W3C home > Mailing lists > Public > www-svg@w3.org > September 2011

Re: luminanceToAlpha values in FEColorMatrix

From: David Woolley <forums@david-woolley.me.uk>
Date: Fri, 02 Sep 2011 23:56:02 +0100
Message-ID: <4E615F02.9010109@david-woolley.me.uk>
To: Dean Jackson <dino@apple.com>
CC: public-fx@w3.org, www-svg <www-svg@w3.org>
Dean Jackson wrote:

>   0.3086 red, 0.6094 green, 0.0820 blue
> Since sRGB is used throughout the SVG specification, and sRGB takes its values from Rec 709, and Rec 709 seems modern (it's designed for HDTV), and the values are really close to what is already specified, I suggest we formally link to Rec 709 and use its values (a very slight change to current implementations).

sRGB is an engineering compromise, based on the behaviour of CRTs. 
Computing Y as a weighted average of gamma corrected values is very easy 
to do in simple analogue TV hardware, but doesn't one really want to 
have a formula that better reflect the subjective brightness to a human 
observer.  (I believe that, to a first approximation, the human eye can 
be modelled with a gamma curve of rather less than x^2.2., but these 
days, if you are using 24 bit colour, you could probably afford to store 
the whole lookup Y=f(R,G,B) lookup table, and for greater colour depths, 
you could interpolate.

(I missed the start of this, but, if one is then going to use it as an 
alpha channel, one also needs to remove any remaining gamma correction.)

I'm only subscribed to www-svg, so public-fx may be delayed or fail.
David Woolley
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Received on Friday, 2 September 2011 22:56:50 UTC

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