iMovie HD Quality Dropping. Help!

Discussion in 'Mac Apps and Mac App Store' started by hcazi, Jul 20, 2009.

  1. hcazi macrumors newbie

    Joined:
    Jun 18, 2009
    #1
    hi im working on a project
    and i have a video that i am importing to imovie to edit then export it out with the best quality possible

    the file being imported is
    960x540
    codec : Integer (little edian) H.264
    total bit rate : 7863


    I imported this into imovie, and when i went to export using QT in H.264 the black color of the movie is degraded and it seems a little blurry from the original file.

    This is the settings i used to export the file as.
    frame rate : current
    Key Frame : auto
    frame reordering : checked
    data rate : auto
    compression type : H.264
    compressor quality : best
    encoding : multipass
    No Filters

    Sound is unused.

    should i be trying to export it even using H.264?

    When i play the original movie vs the exported movie, i see definetely a loss in quality.

    is this imovie failure?

    is there something i need to do during the importing process?
    i cant figure this outt~
     
  2. BoogieTrain macrumors member

    Joined:
    Nov 15, 2008
    #2
    i have not used the new imovie, or even the old imovie extensivley, but its unavoidable to loose quality when you compress an already compressed movie.

    For example, when you have something like a DVD, it is what is considered uncompressed data (though its definitely scaled/compressed from what hollywood has). The data is stored in mpeg-2 streams with little to no compression. This is why we can create 700mb files from a 7GB DVD that still look good, and 1.3GB rips that are very close in quality to the "original" source, the DVD.

    For simple operations like cutting sections out of a video, you can export using something called "direct stream copy" where the original file is copied to a new file, bit for bit, maintaining the original file size, encoding and quality, however this method can only be used when data is removed from the original file. Adding any data to the file, such as transitions, color effects, etc etc, will require the video to be recompressed, and thus video quality is compromised.

    To retain the same quality as the imported video, the edited video must be exported in an uncompressed or lossless format, but this will result in a much larger file size than the original. It is possible to export and compress in a format that is comparable in quality and file size to the original file, though there will always be quality loss when exporting (that or huge file sizes).

    The way compression works is that "useless" or "superfluous" data is discarded, this data is able to be recreated upon decoding so achieve the original video or audio stream. Because compression is not perfect, the decoded and "rebuilt" file will not be 100% identical to the original file; though we are able to achieve "transparent", that is files with no NOTICEABLE difference from the original, with much smaller file sizes than the original file.

    So applying this to the situation at hand, re-compressing a compressed video will degrade the quality of a video that is already reduced in quality, making the re-encode look pretty bad. As multiple encodes are done on the file, the difference between the most recent and the original files becomes very clear very fast. Its like making a copy of a key, then making of the copy, then a copy of the copy that was made from the copy. Soon enough the new copies wont even open your lock.

    To achieve a file that looks pretty similar to the original, you are gonna have to tweak the export settings, and find something that works well for your particular video. Looking at your export settings, there's nothing that really dictates the quality of your video. The bitrate is set to auto, and encoder quality is set to "best", and there's no defined output file-size.

    I would suggest finding the avg bit-rate for each of the streams (audio and video) and set the export settings similar to that, leaving encoder on "best". H.264 is a good encoder, but it may not be suited for this kind of operation, i dont really know. Try different encoders and bitrate settings. Im personally a fan of XviD for exporting.

    Hope this made sense and was of some help to you.
     

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