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  1. Member
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    Jun 2021
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    I post this message because I've a Luma issue on a video tape I've digitized.
    This tape is a 1"C recorded in 1982.
    This tape was digitized 2 times with different equipments.

    I realize the Luma Issue when I start editing the digitized video.
    The problem is the following... The minimum black level is at 0.25V instead of 0.3V as you can see below :

    YC Waveform (Luma in light blue):
    [Attachment 59442 - Click to enlarge]

    YC Waveform (Luma only) :
    [Attachment 59444 - Click to enlarge]

    Video :
    [Attachment 59443 - Click to enlarge]

    I check my equipments and there is no problem.
    The problem is on tape, when it was recorded.

    I try to correct the issue with premiere, after effects and other video software.
    But everything below 0.3V is truncated. And I cannot correct the video.

    I need to adjust the contrast pedestral.
    As the video file is 10 bits Uncompressed YUV SD interlaced (v210), I would like to make this correction with Avisynth.

    In fact I would like to shift up the luma level so that the minimum luma, corresponding to black level, will be at 0.3V (0 IRE)

    As Avisynth wiki has a lot of pages with 404 errors, could you help me in solving this luma issue.
    How to adjust the contrast pedestral in avisynth ? It's not a question to change the black level... The black level is good but its position in scope is wrong.

    Awaiting your replies
    Thanks in advance for your help.
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  2. Upload a short sample.

    ColorYUV(gain_y=m, off_y=n)

    To get black at 64, white at 940.

    Note that ColorYUV works in 8 bit equivalents. So off_y=1 adds 4 to all Y values when working with 10 bit video. The Histogram() function is similar.

    [Attachment 59445 - Click to enlarge]

    On the left is a greyscale ramp ranging from Y=0 to Y=940 (0 to 235 in 8 bit equivalents). On the right is the result of ColorYUV(gain_y=-17, off_y=16). You can see the superblacks are brought up to black (0 to 64), white has remained the same (940 is still 940). The details in the superblacks are now visible in the image.

    Scaling your waveform monitor and overlaying onto AviSynth's Histogram():

    [Attachment 59446 - Click to enlarge]

    It look like you'll want something like ColorYUV(gain_y=-11, off_y=10)

    If you're looking to just crush all the superblacks and superwhites: ColorYUV(opt="coring"). If you want to crush superblacks only use the sequence ColorYUV(off_y=-16).ColorYUV(off_y=16).
    Last edited by jagabo; 15th Jun 2021 at 09:26.
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