VMX Mpeg4 Progress bar
Submitted by Jake Stradling FACT on October 31, 2008 - 1:42pm
FACT'S VMX Computer is running MAC OS 10.5. When I go to transcode my MPEG2 File IN to a MPEG4 file using the VMX software the progress bar that tells the user how much is left to do does not work or is in-accurate. The progress bar will stop working a quarter of the way through the transcoding. If I ignore the progress bar the VMX software will eventually encode the MPEG2 file into an MPEG4 file. I don't think the progress bar works at all.
Also It would be good if the progress bar showed what percentage has been completed.
Yes, this has been the case
Yes, this has been the case all along. Progress bars are never completely accurate. They try to make assumptions based on the first few bytes that are transcoded, and then determine from the length how long it will take. Since you are transcoding a highly compressed file into another highly compressed file, there is no accurate way for the computer to gauge how long it will take. The transcode process in this case, is not a linear process. The computer has to look bi-directionally to know how to compress each frame. So, it may be looking at frame 1, then frame 5, then frames 2-4, then frame10, then frames 6-9, then frame 15, and so on. This makes it difficult for the approximate time to be calculated. The best thing to do is to get an idea for how long a 1 hour program takes to compress, and use it as a benchmark. All files of the same size will take about the same amount of time.
Maybe there is something that can be done to improve the accuracy of the progress bar, but I wouldn't hold my breath.
Hope this helps.
Matt
Matt Goudey
Video Technology Coordinator
Vermont Community Access Media
208 Flynn Ave Suite 2G
Burlington, VT 05401
802.651.9692
The progress bar is
The progress bar is certainly not 100% accurate but it does work and should be working on Leopard just like Tiger. I tested this before the install. Let me try to explain how it works (a lot of this information was brought up by our application developer):
The output log (~/Library/Logs/VMX.log) is used to get a more accurate progress report. The rub here is that ffmpeg’s progress speed, and many other encoders I would now assume, is not “tightly” related to the size of the file to transcode. For instance, PSATurnitOff (16.5 MB) took 299 sec to process while PSA Polar Bears(23.4 MB) took only 264 sec. The log indicates that ffmpeg transcodes each file frame by frame but again the process time varies frame to frame so again it is hard to get an accurate reading (as Matt also points out).
It would seem that any estimated time of completion is hazardous at best. While this does remain a estimate, there are too many differences from file to file to make this useful information to have. Sp what we've done is to simply display the current file process more accurately (following the output log) and not supply the shaky estimated value. The blue bar then displays the portion of the total amount of frames that are actually processed for each file.
Please try to use the VMX application again. Do a few shorter files and see how it's working. Please post your results back here.
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