This episode is brought to you by Curiosity Stream.
Hi, I'm Carrie Anne, and welcome to Crash Course Computer Science!
Last episode we talked about Files, bundles of data, stored on a computer, that are formatted
and arranged to encode information, like text, sound or images.
We even discussed some basic file formats, like text, wave, and bitmap.
While these formats are perfectly fine and still used today, their simplicity also means
they’re not very efficient.
Ideally, we want files to be as small as possible, so we can store lots of them without filling
up our hard drives, and also transmit them more quickly.
Nothing is more frustrating than waiting for an email attachment to download. Ugh!
The answer is compression, which literally squeezes data into a smaller size.
To do this, we have to encode data using fewer bits than the original representation.
That might sound like magic, but it’s actually computer science!
INTRO
Lets return to our old friend from last episode, Mr. Pac-man!
This image is 4 pixels by 4 pixels.
As we discussed, image data is typically stored as a list of pixel values.
To know where rows end, image files have metadata, which defines properties like dimensions.
But, to keep it simple today, we’re not going to worry about it.
Each pixel’s color is a combination of three additive primary colors: red, green and blue.
We store each of those values in one byte, giving us a range of 0 to 255 for each color.
If you mix full intensity red, green and blue - that’s 255 for all three values - you
get the color white.
If you mix full intensity red and green, but no blue (it’s 0), you get yellow.
We have 16 pixels in our image, and each of those needs 3 bytes of color data.
That means this image’s data will consume 48 bytes of storage.
But, we can compress the data and pack it into a smaller number of bytes than 48!
One way to compress data is to reduce repeated or redundant information.
The most straightforward way to do this is called Run-Length Encoding.
This takes advantage of the fact that there are often runs of identical values in files.
For example, in our pac-man image, there are 7 yellow pixels in a row.
Instead of encoding redundant data: yellow pixel, yellow pixel, yellow pixel, and so
on, we can just say “there’s 7 yellow pixels in a row” by inserting an extra byte
that specifies the length of the run, like so:
And then we can eliminate the redundant data behind it.
To ensure that computers don’t get confused with which bytes are run lengths and which
bytes represent color, we have to be consistent in how we apply this scheme.
So, we need to preface all pixels with their run-length.
In some cases, this actually adds data, but on the whole, we’ve dramatically reduced
the number of bytes we need to encode this image.
We’re now at 24 bytes, down from 48.
That’s 50% smaller!
A huge saving!
Also note that we haven’t lost any data.
We can easily expand this back to the original form without any degradation.
A compression technique that has this characteristic is called lossless compression, because we
don’t lose anything.
The decompressed data is identical to the original before compression, bit for bit.
Let's take a look at another type of lossless compression, where blocks of data are replaced
by more compact representations.
This is sort of like “don’t forget to be awesome” being replaced by DFTBA.
To do this, we need a dictionary that stores the mapping from codes to data.
Lets see how this works for our example.
We can view our image as not just a string of individual pixels, but as little blocks
of data.
For simplicity, we’re going to use pixel pairs, which are 6 bytes long, but blocks
can be any size.
In our example, there are only four pairings: White-yellow, black-yellow, yellow-yellow
and white-white.
Those are the data blocks in our dictionary we want to generate compact codes for.
What’s interesting, is that these blocks occur at different frequencies.
There are 4 yellow-yellow pairs, 2 white-yellow pairs, and 1 each of black-yellow and white-white.
Because yellow-yellow is the most common block, we want that to be substituted for the most
compact representation.
On the other hand, black-yellow and white-white, can be substituted for something longer because
those blocks are infrequent.
One method for generating efficient codes is building a Huffman Tree, invented by David
Huffman while he was a student at MIT in the 1950s.
His algorithm goes like this.
First, you layout all the possible blocks and their frequencies.
At every round, you select the two with the lowest frequencies.
Here, that’s Black-Yellow and White-White, each with a frequency of 1.
You combine these into a little tree... ...which have a combined frequency of 2, so we record
that.
And now one step of the algorithm done.
Now we repeat the process.
This time we have three things to choose from.
Just like before, we select the two with the lowest frequency, put them into a little tree,
and record the new total frequency of all the sub items.
Ok, we’re almost done.
This time it’s easy to select the two items with the lowest frequency because there are
only two things left to pick.
We combine these into a tree, and now we’re done!
Our tree looks like this, and it has a very cool property: it’s arranged by frequency,
with less common items lower down.
So, now we have a tree, but you may be wondering how this gets us to a dictionary.
Well, we use our frequency-sorted tree to generate the codes we need by labeling each
branch with a 0 or a 1, like so:
With this, we can write out our code dictionary.
Yellow-yellow is encoded as just a single 0.
White-yellow is encoded as 1 0 (“one zero”)
Black-Yellow is 1 1 0
and finally white-white is 1 1 1.
The really cool thing about these codewords is that there’s no way to have conflicting
codes, because each path down the tree is unique.
This means our codes are prefix-free, that is no code starts with another complete code.
Now, let’s return to our image data and compress it!
Our first pixel pair, white-yellow, is substituted for the bits “1 0”.
The next pair is black-yellow, which is substituted for “1 1 0”.
Next is yellow-yellow with the incredibly compact substitution of just “0”.
And this process repeats for the rest of the image:
So instead of 48 bytes of image data ...this process has encoded it into 14 bits -- NOT
BYTES -- BITS!!
That’s less than 2 bytes of data!
But, don’t break out the champagne quite yet!
This data is meaningless unless we also save our code dictionary.
