About aspect ratios

[cromiX]

Hi!

I'm new in the DVD backup and video editing world. I've been reading Doom9's guides and other stuff to learn more about it.

But one question came up to me when I was reading one of these guides.
In the AVI stretching guide, they start the project with a source that has a resolution of 640x480. They want to convert it to a NTSC DVD, which has a resolution of 720x480.

But let's assume that we have a video with a source that has a resolution higher than 640x480 (4:3 NTSC DVD playback resolution): let's say something approximately the same resolution as DVD resolution (710x480). And we want to convert it to PAL (it would be 720x576). What should we do?? :confused:

1- What would be the best output resolution? Should we stretch it from 480 -> 576, and 710 -> 720 too?

2- If we do that, I think that we wouldn't need to resize it after the stretching and black bar filling (if we need to do that, too).

Could you please explain how we should do? I'm sorry about my English, I haven't been praticing in a while :) Thanks

[wildejedi]

I'm fairly new to this myself but I would say that the best output resolution would be 720 x 520. That is calculated using the basis that PAL is actually 768x576. DVD's need to be 720 but will automatically strech the film during playback.

768 div 710 = 1.0816901408450704225352112676056 x 480 = 519.2. Round this to the nearest even number gives you a height of 520

Hope this helps

[Skelsgard]

The key element is not resolution but aspect ratio (AR). When a given "A "frame size into another "B" frame size, u would want to keep the original aspect ratio in this "container".
710x480 is a frame size, wich can have a given AR that would make it play at resolutions that are independant from the frame size.
If the 710x480 frame is a 4:3 DAR, just stretch to 720x576 with a 4:3 DAR flag.
If 710x480 is a 1:1 DAR instead, then is a 710/480=1.48:1 AR.
To fit this into a 720x576 4:3 DAR, u would calculate:
4:3 DAR means a 768x576 frame size. When u scale 710x480 to 768, the result will be 710*576/768=532.
That will leave u with a 720x532 frame size, so u´ll need to letterbox (576-532)/2 = 22 pixels, on top and bottom to reach 576.
In this case, I use 576 (width size) as the frame is larger in width than height (so when stretching, the width will be "filled" first, before the height is fully reached).
Cheers.

[cromiX]

Thanks for your replies. :)

So, basically, what one should do, according to Skelsgard's calculations is:

1 - Pick the video that we want to resize (710x480);
2 - Then stretch it to the corresponding with and height (720x532);
3 - Finally, black filling the empty spaces (2 x 22), resulting in 720x576 pixels.

http://img129.imageshack.us/img129/6935/videooriginalpf4.th.png (http://img129.imageshack.us/my.php?image=videooriginalpf4.png) http://img207.imageshack.us/img207/9567/videostretchingps4.th.png (http://img207.imageshack.us/my.php?image=videostretchingps4.png) http://img207.imageshack.us/img207/1181/videowithblackbarssp3.th.png (http://img207.imageshack.us/my.php?image=videowithblackbarssp3.png)

So, if we do this, we won't need the BicubicResize line in AviSyth right?

[Skelsgard]

Yes, but remember that only if 710x480, besides being the real frame size, is also the intended display size and aspect ratio (1.48:1) (as in the video being displayed at 710x480 is neither vertically nor horizontally stretched).

So, if we do this, we won't need the BicubicResize line in AviSyth right?
No, the BicubicResize is a resizing method, as there are others, like BilinearResize, Lanczos3Resize or Lanczos4Resize.
You WILL need one of these, wether BicubicResize or any other (I believe u might have the impression that bicubic means something different, just a thought, don´t be offended) if u plan to change the video size.

This is an example of what u would have in the script, for what u need:

AviSource("clip.avi")
ConvertToYV12()
Lanczos4Resize(720,532) <-- I use Lanczos instead of Bicubic, is a little sharper, but it´s only a choice.
AddBorders(0,22,0,22)

Cheers.