The short answer is that Adobe discourages it becuase with certain codecs there is generation loss. PrP exports differently then FCP, just one of many differences. Some good, some not so good.
Here is an entire thread where I explored this issue as what I thought might be a way to speed up the exporting process, which ended up not being the case.
The preview files are made of lesser quality hence the advice not to use them.
The are great for a quick test file, when quality is not a issue.
Ann, thats only partially true. I did a test where i designated Prores as my Preview file. I then outputted using those preview files.
Then I exported my timeline doing a regular export to a Prores file.
I then took both files and following Colin's advice put them on 2 tracks in PrP and selected Difference Blend for the top track. The result was a blank screen, indicating both images cancelled out each other completely , thus were identical.
If I had chose MPEG I frame (a more primitive compression) as a preview file to use for output, then you would be correct in your statement.
I have no real basis for choosing.
If you're preview files are lossless, you're fine using them for export. If they are not lossless, then you're probably better off not using them.
But as dicussed in the thread quoted below, the only advantage in using Preview files is if you have non-CUDA rendered effects in the sequence. Unlike FCP, PrP doesnt use "Smart rendering" which is the process of stitching all the preview files together. According to Adobe, when exporting (using or not using previews) the entire sequence is rebuilt for output. Heres that quote from Wil..
Wil - "What you're looking for is what we commonly refer to as smart rendering, and it doesn't exist in PPro. That's a Final Cut 'ism, where it splices rendered data from the preview QT files into the final render. Keep in mind that's a QuickTime centric feature, & doesn't translate automatically to all file formats.
The 'Use preview files' option in PPro is exactly that - if there's a preview file, it decodes the frame out of it, & re-encodes it into the final file. So while it'll help in (non-CUDA) effects heavy compositions, it also incurs a generation loss. Generally, we don't recommend using it (hence the default to off) unless you really are sure that's what you want."
So if your preview codec is lossless and you have rendered lots of non-CUDA effects on your timeline you could save some time using this process.
Thanks you guys - I'll have to read your answers a few times and think about it all for awhile...I may have a follow-up question or two.
Meanwhile - thanks again!
PrP doesnt use "Smart rendering" which is the process of stitching all the preview files together.
It does for AVI files, which is what lossless previews will be on the PC.
Well Jim, if you choose to disagree with the folks that make the software....good luck with that ;-)
I've always felt more comfortable believing what I see over what I hear (or in this case, read).
For example, Todd stated that hardware acceleration isn't used at all when we queue to AME. It's only used internally by Premiere Pro, which means a Direct Export. That turned out to be incorrect, and running a couple simple tests confirmed that hardware acceleration is indeed available through AME when exports are queued.
Likewise in this case, I choose to form a conclusion based on my own tests. Preview files ARE used to speed up the export process on a Windows machine when they are in AVI format and the box to do so is checked.. This has been true for DV for a long time now, as I'm sure others will attest. (And if this were not true, there'd be no need to caution against generational loss.) The Lagarith codec and UT codec also work. Probably others as well, but those are the three with which I have personal experience. (I just don't preview in any other format.)
So for a Windows user, my advice stands. If the previews are lossless, go ahead and use them. Things will export quicker, and you won't suffer any unnecessary generational loss. If they're not lossless, you're probably better off not using them for export.
Thank you for the spirited debate over the question of using preview files for output.
It sounds like there is a good reason for choosing an uncompressed codec to produce the preview files: there is no degradation if they're used in the final render. This assumes the workstation can handle the data crunch and give the user a satisfactory editing experience.
re: CUDA-enabled effects, preview files and render time - I'm not sure if this correct but here's how I understand the discussion:
Whether the preview files shorten render time is dependent upon the effects used, specifically whether or not they are CUDA-enabled. If the timeline uses only CUDA-enabled effects the render won't finish any quicker by using the preview files. If only some are CUDA-enabled the best course is to use the preview files, provided they've been generated using a lossless codec. There is no "speed penalty" for using the preview files.
Have I got it?
Thanks again for your contributions.
Based on information supplied by Adobe, that sounds about right. If you have a lot of dynamic linked AE files that have already been rendered in the preview files then you save some time there (since they wont be rendered on export, just the preview file will be decoded and recoded as a part of the final export file.