Unwanted moderate distortion of aspect ratio when exporting video

Question

Unwanted moderate distortion of aspect ratio when exporting video

This seems to be a new problem for me, and I don't THINK I've changed any of my parameters. But the result is now wrong... Help!

I use my VCR to record from US broadcasts (good old fashioned plain old TV, NTSC, I presume). Then I transfer snippets onto my DV camcorder (good ol' plain DV, nothing with 16:9 or anything like that). Then I import my DV footage into iMovie HD 6.0.2. It imports fine and looks like a normal aspect ratio.

But when I export using the File > Export... setting, I get a Quicktime video which appears vertically squished. The faces are too wide, bodies are too fat.

Looking at the various aspect ratios, I see that the preview window in iMovie HD is about 720 by 525 -- there are no black bars or letterboxing, and faces and bodies look normal. But the output video is 720 by 480, which is (according to a quick Google search) a normal NTSC output. No cropping has taken place, and no letterboxing, either. So that's why it's squished. But...

So why is iMovie showing me something from right off my TV screen which is the wrong aspect ratio but seems to have the correct proportions? That would seem to imply that my good ol' TV and VCR are not actually operating in NTSC proportions... which would be extremely weird because the footage I happen to have recorded is from an old game show from the 1960s. Do I have some setting wrong somewhere?? Is there something fundamental I don't understand? Help! Like I said, this is a new problem with me and I don't recall having changed any import settings.

Mini, Mac OS X (10.4.6)

Posted on Aug 1, 2006 1:49 AM

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Answer 1

Aug 1, 2006 2:29 AM in response to James Weinrich1

But when I export using the File > Export... setting, I get a Quicktime video which appears vertically squished

Obviously you used the Full Quality DV option which uses rectangular pixels, right?

What you are seeing is normal... 720x480 NTSC rectangular pixel DV should be scaled to 656x480 square pixels.

New versions of QT Player automatically display the (almost) correct aspect ratio of .dv clips.

For more info,check:

http://www.iki.fi/znark/video/conversion/

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Answer 2

Aug 2, 2006 8:20 AM in response to Matti Haveri

Thanks for your help! I had no idea that so many video processes use rectangular pixels. But I'm still confused. What I see on screen in iMovie (and most QuickTime stuff) looks fine, but what I got out of this last set of conversions looks wrong.

Obviously you used the Full Quality DV option which uses rectangular pixels, right?

Uh... I have no idea. I've spent a couple of minutes looking through the menus in iMovie and I don't see anything labeled "Full Quality DV". I'm not specifically trying to export in DV format; I'm trying to compress my NTSC format so that it plays back in nearly full size on my iBook laptop, which cannot keep up with Sorensen video or the H.264 (or two-sixty-whatever) codecs. I found that Cinepak CAN be played back at this size on my iBook. But Cinepak is a compression standard, not a video frame size standard, right?

What you are seeing is normal... 720x480 NTSC rectangular pixel DV should be scaled to 656x480 square pixels.

No, it's not normal. It doesn't look normal to the eye. It's not what normally happened to me in the past. It drives me nuts. By those three definitions, it's not normal.

I am not trying to export in a format that fits some video standard or TV set. I'm trying to export a QuickTime movie which will display properly when played back by QuickTime. So I'm extremely frustrated when iMovie (or something) takes a video clip from an NTSC source and for some odd reason squishes it.

When you say that "720x480 NTSC rectangular pixel DV should be scaled to 656x480 square pixels", I assume you mean that it should be scaled to that number of square pixels in order to display on a computer monitor with square pixels (i.e., basically all computer monitors) in a way which makes faces and bodies appear of normal proportions, right? Are you implying that the problem is that rectangular pixels are NOT being converted/resampled to square pixels, but that the conversion is being done on a (no-resampling) pixel-by-pixel basis? I'm just not sure I understnand the point you are making here.

Since I had different results in the past, there must be SOMETHING different about my current setup. After reading the link you suggested, I see that there are a HUGE number of possible complications out there. The main idea, however, is that I want to produce a QuickTime movie which looks the same as the preview shown in iMovie: Same shape of faces, same shape of bodies, just as it was when I originally viewed the footage on my television.

Thanks!

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Answer 3

Aug 2, 2006 8:34 AM in response to James Weinrich1

Actually, here's the crux of the problem.

In iMovie, the picture looks exactly right. When I do a screen capture and look at the resulting document in Preview, Preview tells me the size of the image is 720 by 525. But when I choose Share... from the iMovie menus, then go to the Movie Settings box, the "Current" choice for Size says the Current dimensions are 720 by 480!

