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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... |
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| 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 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 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 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 has to be applied to stretch the 'width-ways' pixels to an imaginary target width of 768 pixels. |
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Anamorphic MPEG-4 (Part-2/Part-10) Encoding
Currently, when encoding using say: Divx, Xvid, 3ivx, x264 codecs, you will find all the ARS values you'll ever need (depending on the anamorphic shape you require). But in the main, the most common values are: Square 1:1, 4:3 PAL DAR, 4:3 NTSC DAR, 16:9 PAL DAR, 16:9 NTSC DAR, as well as a Custom Option.
Once selected, the ARS 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! |
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| 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. | |
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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 4:3
PAL 12:11
625lines 15:11
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
4:3 PAL 64:60 (or 16:15) 4:3 or 768x576 |
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| 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 |
Thu 23 July 2009 @ 18:00 |