Cropping 720 => 704 but what about top-bottom? (GV-USB2)

[Jokris]

Hello

I am using a GV-USB2 to capture a 720x576 PAL video, here is a snapshot.

[Attachment 73910 - Click to enlarge]

I have read about cropping before and understand that it is recommended to crop down to 704x576, then resize up to 768x576 in order to get 4:3 ratio.
In this case cropping 4 pixels and 12 pixels right seems about right.

However, there are also black borders at the top and bottom of the video that I want to get rid of. It looks like 1 top and 3 bottom pixels
Cropping that out would mean 704x572

Would it be recommended to crop down to 704x572 and then resize up to 768x576 ?

[Sharc]

Caution.
Your video is interlaced, most probably YUV (YUY2) 4:2:2.
- cropping vertically an odd number of pixels will destroy the original interlaced structure (in best case it just reverses the field order, like in the 1,3 case)
- vertical resizing of interlaced video will also destroy the interlaced structure (independent from cropping)

So deinterlace first, then crop and resize correctly as needed such that the aspect ratio is preserved.

[Jokris]

Thanks for pointing that out.

Will cropping in VirtualDub2 reduce quality of original file?

[Sharc]

Cropping as such does not reduce the quality. It just crops the picture. For VirtualDub2 same cropping rules apply. Maybe it throws a warning when cropping interlaced footage incorrectly, maybe not. I don't really remember.
Avisynth+ catches most of wrong cropping attempts (except the reversal of the field order).

Here the cropping rules:

For YUY2 4:2:2 source (your capture most likely):
horizontal: mod2
vertical: mod2 for interlaced video, no restriction for progressive video

For YV12 4:2:0 source:
horizontal: mod2
vertical: mod4 for interlaced video, mod2 for progressive video

With mod4 for all sides you are always on the safe side.

Resizing:
No vertical resizing of interlaced video. Deinterlace it beforehand.

P.S.
mod2=0,2,4,6,8 .... = integer multiple of 2
mod4=0,4,8,12,16... = integer multiple of 4

[Sharc]

Would it be recommended to crop down to 704x572 and then resize up to 768x576 ?

When you deinterlace and crop to 704x572 and then resize to 768x576 you will stretch the picture a tiny bit vertically (about 0.8%). Practically meaningless, just nitpicking

[Alwyn]

When you deinterlace and crop to 704x572 and then resize to 768x576 you will stretch the picture a tiny bit vertically (about 0.8%).

Take an additional 2 pixels off each side.

[Jokris]

You mean 700x568 would be correct?

[Sharc]

700x572 (cropped) -> 768x576 (resized)

[Skiller]

Basically, if you want to have the final video play at exactly 4:3 and not introduce any (tiny) error while doing so, you want the frame to keep the 704:576 ratio (which is 1.222...:1) after cropping.
This (and the chroma subsampling requirements) means you will almost always have to overcrop on purpose a bit.


704:572 ≈ 1.231 (slightly off)
700:572 ≈ 1.224 (good, error is only about 0.9 non-square pixels)

[Sharc]

Personally I don't care much about exact 4:3 Display Aspect Ratio and resizing to square pixels. I crop as needed (according to the cropping rules for interlaced cropping) in order to fully remove the crud from all sides, optionally pad to a standard resolution like 704x576 (for PAL), and encode with x264 using the SAR (Sampling Aspect Ratio) 12:11 (for 4:3 PAL) in .mp4 container, and let the player do the rest.
It's not a recommendation, it's just my preference. I usually don't even deinterlace. I trust the deinterlacer of my TV.

[_Al_]

Same old, same old, crop then resize ...... you do not need that in majority of cases, like in your case

simply as a first step, back up those avis!, then you can deinterlace video, cut 4 left 12 right or whatever works (total 16), resize to 4:3 horizontally only to the same height 768x576. You have exactly the same aspect ratio you started with. I recommend not to crop those lines on top and bottom at all afterwords, no one will notice, guaranteed. If you for some human OCD reason want to crop after resize (I understand that could be a real reason for a human) and get rid of that black lines on top and bottom , ( mind you , then in the future resizing of that video would introduce an ar error). You would take out two lines from top and 4 from bottom. Sharc explanation says you'd need to take 4 pixels in both cases if video is interlaced, not sure about that now, as I said I do not do these tiny crops at all, have no slightest residue of ocd whatsoever luckily

This whole cropping stuff before resize is overrated , all over web and not even necessary in majority of cases.

[Sharc]

Sharc explanation says you'd need to take 4 pixels in both cases if video is interlaced, ...

I said mod 2 for 4:2:2 interlaced and mod 4 for 4:2:0 interlaced sources. The OP's capture is very most likely 4:2:2 interlaced so he can crop mod 2.
If one has no clue what the source format is, one is on the safe side with mod 4.

This whole cropping stuff before resize is overrated , all over web and not even necessary in majority of cases

Well, what one considers necessary is a personal decision. Is resizing necessary? Is deinterlacing necessary? It depends what we want or need. Cropping makes sense when the active picture's borders are not clean but fuzzy. Sometimes there are color distortions (decolorization) close to the borders, or the existing borders are noisy or greyish rather than solid black. For some it is more annoying than for others. Similar with color shifts 601 <-> 709, the infamous 2% AR distortion, and so on.

[Leanoric]

optionally pad to a standard resolution like 704x576 (for PAL)

Sharc, is to pad, the same thing as adding black borders after cropping?

[Sharc]

optionally pad to a standard resolution like 704x576 (for PAL)

Sharc, is to pad, the same thing as adding black borders after cropping?

Yes.

[lollo]

Is resizing necessary?

Not at all

Is deinterlacing necessary?

It depends on who is doing the better deinterlacing between the software and the player.

For both questions, we are repeating the same concept since years...

[Skiller]

When encoding old analog-originated footage for viewing on modern devices, I like to emulate overscan to have the video look similar to what it would like on a CRT. Remember, even when the video was originally shot, in the view finder of the camera there was overscan. It was never meant to be watched without.

Therefore I like to crop to the center 660x540 area and flag it with a 12:11 PAR. 50p deinterlacing with QTGMC and x264 for encoding.

[Leanoric]

When encoding old analog-originated footage for viewing on modern devices, I like to emulate overscan to have the video look similar to what it would like on a CRT. Remember, even when the video was originally shot, in the view finder of the camera there was overscan. It was never meant to be watched without.

Therefore I like to crop to the center 660x540 area and flag it with a 12:11 PAR. 50p deinterlacing with QTGMC and x264 for encoding.

Interesting, I will try this.