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Re: Color fidelity during transcoding

From: Chris Lilley <chris@w3.org>
Date: Fri, 25 Aug 2006 10:03:34 +0200
Message-ID: <1654616867.20060825100334@w3.org>
To: "Glenn A. Adams" <gadams@xfsi.com>
Cc: public-tt@w3.org

On Thursday, July 27, 2006, 5:07:48 PM, Glenn wrote:

GAA> Chris,

GAA> Thanks for your comment. The TT WG has reviewed this comment has agreed
GAA> upon the following response:

GAA> Other than the use of sRGB, DFXP intentionally does not specify
GAA> conformance requirements on fidelity of color conversion from a source
GAA> document to a DFXP document. At this point, we have not identified any
GAA> requirements that would suggest adding this level of specificity.

I am afraid that this response does not satisfy me. I suspect that the technical aspects of sRGB, in particular the corrections for flare and ambient illumination, have been insufficiently considered. 

Note that sRGB, while using a 709-compatible phosphor and white point chromaticity, has different assumptions to 709 regarding flare and ambient illumination. RGB values cannot therefore be simply transferred unchanged between the two systems while maintaining acceptable broadcast quality. For a system aimed at use in studio broadcast equipment, this is unacceptably lax and will lead to interoperability problems.

GAA> Regards,
GAA> Glenn

GAA> -----Original Message-----
GAA> From: public-tt-request@w3.org [mailto:public-tt-request@w3.org] On
GAA> Behalf Of Chris Lilley
GAA> Sent: Saturday, June 03, 2006 12:39 AM
GAA> To: public-tt@w3.org
GAA> Subject: Color fidelity during transcoding

GAA> Hello public-tt,

GAA>  I'm pleased to note that all colors are specified in the sRGB color
GAA> space.

GAA> Does TT AF DFXP express any conformance requirement regarding the
GAA> fidelity of the colors when converting to and from DFXP? I am thinking
GAA> not only of chrominance and white-point adaptation effects - which
GAA> should be minimal for modern equipment given that sRGB uses the 709
GAA> phosphor chromaticities - but more for differing assumptions on headroom
GAA> footroom, allowed colors, and corrections for flare, scene brightness,
GAA> surround and other viewing condition effects.

GAA> There is a useful discussion at
GAA> http://www.srgb.com/srgb709compatibility.html
GAA> How are sRGB and ITU-R BT.709-2 compatible?

GAA> Basically I am wondering how much guidance should be given in this area
GAA> such that reliable interchange between differing systems can be
GAA> achieved.

 Chris Lilley                    mailto:chris@w3.org
 Interaction Domain Leader
 Co-Chair, W3C SVG Working Group
 W3C Graphics Activity Lead
 Co-Chair, W3C Hypertext CG
Received on Friday, 25 August 2006 08:03:47 UTC

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