Latest post Tue, May 14 2013 12:40 AM by Igor Ridanovic. 10 replies.
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  • Wed, Apr 17 2013 3:03 PM

    • RobinDS
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    Colour space and mapping

    Is there a document that explains 'legacy graphics' import? It seems to be matching up with some 709 444 files I have but it would be nice to know a little more about what is going on there. The manual has a nice bit describing which import settings will remap head to full ranges but then I have not yet found out more about legacy graphics and why it actually appears to be one of the mostuseful settings.

    cheers

    R

  • Thu, Apr 18 2013 9:12 PM In reply to

    • slabross
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    Re: Colour space and mapping

    Legacy is simply RGB but without any gamma correction. It works for most computer-based viewing but is not adequate nor the only solution for all video and film outputs.

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  • Thu, Apr 18 2013 11:24 PM In reply to

    • RobinDS
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    Re: Colour space and mapping

    So...... I guess it is working when I import a 'Rec709' file as it is bringing in the file straight as head and not remapping the signal to full?

    I guess that would make sense then but you really have to think around it, the temptation to import a Rec 709 under 'Rec709' import settings is strong but wrong eh?

     

     

  • Sun, Apr 21 2013 5:25 AM In reply to

    Re: Colour space and mapping

    Actually 709 and legacy graphics should introduce very different levels the imported material. I haven't used the legacy option in a long time and can't say if anything has changed.

    DS color manages very diffrently in 4:2:2 from 4:4:4. There is a PDF called "Avid High Resolution Workflow" or something like that which explains DS color management in 4:4:4.

    4:2:2 mode is a little more convoluted. Everything is stored and processed with 2.2 gamma and video levels. In either 4:4:4 or 4:2:2 the 709 option should map:

     

    8 bit code value mV video value
    16 0
    235 700
    0 superblack
    255 superwhite

    The legacy graphics option will map:

    8 bit code value mV video value
    0 0
    16 greater than 0
    235 less than 700
    255 700

    And there is no provision for superblack and superwhite because the full 0-255 (8 bit) range is used.

    4:4:4 mode is easier to use when dealing with sources in multitude of color spaces. It's very straightforward and explained fairly well in the PDF doc.

    Igor Ridanovic

    www.HDhead.com

  • Mon, Apr 22 2013 3:01 PM In reply to

    • RobinDS
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    Re: Colour space and mapping

    I'd like to say it all works out but ..... it doesn't really.

    The job I had was to import a prores 444 film (which says it is a Rec 709 file) ... I import to DS 10bit seq set 444 and Rec 709 colour Space ... if I import the file with import settings as Rec 601/709 then it appears to be stretching that colour space out to full, which makes sense as a 16 (in 8bit) value will be mapped to 0 etc. If, on the other hand I use Legacy Graphics as my import colour space the bars on the sequence now appear to match the DS generated bars in the gamma too. The sequence originated on an FCP I think.

    So it appears to me that if you have a 601/709 file setting the import to 601/709 isn't what you want to do ... but then maybe the fact that the sequence is 601/709 means this is already being applied.

    I know that this really should be simple and much more straight forward than it appears at the moment. Perhaps these files are not what they say they are but I'm fairly certain they are correct.

    I'll read through that article but I don't think it will prove or dis-prove this issue.

     

  • Tue, Apr 23 2013 11:55 AM In reply to

    • jef
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    Re: Colour space and mapping

    All I can add to this is that I remember reading somewhere that ProRes 4:4:4 can be either rec 709 OR RGB.  So that might be part of the issue here.  One would hope that the Meta Data tagging in QT would reflect the reality but it IS QT.

    Jef

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  • Thu, Apr 25 2013 6:49 AM In reply to

    • M-Edit
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    Re: Colour space and mapping

    Here's from a DS10.0.0 help file... (8-bit/10-bit values)

    Color Space and Mapping

     

    Input Values Black point White point Gamma
    Linear 0/0 255/1023 1
    Linear Video 16/64 235/940 1
    ITU 601/709 16/64 235/940 2.2
    SRGB 0/0 255/1023 2.2
    Legacy Graphics 0/0 255/1023 No gamma remapping
  • Thu, Apr 25 2013 10:17 AM In reply to

    • RobinDS
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    Re: Colour space and mapping

    That is a useful bit of info.

