Like the title says, I've wondered if turning on certain codec options like GMC or Q-pel in the first pass, and disabling them for the second pass, serves any purpose. Would the second pass benefit from the first pass having the options set when it analyzed the video, even though the options wouldn't be used in that pass? Thanks.
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No. AFAIK the 2nd pass reads the data of the 1st pass so any changes are irrelevant.
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You will get inaccurate results if settings are different during the two passes. During the first pass the encoder is only checking to see how compressible each frame is (given the current settings). During the second pass it uses that information to allocate bits to each frame so that the final average bitrate matches your requested value. If you change settings between the two passes the information gathered during the first pass may no longer be accurate for the second pass. The more those changes effect the compressibility of each frame, the more inaccurate the second pass will be.
GMC and QPel usually don't have a large impact on compressibility so the errors won't be large overall. But you may find particular frames that look bad. -
GMC and QPel both can be problems for playback on some older DVD players that support Divx playback. If your target is to play on a PC then they're fine to use, but if your target is to play on some kind of standalone DVD, BD or media streamer, you should consider not using them unless you are sure they are OK. GMC should never be set to any value above 1 if you want anything other than a PC to play it. I've not seen even one standalone playback device that can handle GMC > 1. QPel is generally safe to use these days on any modern standalone player but some older ones do reject it. If you haven't personally tested a non-PC playback device it's just safest to avoid both.
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