Does clipping throw off a video's AR?

[Tropican]

I am clipping in DGindex, and occasionally I have to clip a tiny line or two of video in order to completely remove letterboxing. When I resize later will the aspect ratio be slightly off?

Thanks,
Tropican

[neuron2]

It's possible, but unless you specify the clipping and resizing that you are talking about, there's no way for us to determine the effect on the aspect ratio. Let's just say that it is possible to preserve the aspect ratio, but you may have to clip the left and right as well.

If you're clipping only a line or two, the effect on the AR may be imperceptible and thus acceptable.

[Cyberia]

I don't believe clipping in DGIndex will affect the aspect ratio, at least not from a DVD source.

It will affect the 'literal' aspect ratio (ie: width/height) but that's not what matters for aspect ratio. The ratio that really matters is 'pixel aspect ratio' and cropping by any amount in DGIndex won't affect the PAR.

[Wilbert]

It will affect the 'literal' aspect ratio (ie: width/height) but that's not what matters for aspect ratio. The ratio that really matters is 'pixel aspect ratio' and cropping by any amount in DGIndex won't affect the PAR.
Cropping will change the aspect ratio, unless the par is 1:1.

edit: Cyberia is right here, cropping doesn't change the PAR.

[Cyberia]

Originally posted by Tropican
I am clipping in DGindex, and occasionally I have to clip a tiny line or two of video in order to completely remove letterboxing. When I resize later will the aspect ratio be slightly off?

Thanks,
Tropican

I don't want to get into an AR debate because I believe neuron2 and Wilbert and I are on the same wavelength, but if you crop in DGIndex you may have to adjust your resizing to account for the cropped stuff.

A line or two won't make any perceptable difference though.

[Tropican]

Thanks everyone, I was only concerned about 1-line clippings and perceptual Aspect Ratio, if no one can see it than it does not matter to me.

Again thanks,
Tropican

[ukb008]

Why restrict to 1- or 2- lines only? What about cropping more than that?

Many of us do come across DVDs wrongly authored (home-made ones often do). Error of interchanging between 4:3 and 16:9 are easier to correct, perhaps. More subtle errors (e.g., 1.78:1 when it should be 2.35:1) are real headaches.

Maybe what we should learn is

1] How to preserve the picture frame proportions after any amount of crop
2] How to correct small but perceptible AR errors in VOBs by means of DGIndex, if possible.

Regards.

[Cyberia]

Originally posted by ukb008
Why restrict to 1- or 2- lines only? What about cropping more than that?

If you are going to crop before resizing then you need to know what type of video you are starting with, because each video type uses it's own pixel aspect ratio (PAR). Here is a list of common PARs:

NTSC DVD = (72 / 79)
NTSC SVCD = (108 / 79)
NTSC VCD = (72 / 79)
PAL DVD = (128 / 117)
PAL SVCD = (64 / 39)
PAL VCD = (128 / 117)

If you have an Anamorphic DVD, then multiply the PAR by (4/3)

Now calculate the aspect ratio after cropping and multiply by the PAR:

AspectRatio = CroppedAR * PAR

This is your final aspect ratio, so whatever your resize dimensions, make sure you honor this aspect ratio and you'll be fine.

[ukb008]

Yep, it's more generic.

For a given source (e.g. NTSC DVD), the PAR is constant, so that

Aspect Ratio ∞ CroppedAR

which means I can crop to get the AR I want.

Regards.

[Tropican]

Thank you Cyberia. I will always make sure I do that when cropping before a resize.

-Tropican