Crop/Resize/Aspect
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True for the most part; but there is a lot more to it than that.


C. Resolution, Cropping, Resizing, and AR (Aspect Ratio)



Resolution: The width and height of a video frame in px (pixels). ex) 640x480
Cropping: The process of cutting off the edge(s) of an image
Resizing: Changing the Resolution of a video
AR (Aspect Ratio): The intended ratio of a video's width to height. ex) 4:3

- Typically, your footage will have an AR of 4:3 or 16:9, and sometimes 2.35:1 <---(2.35 to 1) - Be aware that there are exceptions to these common conventions-- including DVDs.







Calculating & Expressing AR



Often, AR is Expressed in Different Ways:

by Common Name, Actual Dimension Ratio, Whole Ratio, Decimal Value, or Decimal Ratio.

...I know it's pretty annoying, but they all mathematically mean the same thing.

"FullScreen"
= 640/480, 720/540, etc...
= 4/3 (or 4:3)
= 1.333...
= 1.33:1

"WideScreen"
= 1280/720, 704/396, etc...
= 16/9 (or 16:9)
= 1.777...
= 1.78:1

"NTSC DVD" (before stretching)
= 720/480
= 3/2 (or 3:2)
= 1.5
= 1.5:1

So, regardless of how AR is expressed, the math never lies:

Aspect Ratio = Width:Height = Width/Height = Width÷Height

But, for clarity sake, I personally prefer giving both the Actual Dimensions and the Decimal value.






DVDs & The Exceptions: The Biggest Mistake AMVers Make!



A few of you may have noticed in the above example, DVD Resolutions are not mathematically 4:3 or 16:9 straight off of the disc. As such, the two most Common Types of DVDs you may encounter are:

- NTSC DVD's. Which have a resolution of 720x480 (North America)
- PAL    DVD's. They have a resolution of 720x576 (Europe, Australia...)

And this is how AR is reconciled. ---> via S t r e t c h i n g:


- So, DVD footage must be stretched (or squished) so that it has the correct AR during
  playback. A simple Resize is all you need (see the example cropping/resizing script below).
- But if you leave your DVD footage at 720x480 or 768x576 it will be wrong.


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Other file types may do this same stretch-during-playback-thing using an AR Flag. It is just a piece of information in the file that tells your computer "Hey! Stretch me during playback!" I don't really recommend this method for AMVs... however, there are some advantages explained below for those that are curious.



How to Crop & Resize: An Example



Crop(8,0,-8,-0)         # removes 8px black borders on left/right
Spline36Resize(704,396) # resizes to a smaller 16:9 resolution





Combining Different Resolutions



When combining Multiple Sources with different resolutions/AR's, it is best to crop/resize them so they all have a common resolution. For best results you should do this manually. However, for convenience, I have also written a function called BestFit() that you might be interested in trying out.





Beating a Dead Horse. And for those that want to get Technical.


AR (Aspect Ratio) typically refers to the ratio of width to height of a video during playback. The three AR's you will encounter the most are Fullscreen, Widescreen, and Extra-Widescreen (Scope). Numerically, those ratios would be 4:3, 16:9, and ~2.35:1 respectively. Examples of widths x heights that match these AR's would be 640x480, 1280x720, and 1280x544, if you just do the math/division.

An important note about AR: Some footage, particularly DVD's, stretch during playback to match one of the above AR's. In North America, this usually means footage starts out as 720x480 (which has a 'wrong' AR of 3:2), and it stretches to 720x540 for Fullscreen, or ~854x480 for Widescreen. This is how a DVD can be BOTH Widescreen AND Fullscreen.

As such, this is a very common mistake for beginners and seasoned AMVers alike. (And remember, this is only some footage; however, if it's from a DVD, this is probably the case.)

So then, how do you stretch your footage? Well, one way is to look for "Aspect Ratio" Settings somewhere in your editing program. (Do note, however, that some editing programs differentiate between three separate aspect ratios: the AR of your Footage, the AR of your Project, and in some cases, the AR of your Final Exported File too. If they all match, things usually work out the way you want, but sometimes it takes some experimentation to figure out what the program actually means when they say Aspect Ratio; sometimes AR can refer to the Pixel Aspect Ratio, PAR, which is the ratio of the Pixel's width to height, typically 1:1)

For me, I usually encode an entirely separate file resized to the correct AR so that I get exactly what I want, and I don't have to worry about AR issues, (among some other advantages like frame-accuracy and faster seek-times). The whole process can be a lot of work if you are not familiar, but that's what this guide is for ;)

However, in the end it usually boils down to two options:

1. Crop/Resize it with avisynth before/after editing, using the above scripts; or
2. Flag the final encode with the proper AR so it stretches during playback
(in Zarx264gui there's an Auto-Calculate option in the Advanced Tab)


Personally, I resize everything beforehand so that the AR of text/effects doesn't get ruined by AR stretch.


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Technical Note



Information presented here is slightly inaccurate since it doesn't take into account things like Active Picture Region. However, I would say the reasoning behind it is generally correct. And you will probably get the results you want for making a simple little AMV. However if you really want to take the technical route --more than I have covered here-- feel free to check out these links:


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This Guide is a Work in Progress. For more information use the RELATED LINKS.


TIPS:
  • DVD footage must be Stretched to the correct AR (Aspect Ratio) or it WILL BE WRONG.
  • If you must resize, try to do it so only the vertical or horizontal resolution is altered. This helps preserve the original quality.
  • Using heights & widths that are Divisible by 16 makes processing more efficient. (aka MOD16)


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1 comment:

AMVGuide said...

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