crop and resize for MKV encode?

[spiritgumm]

I'm a novice at making MKV (x264), and am following a guide for Megui. I have a 4:3 dvd video clip with 12 pixel black side borders, and thought I should crop them (no resizing after). I make my scripts in AvsP. I did a test encode with Megui SAR (8:9). Mediainfo DAR for that encode says 1.289.
I encoded again with other SAR (10:11), Mediainfo DAR was 4:3.
I'm confused about which is correct. I thought we're aiming for the 4:3 profile. But I dont want the video stretched out simply to fit a 4:3 box. Which will display the cropped video accurately?
I took screenshots with MPC-HC (with EVR video setting, since that's usually more accurate than System default), and the 4:3 encode is wider (because the SAR ratio was larger, right?) than the 1.289 encode.
I also encoded the original uncropped video (720, 4:3) and compared screenshots. I thought perhaps 4:3 was a specific display size that video would conform to, but the 720 4:3 was wider than the 696 4:3. Which makes sense, as SAR settings are just make existing video bigger or smaller. I guess 4:3 isnt a pre-ordained size.
Long story short - it looks like I dont need to resize (at least in this case)? And I would choose the SAR which makes a DAR show the actual ratio rather than shooting for 4:3?
Btw, does format differences between PAL/NTSC require any resizing?

[spiritgumm]

a kind soul mentioned that the SAR could be determined by opening the Resize Tool in AvsP, so that solve's that question.

[hello_hello]

Which guide are you following for MeGUI? I'm not sure I've seen one which wasn't hopelessly out of date at best, or sometimes so out of date it wouldn't be hard to mistake it for a guide to a completely different program.
I've considered writing one myself as I use MeGUI regularly, but a decent guide would be fairly time consuming.

MPC-HC saves it's screenshots using a 1:1 SAR regardless of the real DAR/SAR, so you can't really compare aspect ratios that way (assuming you were saving screenshots from MPC-HC's File/SaveImage menu and not using the printscreen button on your keyboard). Open a 4:3 or 16:9 DVD with it and save a screenshot. It'll do the same thing. You'll end up with a screenshot with a 720x480 resolution either way for NTSC and the aspect ratio will be 1.5:1.

There's two resizing methods for DVDs. The ITU SAR gives you a slightly wider than 4:3 aspect ratio. Most 4:3 DVDs use the ITU aspect ratio (in my opinion). For NTSC it's 1.367377 rather than 1.333333. Likewise the ITU aspect ratio for 16:9 DVDs is slightly wider than 16:9, but 16:9 DVDs tend to use 16:9 resizing (there's no real way to know for sure which method it used aside from what looks right, and it can be hard to tell).
Have a look here. MeGUI's script creator uses the second (ITU) table if you select an ITU InputDAR. If you don't, it uses the SARs in the fourth table. The script creator will default to using ITU (or not) according to the "Use ITU' option in MeGUI's settings, but you're free to change the Input DAR yourself.

If I'm encoding 4:3 DVDs I usually aim for a 4:3 output aspect ratio. Therefore if I crop from the sides, I also crop an appropriate amount from the top and bottom to return the aspect ratio to it's original state (or exactly 4:3) without distorting the picture. Often it means cropping a bit of picture, but there's no way around that if you want 4:3. In your case if you used the ITU Input DAR (1.367377), 12 pixels from each side only requires cropping 2 pixels from the top and bottom. What's left would be 13497:10126, according to MeGUI, which is 1.3329. Very close to 4:3.

Think of cropping and resizing this way.... if you had a picture with a (imaginary) 10:4 aspect ratio and cropped a total of half of it from the sides, what's left would have to be 5:4. If you then cropped half the height from the top and/or bottom, you'd have an aspect ratio of 5:2, or 10:4 again. Therefore if all you do is crop, the SAR doesn't change because you're changing the display aspect ratio. If all you do is resize, then the SAR must change so the DAR remains the same. Obviously if you do both, then both the SAR and DAR will probably change.

If you open a DVD with MeGUI and index it, it'll open the script creator for setting up an encode. When anamorphic encoding is enabled, the DAR is shown at the top of the script (under the script tab). As you crop that'll change accordingly. Once the script is saved and loaded into the video section for encoding, MeGUI uses the DAR in the script along with the resolution after any cropping and resizing to calculate the correct SAR for encoding.

And having said all that.... I don't use anamorphic encoding any more because not all hardware players support anamorphic video in MKV/MP4 files. I resize to square pixels instead so I don't need to worry about my encodes displaying using the correct aspect ratio (it's all personal preference). MeGUI's script creator will calculate the aspect ratio distortion when you resize to square pixels so it's easy to resize correctly. I'd usually resize 4:3 NTSC to 640x480.

[jagabo]

DVD only specifies the display aspect ratio and the frame size. There are only two display aspect ratios: 4:3 and 16:9. Commercial NTSC DVDs almost always use a 720x480 frame size. You can calculate SAR with the simple equation SAR = DAR / FAR. For a 4:3 DAR 720x480 DVD:

SAR = DAR / FAR
SAR = 4:3 / 720:480
SAR = 4/3 * 480/720
SAR = 1920 / 2160
SAR = 8 / 9
SAR = 8:9

That's at odds with the ITU spec for digitizing analog sources with a 704x480 frame to hold the 4:3 DAR:

SAR = DAR / FAR
SAR = 4:3 / 704:480
SAR = 4/3 * 480/704
SAR = 1920 / 2112
SAR = 10 / 11
SAR = 10:11

But ITU video is usually captured with an extra 8 pixels at the left and right edges (720 pixels total) so as not to lose any of the picture in case the recording is slightly off-center. The SAR remains 10:11 and hence the full 720 pixel wide frame represents a picture slightly wider than 4:3.

When studios master DVDs from analog video sources they usually use their standard ITU capture devices and ignore the difference between MPEG (DVD) SAR and ITU SAR. They just write the 720x480 frame to DVD and label it 4:3 DAR -- because nobody really notices the ~2% difference. DVDs which don't come from analog tape sources tend to use MPEG SAR, not ITU SAR. The only way for you to know for sure which SAR is correct for any particular DVD is to measure something of known aspect ratio within the video. Or do what the studios do, ignore the difference and pick one or the other.