Re: luminanceToAlpha values in FEColorMatrix

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.

> 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?

Dean

> 
> 
> On Thu, Sep 1, 2011 at 6:10 PM, Dean Jackson <dino@apple.com> wrote:
> Get your acronym hats on.
> 
> A colleague has questioned the values used for luminanceToAlpha in FEColorMatrix. The specification (since SVG 1.0) has used:
> 
>  0.2125 red, 0.7154 green, 0.0721 blue
> 
> But where do these numbers come from? It turns out there are a few different specifications for this. The closest to what is specified is in Rec. ITU-R BT.709-5 [1], but it is slightly different:
> 
>  0.2126 red, 0.7152 green, 0.0722 blue
> 
> Yes, a minor difference, but it would be nice to reference an external specification. sRGB is based on Rec 709, so I guess this might be the definitive source. According to [2], sRGB uses the latter values.
> 
> Even then, I'm told that it is more common to use other luminance to alpha values. For example, ITU-R BT.601 (YIQ? NTSC? JPEG?) [3] uses:
> 
>  0.299 red, 0.587 green, 0.114 blue
> 
> And Flash + OpenGL use:
> 
>  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).
> 
> Note that these values are also used in other parts of FEColorMatrix, as well as other parts of the SVG specification (although it seems the spec doesn't always reference explicitly). For example:
> 
> - FEColorMatrix type="saturate" and type="huerotate"
> - 'mask' property
> 
> Dean
> 
> [1] http://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.709-5-200204-I!!PDF-E.pdf
> [2] http://en.wikipedia.org/wiki/SRGB
> [3] http://en.wikipedia.org/wiki/YCbCr#ITU-R_BT.601_conversion
> 
>