to be

uh fast for your transforming

and what that gives us when we add it back in

is a more consistent result with our original image

so you can see here one

two subtracted three

uh added back in

so what we're doing is we're essentially

taking light out

giving it the convolution or the fast

for your transform

and then adding it back on

and what that gives us is this nice glow

any uh or around all of our overly bright areas

now before we end this lesson

there's one thing I wanted to add

I've noticed a little

bit of instability with the lens kernel FFT

without a crop node before it

so before we do this

let's make sure that we

add a crop node before our luminance key

and that will just ensure that the shape or the size

of our lens kernel is consistent

as it is being entered

let's also make sure to turn off our black outside

so that the FFT again

has a consistent input

so that we can use it correctly

so just to wrap this up

there's a lot of different ways

you can create the diffraction effect

you can create a few glows

do that use the convolution node

if you have a lens kernel

or an image you would like to use

or again use that nuke

gizmo that's on Wikipedia dot com

if you want to use accurate images

and do it the right way the first time