Re: luminanceToAlpha values in FEColorMatrix

Hi Dean,

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? 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.

Rik


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
>
>