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Thread: 1888x1062 and "clean aperture" question

  1. #1
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    Default 1888x1062 and "clean aperture" question

    hi all.

    when i capture hdv footage directly from the hv20 into final cut pro, i get different pixel dimension measurements, depending on the application. in final cut pro, the footage is identified as 1080i50 hdv @ 1440x1080 pixels, which is fine. however, when i open the footage file in quicktime pro and go to properties, both "normal size" and "display size" show 1888x1062 pixels (the video track is identified as hdv 1080i50).

    when you export a still image sequence from qtp, the resulting images are 1888x1062 pixels, which is annoying if you're doing compositing, cgi or anything for that matter. saving an image file from after effects gives a 1440x1080 anamorphic frame, which is what i expect.

    i don't understand where this funky 1888 pixel resolution comes from. i managed to google something about "clean aperture" and frame trimming but i'd really appreciate a good explanation but someone knowledgeable about these things...

    thanks.

  2. #2
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    I know where it comes from: SMPTE 274M. That doesn't mean I agree with it!

    SMPTE 274M is the standard which defines 1920x1080 video formats. It has an "informative" annex (i.e. it's not strictly part of the standard) which says...

    E.4 Clean aperture

    E.4.1 The bandwidth limitation of an analog signal (pre- and post-filtering) can introduce transient ringing effects which intrude into the active picture area. Also, multiple digital blanking operations in an analog-digital-analog environment can increase transient ringing effects.
    Furthermore, cascaded spatial filtering and/or techniques for handling the horizontal and vertical edges of the picture (associated with complex digital processing in post-production) can introduce transient disturbances at the picture boundaries, both horizontally and vertically. It is not possible to impose any bounds on the number of cascaded digital processes which might be encountered in the practical post-production system. Hence, recognizing the reality of those picture edge transient effects, the definition of a system design guideline is introduced in the form of a subjectively artifact-free area, called clean aperture.

    E.4.2 The clean aperture defines an area within which picture information is subjectively uncontaminated by all edge transient distortions.
    The clean aperture should be as wide as is needed to accommodate cascaded digital manipulations of the picture. Computer simulations have shown that a transient effect area defined by 16 samples on each side and 9 lines at both top and bottom within the digital production aperture, would represent an acceptable (and practical) worst-case level of protection in allowing two-dimensional transient ringing to settle below a subjectively acceptable level.

    E.4.3 This gives rise to a clean aperture of 1888 horizontal active pixels by 1062 active lines whose quality is guaranteed for final release.
    It's archaic though. HD video rarely if ever goes through analogue stages during production these days, and filtering etc should (and normally is) designed to keep the entire picture "clean", including the very edges.

    Maybe someone else who uses Final Cut Pro can tell you how to get the full image out of it - I would guess that there must be a way.

    Cheers,
    David.

  3. #3
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    archaic indeed. thanks for the info. it's nice to know you always learn something new. i still can't figure out why on earth qtp uses this (seemingly pointless in a digital workflow) "reframing", while after effects and fcp don't.

    ideas anyone?

  4. #4
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    ok, i narrowed it down thanks to your help. apparently this is a new "feature" introduced in quicktime 7.1.

    it seems that clean aperture display can be deactivated, but the apple docs are unclear (to me, at least):

    When opened in QuickTime Player, a movie appears corrected if it has aperture mode dimensions. QuickTime Player respects the other aperture modes as well. The user can toggle the aperture modes to see how the content is displayed in these modes. The default mode is clean.
    they don't mention how to toggle th aperture mode, though (unless you feel like learning api programming).

    the quote above is from this document..

    still looking for a way to disable this behaviour, and for ideas on why apple thought it was useful.

  5. #5
    Valued Member
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    Choose 'Show Movie Properties' from the Window menu. Click the Presentation tab and choose your aperture settings:



    Then don't forget to save the movie.

  6. #6
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    shame on me. thanks a million.

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