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What is... an Anamorphic image?
A Quick Explanation

The first question you have to admit to yourself is... do you know what the word "anamorphic" means?

In most dictionaries the word anamorphic means: "A distorted image that appears to look normal when viewed". 

That said, it may be worth pointing out that in certain professional circles the word "anamorphic" is often used to describe different things in relation to different technologies!

Anyway, in relation to MPEG-2/DVD (or MPEG-2/DVB) images, it's all about how the pixels are allowed to be stretched or 'distorted'. And with MPEG-2/DVD you can only do this, horizontally (ie: left to right/width ways).

To confirm: This means the pixels that make up the 'height' (which is 480 pixels for NTSC and 576 pixels for PAL) are not allowed to 'distort' vertically. But the pixels that make up the 'width' (720 for both NTSC and PAL) are allowed to 'distort' horizontally.

However, because image pixels prefer to be encoded square (in much the same way image dots prefer to be round), if you want to change their shape ie: make them become rectangular, you have to instruct them to do so. And this is where you'll start to hear expressions such as, PAR (Pixel Aspect Ratio), DAR (Display Aspect Ratio) and SAR (Sample Aspect Ratio) - All of which are forms of ARS (Aspect Ratio Signalling).

Mainly, when it comes to generating anamorphic MPEG-4 encodes it's all about calculating the level of ARS you'll require, in order to establish how much each pixel needs to be stretched 'width-ways'.

So it's time to do so maths...
Anamorphic MPEG-2/DVD Images

Okay then, at some time or another, you've probably already worked out that an anamorphic NTSC DVD contains a PFS (Pixel Frame Size) of 720x480 pixels and that an anamorphic PAL DVD contains a PFS of 720x576 pixels. And that if you divide 720 by 480 or 576 you obtain FAR's (Frame Aspect Ratio's) of 1.5:1 and 1.25:1 respectively, which is no where near the 1.7777:1 (aka: 16:9) widescreen TV shape you need. So how many pixels do we need to form a 'true 16:9 frame' image?

In the case of an NTSC 16:9 (Widescreen Anamorphic) DVD Images:
Because we already know the 'height' uses 480 pixels, all we need to do is multiply 480 by 1.7777 and we get a target width-ways pixel quantity of 853.3333. And because you also now know that all NTSC anamorphic DVD's contain just 720 'width-ways' pixels (and not the required 853.333 pixels) ARS signalling has to be applied to stretch the 'width-ways' pixels to an imaginary imaginary target width of 853.3333 pixels.

In the case of an PAL 16:9 (Widescreen Anamorphic) DVD Images:
Because we already know the 'height' uses 576 pixels, all we need to do is multiply 576 by 1.7777 and we get a target width-ways pixel quantity of 1024.0000. And because you also now know that all PAL anamorphic DVD's contain just 720 'width-ways' pixels (and not the required 1024 pixels) ARS signalling has to be applied to stretch the 'width-ways' pixels to an imaginary target width of 1024 pixels.

 

In the case of an NTSC 4:3 (Standard / Fullscreen) DVD Images:
Because we already know the 'height' uses 480 pixels, all we need to do is multiply 480 by 1.3333 and we get a target width-ways pixel quantity of 640. And because you also now know that all NTSC anamorphic DVD's contain just 720 'width-ways' pixels (and not the required 640 pixels) ARS signalling has to be applied to stretch the 'width-ways' pixels to an imaginary imaginary target width of 640 pixels.

In the case of an PAL 4:3 (Standard / Fullscreen) DVD Images:
Because we already know the 'height' uses 576 pixels, all we need to do is multiply 576 by 1.3333 and we get a target width-ways pixel quantity of 768. And because you also now know that all PAL anamorphic DVD's contain just 720 'width-ways' pixels (and not the required 768 pixels) ARS signalling has to be applied to stretch the 'width-ways' pixels to an imaginary target width of 768 pixels.

Anamorphic MPEG-4 Encoding

 

Currently, when encoding using either XviD or 3ivx codecs, you will find all the ARS signalling values you will ever need (depending on the anamorphic shape you require). But in the main, here are the most common ones: Square 1:1, 4:3 PAL, 4:3 NTSC, 16:9 PAL, 16:9 NTSC, as well as a Custom Option.

 

Once selected, the ARS signalling value will be embedded within the video stream as a signalling code, which in-turn will (hopefully) be detected and decoded by your software media player/direct-show decoder filter or hardware device.... I say hopefully because not all software players/direct-show decoders and hardware devices are capable of correctly displaying MPEG-4 encodes containing ARS!

Now if you forget to give your MPEG-4 stream an ARS value or you've given your encode the wrong ARS value don't panic because there's a wonderful little tool called MPEG4 Modifier, that can remedy the situation for you - without you having to generate the MPEG-4 encode all over again!
Please Note: In order to keep my explanation as simple as possible I have only talked about ARS (Aspect Ratio Signalling) here.
For more information about what PAL anamorphic images look like, please click here.
For more information about what NTSC anamorphic images look like, please click here.
For more information about Pixel Frame Sizes and Totals, please click here.

Technical Information
 

For some of you more technically minded people, I thought you might like to know the calculations both XviD and 3ivx use to generate their ARS shapes.

 

Default       Codec              Player Info
PAR Setting   Calculation        Video Size

4:3  PAL      12:11 625lines     15:11
4:3  NTSC     10:11 525lines     15:11
16:9 PAL      16:11 625lines     20:11
16:9 NTSC     40:33 525lines     20:11

 

Unfortunately, if you use the above default settings, the 'video size' will not be reported correctly under your software players properties. This is because they were calculated using the ITU-R BT.601 standard for 'analogue line to digital pixel' conversion based on 704 pixels and not 720 pixels, which is what a DVD uses.

 

In my opinion, the ITU standard is not really necessary when performing 'digital pixel to digital pixel' conversions. So, with this in mind you may find that the following 'Custom Options' will report more accurate information: -

 

Required      Custom             Player Info
PAR Setting   PAR Setting        Video Size

4:3  PAL      64:60 (or 16:15)   4:3  or 768x576
4:3  NTSC     64:72 (or 16:18)   4:3  or 640x480
16:9 PAL      64:45              16:9 or 1024x576
16:9 NTSC     64:54              16:9 or 853x480
For more information about Why I don't follow ITU601 specification, please click here.
Abbreviations Key

PAR = Pixel Aspect Ratio | DAR = Display Aspect Ratio | SAR = Sample Aspect Ratio | MAR = Movie Aspect Ratio | FAR = Frame Aspect Ratio | PFS = Pixel Frame Size | ARS = Aspect Ratio Signalling |
Last Updated

Wed 21 June 2006 @ 18:00