RE: Colour communication beyond sRGB

Thanks Lars and Pierre,



I hope this isn't too difficult to follow - I have added my comments below and updated the spreadsheet to reflect these.



I have not so far made any changes to reflect Lars' email (6th March) which asks broader questions about what content would be useful and more generally how useful this table is in practice. I would also like to hear from other members of this group as to whether this would be useful before putting too much effort into this. It is clearly important to take the reference viewing environment into account when converting between these and other colour spaces.



CR> We can add this. Where does the 'COLOR.6' name come from?



LB> The reference stated for DCI P3 D65/PQ does not use PQ. Is there

LB> a reference for this, maybe in IMF?



PL> COLOR.6 is defined in SMPTE ST 2067-21:2016 (IMF Application #2), which is the

PL> closest to a formal definition of P3D65/PQ that exists, AFAIK.



I have corrected the reference and added an indication that this is also known as 'COLOR.6'.





CR> I think it may be better to include only the necessary primary references for each encoding.



PL> The challenge is that the reference to BT.1886 is easily missed, leading implementers to

PL> invert the OETF specified in BT.709. I would add least add a note.



I have added a note to the table as suggested.





CR> The intent is to describe how the colour data is communicated - in

CR> some cases this is with reference to a display and in other cases it is with reference to a scene.



PL> Ok. The colorspace specified in RP 432-1* is never communicated or encoded. What is

PL> encoded is specified in ST 428-1, which uses XYZ components and does not specify a white point.

PL> I recommend collapsing the DCI P3 lines to a single "D-Cinema Reference Projector" line

PL> (specified in RP 431-2), and moving it to a "reference display" tab.



I am not sure how best to handle this. Lars, can you comment?





CR> What should we show as the reference white luminance for the digital cinema spaces?

CR> Is there another reference besides 431-2?



PL> There is no white point defined in the D-Cinema encoding/decoding equations.

PL> There is a reference projector defined in RP 431-2.



I am not sure what (if any) change should be made.





CR> I have indicated spaces not intended for broadcast by highlighting in blue.



LB> None of the DCI spaces are intended for broadcast AFAIK.

LB> How do we define broadcast here? Over the air only? Including streaming?

LB> Incl digital Cinema?



PL> As Lars pointed out, I am not sure "broadcast" is the right term. I would use "distribution"

PL> instead -- in contrast with "professional" applications, e.g. mastering and production

PL> (and arguably D-Cinema).



Agree that 'distribution' is a better term and have changed to use this throughout.





CR>  in some cases this is with reference to a display and in other cases it is with reference to a scene.



PL> Can you elaborate? For instance, I am not sure why HLG is present on the "to scene" tab, but not PQ.



See explanation from Lars' email. I am afraid that I don't (yet) have a clear understanding of PQ - can you explain the intent of this approach?





CR> On the CSS documentation for 'image-p3' I see that this talks about

CR>'image-p3' [DCI-P3], however I think these both use different transfer

CR>functions. There is also a typo for the y value of the white  chromaticity - it should be '0.3290'.



PL> I would think this should ultimately be filed as an issue against the CSS specification.

PL> Not sure what the timing is, perhaps Chris Lilley can provide some guidance.



I will file an issue against the CSS specification.



Best,



-- Pierre



* Did you mean RP 431-2 instead of EG 431-2 by any chance? RP 431-2 defines a reference projector, whereas EG 431-2 is more of a how-to.



On Fri, Mar 3, 2017 at 4:00 AM, Craig Revie <Craig.Revie@ffei.co.uk<mailto:Craig.Revie@ffei.co.uk>> wrote:

> Hi Pierre,

>

> Thanks for your feedback. I have attached a revised version of the spreadsheet that shows my suggested changes. Please let me know if you think I have misunderstood anything - see my comments below.

>

> You said:

>> - the table is missing the COLOR.6 (P3 primaries, D65 white point and

>> PQ EOTF)

> We can add this. Where does the 'COLOR.6' name come from? Perhaps it would be more consistent (and informative) to refer to this as DCI P3 D65/PQ.

>

>

>> and COLOR.4 (xvYCC709 as specified in IEC 61966-2-4) systems used in

>> IMF App 2E (SMPTE ST 2067-21)

> This is not an RGB space and so can't easily be added to this document. There are a number of such encodings which may be useful to document separately.

>

>

>> - If I am not mistaken, BT.709 (to display) is specified in BT.1886

>> (BT.709 specifies only the OETF)

> You are right. However BT.709 references BT.1886 (see note to table on page 3). There are also other references, for example the reference viewing environment is specified in BT.2035 and it could be confusing to include them all. I think it may be better to include only the necessary primary references for each encoding.

>

>

>> - Shouldn't the "to display" tab should be split into "display

>> colorspaces" and "encoding colorspaces", i.e. pixels are not encoded

>> in the DCI P3 D55 colorspace and many displays that accept Rec.2020 signals cannot reproduce the full Rec.2020 gamut.

> I am not sure that I follow this point. Can you explain further? The intent is to describe how the colour data is communicated - in some cases this is with reference to a display and in other cases it is with reference to a scene. The table shows the relationship between what is communicated and the reference display/scene. As I say, I am not sure I have understood your comment clearly so please clarify.

>

>

>> In fact, perhaps the "encoding colorspaces" tab should further

>> differentiate between mastering/professional colorspaces (COLOR.6 and

>> DCDM) from colorspaces in use for distribution.

> I agree that it would be a good idea to differentiate these. I have indicated spaces not intended for broadcast by highlighting in blue.

>

>

>> - the sRGB reference monitor peak is 80 nits

> Agreed.

>

>

>> - 48 nits is only required for the Xenon reference white in 431-2

> What should we show as the reference white luminance for the digital cinema spaces? Is there another reference besides 431-2?

>

>

>> - 'Display P3' has been renamed 'image-p3' in the most recent CSS

>> draft [1] [1] https://drafts.csswg.org/css-color/


> I understand. I have a request from Apple to refer to the space they use as 'Display P3' but I have added (aka Image-P3) on the assumption that these are the same. It seems from the documentation that they are intended to be.

>

>

> On the CSS documentation for 'image-p3' I see that this talks about 'image-p3' [DCI-P3], however I think these both use different transfer functions. There is also a typo for the y value of the white chromaticity - it should be '0.3290'.

>

> Best regards,

> _Craig

>

> Best,

>

> -- Pierre

>

> On Mon, Feb 27, 2017 at 12:35 AM, Craig Revie <Craig.Revie@ffei.co.uk<mailto:Craig.Revie@ffei.co.uk>> wrote:

>> I would be interested to receive feedback from members of this group

>> on the attached document which provides a summary of a range of

>> colour spaces used for broadcast and display of colour information

>> that I have put together with help from Lars Borg of Adobe.

>>

>>

>>

>> Specifically, are there any important colour spaces that are missing,

>> have we made any incorrect assumptions, is there anything that we can

>> add that would be helpful.

>>

>>

>>

>> Best regards,

>>

>> Craig Revie,

>>

>> FFEI Limited

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