     

    So I guess the issue is :

    If you were supplied a true 601/709 file and want to send it back in the same format do you actually want to import the file using Linear, set the sequence to 709 colour space and then export with 601/709 settings ?

    Is it a case that if you were to import a 709 file using the 709 setting it would actually re-map the levels again? or do I just have an incorrect file in the first place do you think?

    R

  • Sat, May 11 2013 1:12 AM In reply to

    Re: Colour space and mapping

    Sequence LUT has no effect on what you're trying to do, for the most part. You should probably import as 709 an export as 709 in your scenario, even though the levels will be remapped internally. The uncertainty in my answer is due to not having a full picture of what you're attempting to do. I do a lot of color management gymnastics in DS in order to optimize quality and tool utilization.

    The thing to remember is tha 4:4:4 works in linear light. The LUT selection on import is used to linearize the gamma of the source material. If you use "linear" no LUT transformation is applied. Bringing in 709 material with 2.2 gamma in this way (linear) is wrong from the perspective of DS color management as it will alter the color intent of the media, but it can have other benefits. This "wrong" intent can be corrected downstream with either a 709->lin LUT or an external viewing LUT.

    This is all actually far less confusing than it seems.

     

    In 4:4:4

    import "linear"

    export "linear"

    the levels will remain transparent and unchanged for all practical purposes. There is also something DS does internally with captured media but let's not make it too confusing.

     

    import "709"

    export "709"

    the lebels will remain transparent but will undergo log -> lin -> log transformation in DS. Will you see the ill effects of such transformation? Probably not. Process with 32 bit float precision.

     

    In 4:2:2

     

    import "709"

    export "709"

    the levels will remain transparent fully unchanged.

    Igor Ridanovic

    www.HDhead.com

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  • Sat, May 11 2013 10:46 AM In reply to

    • RobinDS
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    Re: Colour space and mapping

    So am I correct to consider the 'sequence LUT' as just a LUT for the monitoring?

    Everyone keeps saying how simple and straight forward 444 is in DS yet I am really struggling to grasp how many LUTs are being applied and where!

    So the LUT on import designed to Linearise the supplied file, then DS works internally in a Linear colour space which makes perfect sense.

    Then to view this Linear material ... in my case I want the sequence LUT set to 709 as this will apply the correct LUT for a broadcast monitor (I have a Sony OLED which works in HD 444 and 2K via the graphics link).

    Then I just have the question of what file someone would like at the end ... if they want an RGB Linear space or more probably a 709 gamma adjusted file.

    If the above is correct then all that is left is to determine if they want to keep the image within the 709 defined space of 64 to 940 rather than extended.

    Have I got this right do you think? I guess in shake and Nuke these things seem more exposed and you can actually see the workflow of colour space in your script. 

     

  • Tue, May 14 2013 12:40 AM In reply to

    Re: Colour space and mapping

    Yes, you are correct. That's how it works. There is a very clear diagram in the Avid Hires Workflow Guide PDF.


    Sequence LUT is a viewing LUT for the AJA video output. And sequence LUT is also used for lin -> log conversion that happens when DS .GEN caches are generated and stored. The caches are actually stored with log gamma in order to maximize the dynamic range at any given storage bit depth. It's completely transparent from users.

    In addition to sequence LUT there is also the montitoring LUT which is accessible through Viewer Properties. This LUT is most often set to sRGB if that's how your computer monitor works. It can be turned off as well.

    The only export option that gives you full control of LUTs on export is DPX. All other file types go through either under-the-hood voodoo like Quicktime, or a limited range of LUTs like let's say TIFF.

    However, this can be circumvented by applying an appropriate 3D LUT as a track effect over the timeline and exporting linear.

    Igor Ridanovic

    www.HDhead.com

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