You know how important it is to have
good quality footage to get good quality keying results, green screen/blue screen. In this video, I want to
show you the impact of color compression on the keying process. So of the chroma sub sampling,
this means are you working convert the raw footage or 4444, 422 or
worst case H.264 encoded footage. Depending on what level of color
compression or chromosome sampling is happening, you have quite an impact
on the keying and I want to show you how that looks like, how you can
analyze it and how, in some cases you can even mitigate it a little bit. This lesson is taken from my full
composite in course where I have two full chapters on keying with lots of
discussion on pre-processing impact of noise, how to do advanced keying
processes for hair and so on and so on. Feel free to have a look on my
website for the full course, but first enjoy this lesson. When we talk about color subsampling,
we know that our human eye is very perceptive to changes in luminance,
but not so perceptive to small changes in the color, in the chrominance. And that is what all
compression codecs make use of. So if you have an H.264 codec or H.265,
for online internet viewing, you have the Luminance channel very accurate,
but the chrominance is compressed. So you have four times the luminance
information compared to each of the chrominance channels and the same as
being used in many cameras codecs. If you recall, ProRes 4:2:2, you
have, for each four parts of luminance information, we have only two parts
in each of the chrominance channels. And this may not be visible to our eye,
but it is very visible to the keyer. So let's have a look at some example
footage, see how it looks like, and I'll also show you some tricks, what
you can sometimes do to mitigate the issue a little bit, If you do
not have good chroma subsampling. Let's have a look. I'm looking here at this test
footage and on my screen, I'm looking at it at full resolution. For you I will have to zoom in a bit. And on the left side of my AB Split
View, you see the ProRes HQ and on the right side, the H.264 encoding. Now, because this is again a streamed
for you as the training video, you get everything in H.264 encoding, so whenever
you cannot see exactly the differences I'm pointing out, I really recommend load
it up on your own display in your own machine in your own Resolve installation. You have the footage, so
just, I look at it offline. But if I zoom in and
exaggerate a bit, you will see. And the first thing you will see
is that here between the left and the right part, when I play this
back, I hardly see a difference. Now, if I pay close attention, I see
that here the hair is a little bit more blurry and the more I zoom in
the more I will see the difference between the ProRes and the H.264. Again, H.264 on the right, you see
it gets tiny, little bit more blurry, but I really have to zoom in and pay
very close attention to see that. Let me zoom in on an edge and a zoom
in really far to really see this. So I have here the shoulder edge. So let's just look at this and. You can hardly see a difference. What I can see is that the ProRes
footage captures much more noise here. And if I look at the H.264, it looks
almost like it's a bit denoised, but actually it's not really a denoise,
it's more like it's getting more blocky, so it's not capturing this
very fine level of noise from frame to frame, and that's not a big issue. I mean, artistically, of course,
maybe I want to see it, but it won't be a big issue for the keyer. It might even be desirable
as we see in the next lesson. But all in all, just from looking
at it, it looks pretty good. And I don't really see a
difference if I just look at the edge, hardly any difference. But now let's see what
this means to the keyer. So I will just add a Delta Keyer here. And I won't do like a complete keying run. I will just do very quickly sample the
green here and let's bring this on the left and I bring the exact same Delta
Keyer just with an instance copy, I added to the right for my H.264 image and let's
look directly at the Alpha channel and bring the, maybe I bring the Gain up a
bit, just tiny little bit to get a bit rid of some of the background noise. And that's all I'm doing now. I'm not refining this. Just to see the immediate
difference between the two images. So I have left and right. The Delta Keyer, both the same
version of the keyer, once on the HQ ProRes footage, once on H.264 and
now let's have a look at the edge. So this is the edge from ProRes,
and now I move over and my edge is completely ruined in the H.264. So something that was hardly noticeable
in just looking at it with my human eyes in the full color image becomes
very noticeable to the keyer. And you can look further
if you look at the hair. So here we get some hair detail,
and if I look at the H.264 image, the hair detaile is basically gone. So, all in all, I really have a
strong impact of my compression. So let me try to understand what's really
happening here a little bit better by looking at the chrominance channels and
to do this, I want to look at a even more extreme example because that increases,
again, the chances that you can see that even here in the streaming recording. Again, try it yourself to see
it on your own monitor without any additional encoding. So let me open an example here
from Hollywood Camera Work back in the color channel. So I'm looking at this image here, and
this image has also exposure issues and lots of issues, but it has a really
terrible edge if I look at it, right? So let's see what's happening here. So one thing is, I mentioned that earlier,
this edge, and I can use the Gamma slider here, maybe to see it even better. So this edge here, there's some
artificial edge sharpening going on, some sharpening filter, which
makes the outside edge darker. But that's not the only thing that you
