Re: cICP wording feedback

Hi Simon,

The cam painting documents seem to require a login.

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________________________________
From: Simon Thompson - NM <Simon.Thompson2@bbc.co.uk>
Sent: Friday, February 2, 2024 4:39:50 AM
To: jbowler@acm.org <jbowler@acm.org>; public-png@w3.org <public-png@w3.org>
Subject: [EXTERNAL] Re: cICP wording feedback


Hi John, all,

>>Narrow range images are found in video workflows where the interpretation of sample values below reference black >>(0% signal level) or above nominal peak (100% signal level).

>Should "where the interpretation of" be "with"?  The base Kodak
>PhotoCD format does the same thing in an 8-bit encoding; IRC black is
>'16' and white '235' (but don't quote me).


Agreed, the sentence is a bit long and could be shortened with “with”.

>>overshoots and undershoots exist below reference black and above nominal peak in order to preserve processing >>artifacts caused by filtering/compression

>If original image data is "normalized" to the range 0..1 then
>resampled to a sub-pixel offset it will typically produce values above
>1 and below 0 and these are not "artefacts" they are correct (you need
>to use bicubic with the correct coefficient or better to see this
>happening.)  In essence the normalization maps an absolute value >0 to
>"black" and an arbitrary absolute value to "white" then resampling,
>particularly with just a pixel offset, recovers the original values at
>the intermediate locations.



I think the range 0-1 is a target in live video production, not necessarily always possible, if you think of a sports game under natural lighting or an outdoor interview, then as the lighting changes, the camera operator will slowly adjust the iris to prevent any large shifts being visible.  Any still images exported from a live video will probably include values outside the 0-1 range.  The various regional production specifications prevent clipping to the range 0-1 as this would cause ringing when filtering and increase bitrate requirements in DCT based encoders.


>>The use of undershoot/overshoot has also been used to preserve additional color volume (both light and color)

>Indeed.  scRGB; what is the transfer function for that?  The list of
>transfer functions seems to be absent.  I suggest it needs to be there
>along with the way to register a new value.  I assume values >=128 are
>private.  In any case what is the value for scRGB?



It’s also commonly used when converting a video feed from HDR to SDR.


>Referencing ISO or EBU or ITU specs does not help.  They are
>expensive; outside my reach.  I'm pretty dubious about the sRGB
>approach too; the spec was meant to be public but only the WDs were
>actually released.  At least the ICC seems to do it properly.



EBU and ITU specifications are publicly available and free.  If there are any that you haven’t been able to access, let me know and I’ll ask the rapporteurs to ensure they’re available.



Best Regards



Simon



Simon Thompson
Senior R&D Engineer

BBC Research & Development