This, obviously, is what is doing the squishing. Why does this box tell me the size of my Current clip is different from what it obviously is? If this box is talking about non-square pixels, then that is probably the problem; it's squishing 525 scan lines into 480 scan lines... it's turning rectangular pixels into square pixels without resampling... right?

But iMovie, obviously, knows how to display this video correctly. I'm not trying to fit anything into anything; I just want to produce a QuickTime video which turns an image which is 720 by 525 into something which is 720 by 525.

Square, computer-monitor pixels.

I'm going to go try chainging the Export Size Setings box from Current 720 x 480 to 720 x 525. But NOW I'm worried that I'll be introducing an extra degradation of image quality if, for example, the correct number should be 524 or 526 or 520 or something. The image size reported by Preview reflects the size of my screen shot, and I am not sure I told the screen capture utility exactly correctly where the iMovie image's "true" edges are.

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Answer 4

Aug 2, 2006 9:34 AM in response to James Weinrich1

No, it's not normal. It doesn't look normal to the eye. It's not what normally happened to me in the past. It drives me nuts. By those three definitions, it's not normal.

That not quite what Matti said. He's not saying your Full Quality movie looks normal. He's saying that it's normal for a Full Quality DV movie to look that way.

Let's back up a minute. iMovie's use of the term "Full Quality" as an export option is a bit confusing. (Thousands have preceded you.)

The term means that iMovie will export the project to a QuickTime movie that retains all the quality of the original source material. There will be no reduction in quality — it will be full quality. You can import the full quality movie to another iMovie project, edit it in QuickTime Pro, or use it in QT Pro to export to a smaller compressed version.

It does NOT mean a Full Quality movie is suitable for sharing with others. For one thing it's usually way too large. Then there's the pesky playback issues like the one you mentioned. (There are several others too that haven't been raised.)

But to preserve the quality, the DV movie isn't focused on looking good. It's focused on preserving all the quality internally so it can be used elsewhere. The problems you see are handled internally when the movie is imported to iMovie, for example,

(Historical note: This large, unwieldy, DV movie was impossible to play on yesterday's Macs. The movie was designed as a transfer vehicle, not suitable for playing. Today's processors are powerful enough to play the movie, which indirectly adds to our confusion. When we play a Full Quality movie we understandably focus on its obvious playback deficiencies. In the past, the Mac would just choke, unable to play the movie at all, or not play it smoothly.)

For sharing your project with others, there are better export formats. Those that use the H.264 codec, for example, both compress the movie and offer excellent playback. That's the default iMovie 6 export codec for the other choices offered. You can pick one of those, or use Expert Settings to configure your own H.264 settings.

Karl

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Answer 5

Aug 2, 2006 11:25 AM in response to James Weinrich1

When you say that "720x480 NTSC rectangular pixel DV should be scaled to 656x480 square pixels", I assume you mean that it should be scaled to that number of square pixels in order to display on a computer monitor with square pixels

Yes, 720x480 NTSC rectangular pixel DV should be scaled to 656x480 so it appears normal on a square pixel computer monitor. You can do that by entering those figures to the export options, but often the exporting app may automatically scale it for you if the used export codec commonly uses square pixels. The new version of QT Player also scales its display automatically with DV material.

That 656x480 can then optionally cropped to 640x480. BTW, many apps cheat by scaling 720x480 straight to 640x480 but luckily the error is so small that it goes unnoticed unless looked for.

AFAIR iMovie uses some other similar scaling figures behind the scenes (720x540, 720x525 or something like that). But this is only cosmetic because the underlying video is 720x480.

When you export, do notice that for progressive material it is OK to scale vertically, but with interlaced material it is best to scale horizontally (or deinterlace) because otherwise interlacing is badly distorted (unless interlaced scaling is used). So usually it is best to leave 480 lines intact and scale the 720 figure instead.

I hope that made sense. This is just one of the gotchas in video work 😉

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Answer 6

Aug 2, 2006 1:09 PM in response to Matti Haveri

Thanks, Matti! I guess I'll just have to enter the export dimensions manually. I still don't understand why my earlier ones exported at native size without my having to specify anything, and why my recent ones naturally export in the distorted, squished way. A software update or something??

Again, just for clarification, my question had nothing to do about DV format. I was not trying to export to Full DV or anything specifically DV or Old Fashioned Plain Old TV or Weird French PAL or anything... just wanted to get a QuickTime video that looked like the one on screen in iMovie, that's all. I don't care how QT represents it internally and/or if that representation has anything to do with anything else; I just wanted to get QT to do to my footage what it had done perfectly well with others before.

By the way, in response to another poster, I have not been able to get my iBook to play back H.26whatever without HUGE frame dropping at normal-ish video sizes. The tiny 320 x whatever works OK, of course. Cinepak is the only non-humongoid codec that plays back well at 640-ish size, in my experience.

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