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Why does MPG change CUTS into FADES?
Gentlemen,
I have asked this question a couple of times here buried within other threads, and never gotten a satisfactory answer. So I will ask it in its own thread in hopes that someone can address it.
WHY does a render from an HDV project into a DVD Architect Widescreen 24p .mpg file turn CUTS into FADES?
To repeat: when I make the file, it renders some (many) of the hard cuts within the ~70 minute project as quick FADES.
You can SEE the fade being built into the .mpg in the preview file as it renders slowly, and when you view it at speed it gives me this sort of “ghostly” effect where the new is visible for one frame before the cut and then the old frame is visible for one frame after the cut.
First I thought this might be a property of DVD video that I had not noticed before.
However, I examined several commercially released DVDs and found that that this ghostly fade does NOT occur, even when the playback is slowed to 1/10th speed.
Clearly, this fade is some form of compression or way to save space. Is there ANY way to turn this off so that my cuts are clean, as in motion picture film? One image onscreen, CUT, next image onscreen, no fade, no ghost?
EVEN when I render the project as .AVI, or as a 9,800,000 constant bit rate mpg, there is STILL the fading problem!
Any and all help will be appreciated.
Some info.
1) I have just upgraded to Vegas 9.
2) I don’t know if this is a codec issue, I have only the codecs that came with Vegas and also quicktime.
3) I am running a fairly decent PC, (twin 3.2GHZ pentium I believe), and my video card is an ATI 256MB X1300 PRO. Is my video card the problem? Do I need to upgrade to a better one to get a fade-free render?Thanks to all for help. This is a project I have been working on for several years and am finally ready to be “done” with it, however I am unwilling to release the product with this cut-fading problem.
Best,
R Gould -
2 Replies
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> Clearly, this fade is some form of compression or way to save space. Is there ANY way to turn this off so that my cuts are clean, as in motion picture film?
Very observant. What you are seeing is the way that MPEG2 compression works. It is assembled in a Group of Pictures (GOP) that contains 15 frames. The first is an I-frame and it contains a complete image. The next 14 frames are composed of B-frames and P-frames which are delta frames and predictive frames. These only contain the information that has changed since the last I-frame. If you make a hard cut on a B or P-frame, the encoder will probably not give you enough bits to represent an entire frame so you watch that frame fill in over several frames. This is the nature of the MPEG2 GOP.
There are two ways that I can think of to fix this. One thing you can try is rendering with the 2-pass option. This will allow the encoder to be smarter and use a higher bit-rate for the frames that need it for these cuts. The other is to only make cuts on I-frames which gives the encoder the entire frame to represent the cut. To accomplish this, you could try setting your timeline ruler to Frames. Then only make cuts on frames that are evenly divisible by the GOP length (15).
I have not tried either of these to fix this problem so let us know how it works.
~jr
http://www.johnrofrano.com
http://www.vasst.com -
There is a third option too, which is that you can drop I-frames on the last frame of the one clip and the first frame of the next, so you get two I-frames in a row, sandwiching the cut. That clearly bumps the data rate way up momentarily and breaks the cadence, but it shouldn’t cause problems.
I’m afraid the last time I remember doing this was a decade ago, so I don’t even know what tool you’d use today, but advanced compression apps have an automatic “scene detection” algorithm that will sometimes take care of this automatically as well as the ability to drop compression keyframes (that’s what the app I used called them) wherever you want. Sorenson Squeeze might be something to look at? Sorry I don’t have more info other than to get you started looking for an answer, but this is the sort of stuff a professional DVD compressionist in Hollywood would do full time.
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