[CinCV TNG] VideoScope and CinelerraCV

Pierre p.autourduglobe at gmail.com
Sat Nov 14 07:24:58 CET 2015


Igor,

Thank you for your detailed explanations and links to some good 
documentation. This allows me to better understand and properly adjust 
my video levels with the waveform of the videoscope in CinelerraCV.

Now, if it were possible to enlarge the frame size of the Videoscope, I 
would not have to regularly use a magnifying glass to clearly identify 
at what precise level reaches some isolated points of my videos ...

Pierre


Le 2015-11-13 15:32, igor_ubuntu a écrit :
> Hi everyone !
>
> VideoScope:
> Patch by Craig Lawson (7 Sep 2007)
>
> http://thread.gmane.org/gmane.comp.video.cinelerra-cv.general/6620/focus=6623
> http://git.cinelerra-cv.org/gitweb?p=CinelerraCV.git;a=commit;h=34f9b4a519fcb151cad8f42d16f617d42e05c18a
>
>   Quote:
> /" Add optional waveform graduations for IRE 7.5% black limit, ITU-R B.601
> limits, ITU-R B.709 limits.  Add checkboxes to control their display."/
> ------ ----------------- ---------------
>
>   At present we have in CinCV-interface:
>
> ITU-R B.709 limits - checkbox `HDTV`
> ITU-R B.601 limits  - checkbox `MPEG`
> IRE 7.5% black limit - checkbox `NTSC`
>
> https://www.dropbox.com/s/a50xmubt08xht2k/labels_cv.png?dl=0
> -------------------------------
>
> Ok. Here goes! :-)
>
> PART - 1
> --------^^^^^^^^^^^^^^^^^^^^^^^^
> Quote from source code:
>
> http://git.cinelerra-cv.org/gitweb?p=CinelerraCV.git;a=blob;f=plugins/videoscope/videoscope.C;h=46e194645620639155e7fe1c9a664a5f155ab357;hb=34f9b4a519fcb151cad8f42d16f617d42e05c18a
> -------------
> /655   set_tooltip("Indicate ITU-R BT.709 limits. Use when rendering 
> to HDTV and sRGB.");/
>
> -----------^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> But these limits (in CV) with patch by Craig Lawson  are incorrect !
>
> BT.709 does not define sRGB encoding.  sRGB is standartRGB.
> BT.709  defines `Studio RGB`, which is RGB with black at 16 and white 
> at 235.
> "sRGB" is not an abbreviation of "Studio RGB"
>
> -- Rec. 709 defines an R’G’B’ (StudioRGB) encoding and a Y’CbCr encoding
> -  Black level -  R, G, B, Y = 16
> – Nominal peak -  R, G, B, Y  = 235
>
>
> **********************************
> https://books.google.com.ua/books?id=6dgWB3-rChYC&pg=PA16&lpg=PA16&dq=sRGB+range+16+235&source=bl&ots=1KAjEGJxZ2&sig=raDau0hSC_GATXuvO8ADggqKNlA&hl=ru&sa=X&ved=0CF4Q6AEwB2oVChMIyYPdkJSdyAIVxP1yCh0NSgTq#v=onepage&q=sRGB%20range%2016%20235&f=false
> ***********************************
> /Quote from Rec. ITU-R BT.709-5 
> https://www.itu.int/rec/R-REC-BT.709-5-200204-S/en
>
>     Digital representation
> Coded signal R, G, B, or Y, CB, CR
> Quantization levels(
> – Black level R, G, B, Y 16
>   – Achromatic CB, CR 128
>     – Nominal peak
>       – R, G, B, Y 235
>         – CB, CR 16 and 240
> Quantization level assignment(3) 8- bit coding
> – Video data
> – Timing references(2)
> 6.12
> Filter characteristics(4)
> – R, G, B, Y 1 through 254
> – CB, CR 0 and 255/
> ***********************************
>
> But in CinCV-Waveform monitor of VideoScope  ITU-R B.709 limits (HDTV 
> checkbox) = 0-100 (0-255)
> It is not right !
>
> CinelerraCV represents the [16 to 235] range as 6.3% to 92% in it’s 
> waveform monitor (VideoScope).
>
> https://www.dropbox.com/s/jqxcuj81gmss58k/16-235_cincv.png?dl=0
> https://www.dropbox.com/s/ezbwlcafmsa858i/cincv_labels-limits_16-235.png?dl=0
>
> In the software, apparently it is the percentage of 255.
> 16/255 = 6.3%
> 235/255 = 92.2%
> Or it is very likely to be one of these two possibilities:
> 1)  The software uses integer values 0-255, and when reading a Rec.709 
> Y'CbCr file, it decodes that to black at 16,16,16 and white at 235,235,235
> 2) The software uses floating point values 0-1, and when reading a 
