Latest post Fri, Jan 26 2018 7:19 AM by doxilia. 2 replies.
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  • Mon, Jul 25 2016 10:01 PM

    • voxen
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    Editors are working in "10 bit" mode in dnxHD 36

    A few  "A list" Film Editors out here in Hollywood believe by selecting "10 bit" all-green mode they are seeing a clearer "more high resolution" image while they are working away on dnx36/115. This can't be possible since the resolutions themselves are 8 bit. Am I correct? What could the Nitris DX box be showing them that would make them think that ? 

  • Tue, Jul 26 2016 1:18 AM In reply to

    Re: Editors are working in "10 bit" mode in dnxHD 36

    Resolution and bit depth are 2 different things. Resolution refers to the actual amount of pixels in terms of width by height. 1920 pixel wide with 1080 pixels height.

    Bit depth refers to the granularity of the color information from point color A to point color B. You have 256 small steps for 8 bit video. 10 bit video have 1024 steps for the "same distance". It is like measuring a distance in feet or inches. One is more precise.

    10 bit mode helps if you are using 10 bit media AND your monitor is capable of displaying 10 bit video.

     

    DNX 36 and 115 are both 8 bit video.

     

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  • Fri, Jan 26 2018 7:19 AM In reply to

    • doxilia
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    Re: Editors are working in "10 bit" mode in dnxHD 36

    voxen:

    A few  "A list" Film Editors out here in Hollywood believe by selecting "10 bit" all-green mode they are seeing a clearer "more high resolution" image while they are working away on dnx36/115.

    Yes, we ought to be careful to not mix apples and oranges. In video parlance, resolution refers to the number of horizontal and vertical pixels in the frame. Full HD, for example, being 1920x1080 pixel resolution. 

    Pixel bit depth refers to the colour sample space used to define those pixels. So 8 bit means 2^8 = 256 colours. 10 bit means 2^10 = 1024 colours. So 10 bit contains 4 times more colour information that 8 bit (4 x 256 = 1024). This higher bit depth results in colour images looking smoother or less “banded” in regions where there are strong luma gradients (contrast) within relatively narrow colour differences (range or gamut). The sky at dawn or dusk is a good example of where 10 bit “quality” or “depth” can be seen and appreciated. Further, 12 bit images have 2^12 = 4096 colours so 16 times more information! In other words the scale is exponential and every bit increase in depth, exponentially increases the quality both factually and perceptually. At 24 bits one has reached the so called “millions of colours” depth of 16M+ quality which, in a static (still) image, appears to the human eye to produce continuous non banded gradients. Interestingly, moving pictures, don’t require such high bit depth to make the eye (or brain rather) perceive the same smooth gradients. Simply put, the brain is better at “interpolating” colour information when it’s viewing dynamic images.

    This concept also leads to colour “sample rate” within YCbCr signals where we talk about using 4:4:4, 3:1:1, 4:2:2, 4:1:1, 4:2:0, etc. “subsampling”. The “reduced ratio” sampling systems contain some lower ratio of pixel colour information for every pixel of luma information. So while all pixels contain the luma information, not all of them contain chroma information. The subsampling ratio defines the pattern of chroma information of the pixel grid. Why do we do this? Well, because the brain is also good at “filling” the missing colour samples provided it’s got good luma samples. Otherwise put, the human eye/brain cant “see” colour without light. Scotopic vision must be engaged to witness chroma. Photopic vision is sufficient to witness luma.

    Now, the 10 bit green timeline dot indicates to the Nitris hardware (or software absent display hardware) how colour in the images should be handled by the display engine. This is different from the colour depth that is contained by the images themselves. If one has a timeline which contains a mix of DNX 36, 115 and 1:1 UC (all 8 bit) as well as some images in DNX 175X and graphics in 1:1 10b UC then having the 10 bit green dot enabled will display all 10 bit media as such (assuming the display is also 10 bit capable - most are) and “bit interpolate” any 8 bit media. This doesn’t actually improve the 8 bit images but rather applies a “colour smoothing” algorithm if you will. Depending on the image this may result in a perceived better but also sometimes worse appearance of the image. There are good example situations of both these scenarios.

    That said, editors/colourists will often engage the 10 bit engine during online in order to view any and all 10 bit depth images in the highest possible quality which is important during grading. 8 bit images may or may not benefit depending on the particular image type and colour content. At the end of the day, one has to “bake in” bit depth into a mixed depth program so either one renders (or mixes down) all 10 bit to 8 bit or, more commonly, all 8 bit to 10 bit. The latter being chosen to cater to the highest quality available even if intermittent in the program.

    Note that reduced ratio YCbCr signals are typically “upsampled” by NLE’s on playback/display to a 4:4:4 “full gamut” signal via chroma interpolation in a similar fashion (though technically different process) to the chroma interpolation that takes place when augmenting bit depth. So bit depth as well as chroma sampling both affect our perception of colour in images. Both processes apply some form of smoothing to the images chroma information.

    This can't be possible since the resolutions themselves are 8 bit. Am I correct? What could the Nitris DX box be showing them that would make them think that ? 

    I hope the preceding sheds some light (pun intended... :-) on your question above. However note that the resolutions are not 8 bit, but rather the image encoding or codecs are. You can have an 8 bit coded image displayed by a 10 bit engine by interpolating up and also, by setting the green dot to 8 bit mode, effectively extrapolate 10 bit media down.

    In summary, image resolution, pixel bit depth and signal colour subsampling all affect the quality of the images displayed.

    David

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