How do you re-encode h.264 video without artifacts?

Question

Hi
I have a video that was exported out of Premiere using h.264 at max 20mbps. All is good. Since the export takes 10+ hours I now want to create a version for Vimeo and Youtube. Becuase of the file size I don't want to upload the entire 2GB file to these services (I have a 5GB week limit upload at Vimeo) so I'll simply bring the file back into Premiere and export it using the Vimeo or Youtube setting with the max mbps around 5mbps or so. When I do this the resulting output file is unplayable, lots of artifacts and stutters. Some blacks are white even. I'm guessing Premiere is having an issue becuase it's re-encoding an already coded h.264 file, or something like that. Any ideas why this would happen and any solution to producing a good output?
Cheers.

Answer 1

As Dave stated, re-compressing already lossy-compressed video directly into another lossy compressed format is never a good idea: In this instance, image quality will degrade severely from that of the original AVCHD video. And all of the suggested intermediate files for Windows habe their disadvantages: DNxHD (and possibly UT) invoke the Apple QuirckTime API that induces a severe gamma shift in the encoded video (in the worst cases, whites come out medium-gray and middle tones come out pitch black) and is very slow to process due to it being restricted to 32-bit operations. Lagarith requires a massively powerful CPU - far more powerful than even a heavily overclocked i7-3960X - just to even play back well (this is based on my experience with Lagarith and my heavily overclocked i7-2600K, which dropped frames massively on playback of Lagarith-encoded material).