> Rec.709 Y'CbCr file, it decodes that to black at 16/255,16/255,16/255 
> and white at 235/255,235/255,235/255
>
> CinelerraCV represents the [0 to 255] range as 0% to 100% in it’s 
> waveform monitor (VideoScope).
>
> https://www.dropbox.com/s/z6sl41qehu0hsiy/0-255_greyscale_8bit_cincv.png?dl=0
>
> Because of the nature of how CinelerraCV handles media on the 
> timeline, it is possible to have a RGB- range (0-255) clip on the same 
> timeline next to a YCbCr-clip (16-235) and levels will be correctly 
> scaled/displayed and preserved for each in WFM.
>
> ***********   *****    **********
>
>         PART - 2
>  About   CinCV-checkbox `NTSC`  - IRE 7.5% black limit
>
> -------^^^^^^^^^^^^
> /Quote from source code:
>
>  71         int show_IRE_limits;   // Black = 7.5%
>  107  int  limit_IRE_black;  // IRE 7.5%
>  687       set_tooltip("Indicate IRE 7.5% black level.");
>   684  : BC_CheckBox(x, y, plugin->config.show_IRE_limits, _("NTSC"))
> 685 {
> 686         this->plugin = plugin;
> 687         set_tooltip("Indicate IRE 7.5% black level.");/
>
> --------^^^^^^^^^^^^^^^
>
> But:
>
> - Digital video levels are not measured in IRE.
> - IRE refers only to analogue video.
> - `7.5 IRE Setup` is a purely analogue phenomena, and is only ever 
> relevant for North American NTSC.
> - `7.5 IRE Setup` should only ever be removed or added to an analogue 
> signal - never a digital one.
> - The Waveform monitor of CinCV-VideoScopes does not show what our 
> analog levels are like.
> - CinCV-WFM is not a CinCV-WFM analog waveform monitor.
> - CinCV-WFM  doesn't measure IRE-analog video levels, so it can't 
> accurately represent them.
> - CinCV-WFM is just a *digital* waveform monitor.
> - 0 IRE or 7.5 IRE, the black levels in the digital data should be the 
> same: 16.
>
> Thus, the  CinCV-checkbox `NTSC`(IRE 7.5% black limit)  is not 
> meaningful, it does not correspond to the theory and misleads users.
> It is just not right.
>
> https://www.dropbox.com/s/ri8b6qsjdjizxxr/labels.png?dl=0
> -------------------------------------------------
>
> Igor
>
> -----------------------
>    BASIC THEORY
>
> Useful links:
> http://www.adamwilt.com/DV-FAQ-tech.html#Setup
> http://www.tri-sysdesigns.com/Articles/BlackisBlack.html
>
> http://www.provideocoalition.com/luminance_ranges/page-5
> http://web.archive.org/web/20020124025939/http://www.dv.com/magazine/2000/0400/luminanceranges0400.html
>
> http://www.kenstone.net/fcp_homepage/video_levels_nattress.html
> http://www.glennchan.info/articles/technical/setup/75IREsetup.html
>
>
> http://www.tri-sysdesigns.com/Articles/BlackisBlack.html
>
> Most of the confusion occurs when analog to digital and digital to 
> analog conversions take place.
> In NTSC, a composite analog signal enters a decoder and the 7.5 IRE 
> black levels are mapped to a digital value of 16.
>  In PAL systems, 0 millivolt (black) is also mapped to a digital level 
> of 16.
>  NTSC-J systems are similar to NTSC except that there is no set-up. 
> Black (0 IRE) levels are also mapped to 16.
> ------------------    -------------------- ---------------
>
> http://www.adamwilt.com/DV-FAQ-tech.html#Setup
>
> Going from analog to digital and back again, if you're following the 
> spec, analog black goes to digital black and vice versa.
> Whether digitizing from NHK in Tokyo (0 IRE setup) or NBC in New York 
> (7.5 IRE setup), the black levels in the digital data should be the 
> same: 16.
> Likewise, the blackest black in a picture coming out of the camera 
> section of a camcorder should always be laid to tape with a luma level 
> of 16, regardless of what part of the world the camera is designed for.
> And that same tape should play back in Japan with 0 IRE setup, and in 
> the USA with 7.5 IRE setup.
>
