Aspect Ratios

Aspect ratios are one of the more confusing things about video. They rarely make as much sense as they probably should. Close investigation into aspect ratios will just get you more confused than you were to start off with, so I'm going to try and keep it simple whilst keeping it accurate in a relativistic sort of way.... I have no idea if this is possible but here goes.

There are three main ways of describing aspect ratio - Pixel Aspect Ratio (PAR), Display Aspect Ratio (DAR), and Sample Aspect Ratio (SAR).

Pixel Aspect Ratio

A pixel is a pixel, it is a small block of color information. However, some playback devices have different shaped pixels than others. Computer monitors have square pixels and hence everything that is designed for display on a monitor should have a PAR of 1. TVs however have 'rectangular pixels' which have a different PAR depending on the format (NTSC or PAL).

Full screen NTSC dvd footage is usually 720x480 and fullscreen PAL dvd footage is usually 720x576. This footage is supposed be shown on a 4:3 TV... but these resolutions aren't 4:3 mathematically, so what's going on?

The reason for this (I'm simplifying here) is due to the fact that TVs have rectangular pixels and when you show the above resolutions resolutions on a TV they will look 4:3.

In practical terms it means that when you look at dvd resolution images (720x480, 720x576) on a PC monitor they do not have the correct pixel aspect ratio so they look a tiny bit squished/stretched in comparison to the way they look on a TV. This is normal. Programs that are designed for playing DVD video on your PC will correct this when they are displaying the footage by resizing it so it looks right, but if you are viewing it with a "Square Pixels" option then it will look slightly wrong. The good thing is that it doesn't really matter until you display your final product as to what PAR something has. You have a video of a certain resolution to edit with and no matter what PAR you display it in you still have that many pixels.

NTSC footage technically has a PAR of 0.911 and PAL has a PAR of 1.094 but we are going to work from the following principle - on a PC monitor, NTSC dvds need resizing to 640x480 to be 4:3 and PAL DVDs need resizing to 768x576. This is a simplified version of the truth but it suits us well enough.

[You may notice that NTSC is downsized and PAL is upsized - this is purely and simply to keep the vertical resolution the same in case someone uses these values on an interlaced source]

Now at this point, I have yet another surprise to spring on you. You know how I said NTSC DVD resolution is 720x480? Well, that's not entirely true. 8 pixels on each the left and right side of the image are supposed to be just empty space, and aren't really even supposed to even be considered! Got that?

Here is a simple math equation just to make sure you see what is happening here:

A DVD is 720x480, but taking 8 off each side, we get 704x480.
704*0.911/480 = 641.344/480 = close enough to 640/480
640/480 = 4/3 = 1.333...

Display Aspect Ratio

In mpeg video this is often stored as a flag that basically says "this is the aspect ratio that you should display me at". There are two possible Display Aspect Ratios (DARs) on a DVD - 4:3 and 16:9. TV shows will generally be 4:3 and movies will generally be 16:9 unless they are pan&scan or letterbox (see below). Movies with a 16:9 DAR are what we refer to as Anamorphic  (or sometimes listed as "optimised for widescreen TV" etc).

So, what is anamorphic?

Let me demonstrate with an NTSC dvd source: Revolutionary Girl Utena the Movie (aka Adolescence of Utena)

The resolution of an NTSC dvd is 720x480 and is generally designed for 4:3 footage. However, in order to maximise the amount of pixels being used, a widescreen movie is sometimes scaled to fit that resolution like this (images are 1/2 size):

The display aspect ratio for the image is actually 16:9 so the image, when shown corrected, will look something like this:

That image was made by stretching the image horizontally, just like a widescreen TV would do. If you were watching it on a 4:3 display, you might be able to set it so it adds letterboxing to display the 16:9 image like this:

Sometimes sources will be already letterboxed. Those have a 4:3 DAR and the footage itself has copious black borders on the top and bottom just like the image above. This is a waste of good resolution. However, it is a useful technique when producing footage for devices (and anime conventions) that cannot display or compensate for 16:9 DAR.

Another way of dealing with a widescreen source and keeping a 4:3 DAR is to do something called Pan and Scan which will extract a 4:3 image from the source by cropping the edges like this:

You can see the amount that has been removed from the image by comparing it with the stretched widescreen image above. The amount of footage lost is even worse when you have big budget Hollywood movies that actually have an aspect ratio of 2.35:1 (compared to 16:9 which is only 1.778:1). On DVDs movies with an aspect ratio like 2.35:1 are usually presented anamorphically with a 16:9 DAR but still need some letterboxing because they are not 16:9. The principle exactly is the same as letterboxing 16:9 footage in a 4:3 DAR source and it all comes out right in the end.

Now, as if Pixel Aspect Ratios and Display Aspect Ratios weren't enough, now we have another one to throw into the mix. Sample Aspect Ratios were introduced with the MPEG4 container. As such, they have absolutely nothing to do with DVDs! Since you might want to distribute your AMV as an MPEG4 though, it is worth learning what they are. Sample Aspect Ratios are actually very simple to understand - they are just a little hard to calculate. Fortunately, you probably wont ever need to calculate an SAR by hand!

The SAR basically just says "stretch the video by this ratio". So lets say for example, we have a 704x480 video (we took a dvd, and simply cropped off those annoying bits on the sides). If we want this video to be displayed at an aspect ratio of 4:3, then it would need an SAR of 10:11.

Why 10:11? Just look:

(704*10)/(480*11) = 4/3

See, it *is* easy to understand, right? If we wanted that same 704x480 video to be displayed at an aspect ratio of 16:9, then we would use an SAR of 40:33, as (704*40)/(480*33) = 16/9.

But like I said, you wont need to be calculating these manually, so all you really need to worry about is just understanding how the SAR differs from the PAR and DAR.

August 2010