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Re: luminanceToAlpha values in FEColorMatrix

From: Chris Marrin <cmarrin@apple.com>
Date: Fri, 02 Sep 2011 15:07:03 -0700
Cc: Rik Cabanier <cabanier@gmail.com>, public-fx@w3.org, www-svg <www-svg@w3.org>
Message-id: <67ACA8EB-5754-4F50-A1B9-E78517ACBA78@apple.com>
To: Dean Jackson <dino@apple.com>

On Sep 2, 2011, at 10:44 AM, Dean Jackson wrote:

> Hi Rik,
> 
> On 02/09/2011, at 2:53 PM, Rik Cabanier wrote:
> 
>> PDF (and all Adobe apps that create luminosity masks) use 0.03 Red, 0.59 Green, 0.11 Blue which is close to ITU-R BT.601
>> 
>> Where did you get the values for Flash?
> 
> Good catch! It turns out that I thought I was reading the Flash docs, but I was instead reading the docs for a Flash library. My mistake.
> 
> Also, while admitting that error, I found another since sending that email. The OpenGL values come from informative notes on the OpenGL site - not an official value.

Those OpenGL values are computed assuming a Gamma value of 1.0, which is what GL always assumed long ago. I don't think those numbers are meaningful here.

> 
>> Flash content generated from our applications (such as InDesign and Acrobat.com) also use these values.
>> I can ask internally where these numbers came from.
> 
> Maybe the question to ask is if the BT.709 values are acceptable? Given that they've been in the spec for a long time (although with a minor difference/error) I think we'd need a good reason to change them dramatically. I wonder what the Adobe SVG Viewer used - the Adobe values or the SVG values?

BT.709 is a very well defined spec, used for HDTV. So it seems like a reasonable approach, both for that reason and because it would be the low impedance path. But we should change the numbers to match BT.709. I can't imagine the slight differences in values being an issue. 

-----
~Chris
cmarrin@apple.com
Received on Friday, 2 September 2011 22:07:44 GMT

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