x264 CRF Guide

Along with x264 came a new rate control method Constant Rate Factor(CRF) to me this is one of the best ways to encode videos. Unfortunately there isn't a lot of data on how CRF actually work and the results it creates. The reason for this is because it is based off of Constant Quantizer(QP) to understand this one must know how QP works first.

Constant Quantizer(QP)
Quantization works by breaking large input sets into smaller ones by removing information that might not be as relevant. A small example of this is, instead of storing data like: 28.56 we store it  28.56204234672.

So QP work by keeping the information it throws away consistent, thus resulting in a constant quality. In the H263 (xvid,divx, mpeg part 4, 3ivx) days a QP of 1 = 100% quality and every step was 5 less, so a QP 3 would be 90% and QP 5 = 85% and so on.

In H264 a QP 0 = 100% and there are 51 steps suggesting that every step is roughly 2% quality loss of course that is all mathematical and in the real world we deal with perceptual. And that is where CRF comes in. 

Constant Rate Factor
CRF works similarly to QP in that it tries and mimics the perceptual quality output of QP, such that a QP 21 would look to the human eye near identical to a CRF 21, it does this by reducing information in areas the human eye can't perceive and redistributes them in other areas that might need more data.

CRF-25

QP-25

As you can see the images have a very similar image quality even during playback but has a significant difference in file-size:
QP: 2.6 GB
CRF: 2.1 GB(24% smaller)

Resolution scaling

So lets take a look at how CRF scales with different resolution sizes. The y-axis represents file size and the x-axis the vertical lines from 96x52p to 1920x1040p. The first step is 5% pixels of max and the second is 10%, the 3rd 15% and so on until 100% of/or 1920x1040.

As you can see the there is a slight indication that the amount of data needed to represent an image with CRF scales slightly exponentially in regards to the number of pixels.

And if we look at the encoding performance above we can see that as the resolution increases there is a sharp drop in encoding speed that tapers of has the resolution reaches the higher end.

So what is a good CRF value? It mainly depends on what you want, if you want a near identical copy then 18, High Quality 19-21.

But lets see how various CRF values impact the compression of a file. y-axis is file-size and x-axis is the CRF value

Again an exponential trend occurs, the lower the CRF value the significantly larger the file-size. As for image quality its hard to see the exact difference between still images when you compare the visual quality of CRF value with each other, you only notice a significant difference when its in motion. However I would suggest no higher value than 25.

CRF 21

CRF 25

CRF 30

Hollywood still uses film

It might come as a surprise to know that the majority of major hollywood films are still shot using analog film. Inception, Dark Knight, Star Trek into the Darkness, Man of Steel are a small sample of movies still shot in film [link]. 

And at first glance you might think its silly to still use film, after all film productions is significantly more expensive. The film needs to be developed(creating the negatives) then turned into digital content via a film scanner, before they can add in any CGI. Not to mention the costs of developing the film alone to be displayed.

“It costs about $1,500 to print one copy of a movie on 35 mm film and ship it to theaters in its heavy metal canister. Multiply that by 4,000 copies — one for each movie on each screen in each multiplex around the country — and the numbers start to get ugly. By comparison, putting out a digital copy costs a mere $150.” [Link]

If film is more expensive and harder to work with, why do they still use it? One reason is that film is analog and is not really subject to resolution boundaries like digital. The reason why they can remaster Indiana Jones, Star Wars (4,5,6) and all those other old films to HD(1920x1080) is because all they have to do is rescan the negative at a higher resolution. Where as with digital you're stuck with the resolution you shot in. So when 4k(3840 x 2160) comes out the old movies will be rescanned at sold again and when 8k(7680 x 4320) comes out its the same process.

Of course there is a limit to how much resolution you can get out of an analog film. Theoretically all you have to do is:  ((lpm * w * 2) * (lpm * h * 2)) / 1 000 000 = Mega Pixel count [link]
Where:
lpm = lines per millimeter
w = width of film in millimeter
h = height of film in millimeter

So according to that a 35mm(academy ratio) film at 160 lines per millimeter would result in ((160 * 22 * 2) * (160 * 16 * 2))/ 1000000 = 36MP

This is of course assuming perfect conditions which never occurs in the real world. And other have done side by side comparisons between the two. It turns out that in practice 35mm film is about 9MP and if you take Kenrock Well's lie factor into account its 18MP [link]. Digital film cameras are not yet at that level, although the gap is narrowing and more films are being shot in Digital [link] such as the Hobbit shot using the RED digital camera.