> --------------------    --------------------- ------------------
>
> http://www.provideocoalition.com/luminance_ranges/page-5
> http://web.archive.org/web/20020124025939/http://www.dv.com/magazine/2000/0400/luminanceranges0400.html
>
> Some people confuse the digital luminance value issues discussed in 
> this article with the analog measurement of brightness, known as IRE 
> (Institute of Radio Engineers) units. They are indeed both definitions 
> of how to represent black and white. But they operate in different 
> worlds and are not interchangeable, nor is one a replacement for the 
> other.
>
> In analog video, the electrical reference for white is generally 
> defined as 100 IRE, using near-identical electrical levels regardless 
> of format. The electrical reference for black depends on what video 
> format you are using, and even what country you are in: PAL and the 
> Japanese version of NTSC always use 0 IRE to define black. Composite 
> NTSC video in North America uses an electrical value of 7.5 IRE, which 
> is sometimes referred to as set-up. Component video in North America 
> can use either 0 or 7.5 IRE for black; 7.5 seems to be more common.
>
> The analog-to-digital and digital-to-analog conversion portions of 
> your video card or deck decide how to translate luminance values 
> between these two worlds. For example, if set to 0 IRE and digitizing 
> to a 601-specification data stream, a video signal at 0 IRE would be 
> translated to a value of 16, and video at 100 IRE would be translated 
> to 235. Hot spots brighter than 100 IRE would be captured as values 
> above 235.
>
> If this same data stream was output through hardware with 7.5 IRE 
> setup enabled, a value of 16 would create a video signal at 7.5 IRE, 
> and a value of 235 would create a 100 IRE signal.
> Most video editing systems have a 0/7.5 IRE switch, but note that some 
> - such as the Media 100 - actually only change this reference on the 
> output, always keeping the input at 7.5 IRE for NTSC systems.
>
> Some mistakenly assume that a luminance range of 16-235 means 7.5 IRE 
> setup was used, but this is not true. For example, a value of 16 still 
> defines black in any 0 IRE system, including PAL and Japanese NTSC. If 
> a videotape was recorded with 7.5 IRE setup, but digitized through 
> hardware that uses 0 IRE for its black reference, and then translated 
> through a codec that stretched video black down to a numeric value of 
> 0, "black" on the 7.5 IRE tape would indeed decode at a value around 
> 16 - but only because someone made a mistake.
>
> ------------------    -------------------- ---------------
> http://www.glennchan.info/articles/technical/setup/75IREsetup.html
> Please note that application-based waveform monitors (i.e. in your 
> NLE) are unable to measure analog levels!
> They cannot tell if your digital-analog converter will convert levels 
> properly or not.
> Don't get confused by any settings that mention 7.5 IRE!
>
> --------------          --------------- -------------------
> https://forums.creativecow.net/archivethread/8/459306
>
> Digital SMPTE 601 standard video is 16-235. It's always 16-235 - it 
> has nothing to do with setup, and it's the same for PAL, NTSC and 
> matian (if martians have digital video).
>
> DV and the Apple DV codec are both 16-235. In PAL land 16 is mapped to 
> black which is 0 IRE. In NTSC North America 16 is mapped to 7.5 IRE 
> which is black, and in Japan, it's mapped to 0 IRE which is black. All 
> map 235 to 100 IRE white.
>
> ************************************************
>
>
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> Cinelerra at lists.cinelerra-cv.org
> http://lists.cinelerra-cv.org/cgi-bin/mailman/listinfo/cinelerra

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