In the video space, there’s always a lot of talk about these number
ratios – 4:4:4, or 4:2:2, or 4:1:1, but what exactly do they mean?
Recently, someone argued with me that it was better to convert every
video clip from my Canon Rebel T2i DSLR camera into a 4:4:4 intermediate
codec before editing; that this would make the color magically “better”
and that editing natively was somehow bad. They were wrong, and I’m
going to explain why.
One trick that video engineers have used for years is to toss away a
lot of the color information. Basically, they can toss away the color
values on every other pixel, and it’s not very noticeable. In some
cases, they throw away even more color information. This is called Color
Subsampling, and it’s a big part of a lot of modern HD formats for
video.
When looking at color subsampling, you use a ratio to express what
the color subsampling is. Most of us are familiar with these numbers:
4:4:4, or 4:2:2, or 4:1:1, and most of us are aware that bigger numbers
are better. Fewer people understand what the numbers actually mean. It’s
actually pretty easy.
Let’s pretend that we are looking at a small part of a frame – just a 4×4 matrix of pixels in an image:
In this example, every pixel has a Y value, a Cb value, and a Cr
value. If you look at a line of pixels, and count how many Y, U, and V
values, you’d say that there are 4 values of Y, 4 values for U, and 4
values of V. In color shorthand, we’d say that this is a 4:4:4 image.
4:4:4 color is a platinum standard for color, and it’s extremely rare
to see a recording device or camera that outputs 4:4:4 color. Since the
human eye doesn’t really notice when color is removed, most of the
higher-end devices output something called 4:2:2. Here’s what that 4×4
matrix would look like for 4:2:2:
As you can see, half of the pixels are missing the color data.
Looking at that 4×4 grid, 4:2:2 color may not look that good, but 4:2:2
color is actually considered a very good color standard. Most computer
software can use the neighboring color values and average in the values
of the missing color values.
Let’s look at 4:1:1 color, which is used for NTSC DV video:
Bleaccch. 75% of the color for each pixel is tossed away! With bigger
“gaps” between color information, it’s even harder for software to
“rebuild” the missing values, but it happens. This is one of the reasons
that re-compressing DV can cause color smearing from generation to
generation.
Let’s look at one other color subsampling, which is called 4:2:0, and is used very frequently in MPEG encoding schemes:
This diagram shows one of many ways that 4:2:0 color subsampling can
be accomplished, but the general idea is the same – Luma samples for
each pixel, one line has Cb samples for every other pixel, and the next
line has Cr samples for every other pixel.
With a color subsampled image, it’s up to the program decoding the
picture to estimate the missing pixel values, using the surrounding
intact color values, and providing smoothing between the averaged
values.
Okay – we’ve defined what color subsampling is.
Now, how does that relate to my friend’s earlier argument?
Well, in my DSLR camera, the color information is subsampled to 4:2:0 color space in the camera. In other words,
the camera is throwing away the color information.
It’s the weakest link in the chain! Converting from 4:2:0 to 4:4:4 at
this stage doesn’t “magically” bring back the thrown-away data –
the data was lost prior to hitting the memory card.
It’s just taking the data that’s already there, and “upsampling” the
missing color values by averaging between the adjoining values.
Inside Premiere Pro, the images will stay exactly as they were
recorded in-camera for cuts-only edits. If there’s no color work going
on, the 4:2:0 values remain untouched. If I need to do some color
grading,
Premiere Pro will, on-the-fly, upsample the footage to 4:4:4, and it does this very well, and in a lot of cases, in real-time.
Going to a 4:4:4 intermediate codec does have some benefits – in the
transcode process, upsampling every frame to 4:4:4 means that your CPU
doesn’t have as much work to do, and may give you better performance on
older systems, but there’s
a huge time penalty in transcoding.
And, it doesn’t get you any “better color” than going native. Whether
you upsample prior to editing or do it on-the-fly in Premiere Pro, the
color info was already lost in the camera.
In fact, I could argue that Premiere Pro is the better solution for
certain types of editing because we leave the color samples alone when
possible. If the edit is re-encoded to a 4:2:0 format, Premiere Pro can
use the original color samples and pass those along to the encoder in
certain circumstances. Upsampling and downsampling can cause errors,
since the encoder can’t tell the difference between the original color
samples and the rebuilt, averaged ones.
I’m not trying to knock intermediate codecs – there are some very
valid reasons why certain people need them in their pipeline. But, for
people just editing in the Adobe Production Premium suite, they won’t
magically add more color data, and may waste you a lot of time. Take
advantage of the native editing in Premiere Pro CS5, and you’ll like
what you see.
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