also see these kind of steps, right? You see, like steps going on like this. So let's start to analyze this further
and look into the chrominance channels. And I can do this with the Color Space
tool, let me add a Color Space tool here. So the Color Space tool can
transform my color image. If I say to Color from the current
RGB into a YUV, which is the typical Luminance, and then U and
V are two chrominance channels. So this is the typical breakdown
that is being used by most cameras and by encoding codecs like H.264. And if I just look at it, it looks ugly. Of course it's, I don't see anything. But I can now go into these
individual channels and let's look into the Luminance channel first,
which is now in the Red channel. So the first one, our Luminance, and
this is what my edge looks like here. Okay. Not as bad as in the full image, right? So I mean, it's not super
resolution, but I do see an edge. It's a little bit blocky,
but all in all, okay. So now let's go into the
chrominance channel, into the Green channel, the first one. Oh, what's happened now? Now you see the same edge got much wider. So I had here the Luminance,
and here the chrominance, and suddenly it looks like stretched. You see the edge is not as sharp
here, it's spread out wider. And then in the second
chrominance channel the same. So again, Luminance, first chrominance
channel, second chrominance channel. So you really see that the edge is
somehow, has less resolution at the end. And here you even see like the
spatial orientation of that issue. So, depending on how your sensor pixels
are arranged and how the encoding happens, you can actually sometimes really see
the blockiness of the lower resolution. So you can see that in one orientation,
it's only capturing like every second pixel compared to the luminance. So it's capturing this with
less resolution, even though there may be some interpolation
still going on from afterwards. But you can see these kind
of steps in the image. Now in this case, there is actually
something that we may be able to do. Now, we cannot create information that
isn't there, but at least we can smooth out some of the problems that we see here. So, because we see these kind of steps,
which get a really bad edge we can try to smooth them out a little bit. And what I do for this, I just add
a Blur node here, after my Color Space and I will blur those two
chrominance channels a tiny little bit. So what I'm doing, let
me bring in the Blur. I don't want to blur in general. So let me first go to 0. So this has without blur. And then I want to do this, I'm
fine with the Luminance channel. I don't want to blur this, which
is currently in the Red channel. So I turned this off,
Alpha doesn't matter. So I'm only looking at the
two chrominance channels. And here you see, I have resolution
in the Y direction, but in the X direction, I'm having these
jumps, sorry, the other way round. So I need to blur in this direction,
in the X direction in order to avoid these kind of steps I see. So let me bring in a Blur of 1. And you see, I mean, I'm not getting a
super clean edge now, but just by doing a 1 pixel step, you see, I can kind of
avoid these extreme jumps that I'm seeing. Maybe even 2. Let's try. Yeah. So this way I'm getting slightly blurred,
but it's not as zig-zag as it was before. Let me also check in the other channels. So this was the Blue channel, Green
channel, the other chrominance channel, with blur, without, so a 2
pixel blur in this case, just in this direction and only on the chrominance
channels, it makes those channels on the edge here a bit smoother at least. Let's see, I add the Color Space tool
again at the end to go back and now I'm going from Color back to RGB. So I should go back to
my original image here. Let me see in Color View. Now let me have a quick
look at before and after. So I still have my Gamma slider down, by
the way, just to make it very obvious. So this is the edge after my tiny
little bit of blur in the chrominance, and this was the edge before. So I hope you agree before, after. So, I got rid of this like
extreme strange artifacts. Now I'm not getting a
great image from this. I still have issues with this footage. But at least I have mitigated
this a little bit and this will be very beneficial for the keyer. Let me very quickly demonstrate
again with a Delta Keyer, I just bring the Delta Keyer in and again,
make an instant copy right away. Let's try directly here
on the smooth edge. If I take the Delta Keyer here and key it,
Alpha channel, let's have a look at it. Okay. And I'm just not even
changing any settings. So this is my edge that I'm getting from
my slightly blurred improved version. And this was my original edge. So you see first edge, second edge. So this is definitely quite
an improvement in this case. Alright, hope this helps to
understand the impact of chroma subsampling a bit better, and you can
really see it and, and go into it. In some cases, like what I have shown
here, in this extreme case, I was able to get a slight improvement on my keyer
with a little bit of selective blur. I can't do magic. I cannot create information that
wasn't there in the file, obviously. So also these are like small tricks
that can sometimes help and that you can add to your tool arsenal. Thank you for watching this lesson
from my compositing training course. This course builds strong
foundations in the art and craft of compositing with Fusion. It covers topics from Rotoscoping,
Keying, Integration, 3D Camera Tracking, all the way up to topics like
projection and multipass compositing. And it goes into a level of depth
that very few online courses reach. If you are interested, check out
the course page for further details, have a look at what other students
are saying about this course and I’d be very happy to welcome you inside.