So, we’ll need to append it to the front of the image data, like this.
Now, including the dictionary, our image data is 30 bytes long.
That’s still a significant improvement over 48 bytes.
The two approaches we discussed, removing redundancies and using more compact representations,
are often combined, and underlie almost all lossless compressed file formats, like GIF,
PNG, PDF and ZIP files.
Both run-length encoding and dictionary coders are lossless compression techniques.
No information is lost; when you decompress, you get the original file.
That’s really important for many types of files.
Like, it’d be very odd if I zipped up a word document to send to you, and when you
decompressed it on your computer, the text was different.
But, there are other types of files where we can get away with little changes, perhaps
by removing unnecessary or less important information, especially information that human
perception is not good at detecting.
And this trick underlies most lossy compression techniques.
These tend to be pretty complicated, so we’re going to attack this at a conceptual level.
Let’s take sound as an example.
Your hearing is not perfect.
We can hear some frequencies of sound better than others.
And there are some we can’t hear at all, like ultrasound.
Unless you’re a bat.
Basically, if we make a recording of music, and there’s data in the ultrasonic frequency
range, we can discard it, because we know that humans can’t hear it.
On the other hand, humans are very sensitive to frequencies in the vocal range, like people
singing, so it’s best to preserve quality there as much as possible.
Deep bass is somewhere in between.
Humans can hear it, but we’re less attuned to it.
We mostly sense it.
Lossy audio compressors takes advantage of this, and encode different frequency bands
at different precisions.
Even if the result is rougher, it’s likely that users won’t perceive the difference.
Or at least it doesn’t dramatically affect the experience.
And here comes the hate mail from the audiophiles!
You encounter this type of audio compression all the time.
It’s one of the reasons you sound different on a cellphone versus in person.
The audio data is being compressed, allowing more people to take calls at once.
As the signal quality or bandwidth get worse, compression algorithms remove more data, further
reducing precision, which is why Skype calls sometimes sound like robots talking.
Compared to an uncompressed audio format, like a WAV or FLAC (there we go, got the audiophiles back)
compressed audio files, like MP3s, are often 10 times smaller.
That’s a huge saving!
And it’s why I’ve got a killer music collection on my retro iPod.
Don’t judge.
This idea of discarding or reducing precision in a manner that aligns with human perception
is called perceptual coding, and it relies on models of human perception,
which come from a field of study called Psychophysics.
This same idea is the basis of lossy compressed image formats, most famously JPEGs.
Like hearing, the human visual system is imperfect.
We’re really good at detecting sharp contrasts, like the edges of objects, but our perceptual
system isn’t so hot with subtle color variations.
JPEG takes advantage of this by breaking images up into blocks of 8x8 pixels, then throwing
away a lot of the high-frequency spatial data.
For example, take this photo of our directors dog - Noodle.
So cute!
Let’s look at patch of 8x8 pixels.
Pretty much every pixel is different from its neighbor, making it hard to compress with
loss-less techniques because there’s just a lot going on.
Lots of little details.
But human perception doesn’t register all those details.
So, we can discard a lot of that detail, and replace it with a simplified patch like this.
This maintains the visual essence, but might only use 10% of the data.
We can do this for all the patches in the image and get this result.
You can still see it’s a dog, but the image is rougher.
So, that’s an extreme example, going from a slightly compressed JPEG to a highly compressed
one, one-eighth the original file size.
Often, you can get away with a quality somewhere in between, and perceptually, it’s basically
the same as the original.
The one on the left is one-third the file size of the one on the right.
That’s a big savings for essentially the same thing.
Can you tell the difference between the two?
Probably not, but I should mention that video compression plays a role in that too, since
I’m literally being compressed in a video right now.
Videos are really just long sequences of images, so a lot of what I said about them applies
here too.
But videos can do some extra clever stuff, because between frames, a lot of pixels are
going to be the same.
Like this whole background behind me!
This is called temporal redundancy.
We don’t need to re-transmit those pixels every frame of the video.
We can just copy patches of data forward.
When there are small pixel differences, like the readout on this frequency generator behind
me, most video formats send data that encodes just the difference between patches, which
is more efficient than re-transmitting all the pixels afresh, again taking advantage
of inter-frame similarity.
The fanciest video compression formats go one step further.
They find patches that are similar between frames, and not only copy them forward, with
or without differences, but also can apply simple effects to them, like a shift or rotation.
They can also lighten or darken a patch between frames.
So, if I move my hand side to side like this the video compressor will identify the similarity,
capture my hand in one or more patches, then just move these patches around between frames.
You’re actually seeing my hand from the past… kinda freaky, but it uses a lot less data.
MPEG-4 videos, a common standard, are often 20 to 200 times smaller than the original,
uncompressed file.
However, encoding frames as translations and rotations of patches from previous frames
can go horribly wrong when you compress too heavily, and there isn’t enough space to
update pixel data inside of the patches.
The video player will forge ahead, applying the right motions, even if the patch data
is wrong.
And this leads to some hilarious and trippy effects, which I’m sure you’ve seen.
Overall, it’s extremely useful to have compression techniques for all the types of data I discussed today.
(I guess our imperfect vision and hearing are “useful,” too.)
And it’s important to know about compression because it allows users to store pictures,
music, and videos in efficient ways.
Without it, streaming your favorite Carpool Karaoke videos on YouTube would be nearly
impossible, due to bandwidth and the economics of transmitting that volume of data for free.
And now when your Skype calls sound like they’re being taken over by demons, you’ll know
what’s really going on.
I’ll see you next week.
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