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Old August 7th, 2008, 10:01 AM   #1
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Wavy video when scaling HD to DVD

Here's the quick background of what I'm doing. I have some video shot with Canon HF100 (AVCHD 1920x1080, 60i) and some video shot with an older Sony DVD camera (mpeg-2 720x480, 60i). I'm combining them into one project in Vegas Pro 8.0b, and rendering to 720x480, 60i video (using Render As... / Mpeg-2 / NTSC DVD template).

Now, this part works like a charm, everything renders and looks great. Now, the HD video is 16:9, so it has black bars at the top and bottom in this final render, as expected. Fixing this is where I run into problems...

I used the "pan/crop" option to crop some of the sides of the HD video to get the aspect ratio to match the 720x480 video. I selected the "4:3 standard TV" preset, and then expanded it a little bit on the sides to get the proper aspect. When previewing this, everything looks great.

When I now render this video again (using the same options), the resized HD video now has very exaggerated wavy artifacts on the edges of moving objects. It's similar to playing interlaced video without de-interlacing, except the wave artifacts are at least 10-20 pixels high, not a single line as with the "combing" effect.

I'm totally puzzled here, I don't have enough video knowledge to even guess what is going on here -- the preview is fine, render without cropping is fine, but render with cropping has wavy artifcats.

Playing with different aspect ratios didn't make a difference -- choosing 4:3 preset and just having some black bars on the sides also produces the wavy artifacts -- it seems like any resizing of the original AVCHD does this.

Does anyone have any ideas or thoughts on what is happening here?

Thanks!
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Old August 7th, 2008, 11:49 AM   #2
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It sounds like the wavy lines are being caused by cropping. Rather than selecting a crop preset, I always use "Match Output Aspect". You said you were further resizing the crop to get it right and this may have introduced the problem. Using "Match Output Aspect" will always make the perfect crop.

Also go into the clip properties and select Force resample. This will make sure the video gets resampled properly at it's new cropped dimensions.

~jr
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Old August 7th, 2008, 01:35 PM   #3
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Thanks for your suggestions, John. I never noticed the "match output aspect" option, that's a good one to know. Unfortunately, other than making the cropping easier for me, it didn't do anything else :(. I tried forcing the resampling, too -- each of two options individually, and both together, and I still get the same wavy artifacts on the edges of moving objects.

I guess it must be related to interlacing somehow, I just can't figure out what the cropping/resizing has do with it. It certainly previews fine even after the cropping.
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Old August 7th, 2008, 02:02 PM   #4
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Hmmm... if it looked like an interlacing field order problem, I'd say the crop might start on a lower field for an upper field first video which would throw the field order off, but you said they were big wavy lines so that doesn't sounds like a field order problem. Still, you could try moving the crop up or down one pixel (using the data entry fields on the left). Other than that, I'm out of ideas (sorry).

~jr
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Old August 7th, 2008, 02:12 PM   #5
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Well, it looks like I figured it out. What seems to be happening is the every time there's a resize, Vegas de-interlaces the video, resizes it, and then (as I'm rendering interlaced video) interlaces it again. I didn't expect this, as I thought it would stay in the "interlaced" domain -- you don't really need to de-interlace video to be able to scale it (unless I'm missing something).

I had my de-interlace method set to 'none' in the project, as I didn't expect to need any de-interlacing, so that's the method Vegas used prior to resizing. The resizing is what caused normal interlacing artifacts to "grow" into thick lines that I'm seeing.

Once I enabled the interpolation in the project properties, I got the rendered video to look fine.

I still find it strange that Vegas needs to de-interlace to resize/crop, though, I wonder if that's due to some other setting that I have somewhere...
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Old August 7th, 2008, 10:24 PM   #6
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Glad you found it. I never though to ask if you change the project properties from the defaults. Deinterlacing before resizing makes sure you don't drop any fields in the resize step.

~jr
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