SVG Masking and different implementations

Hi,

I tried
http://trac.webkit.org/export/40986/trunk/LayoutTests/svg/custom/grayscale-gradient-mask.svg on different svg-viewers/browsers and find that the drawing of Opera, librsvg, Incscape differs to firefox, Safari, Batik.
If you look to the source of that SVG, you'll notice, that the alpha
channel is 255 and has no affect on the masking, therefore premultiplied
or unpremultiplied doesn't care here.
Doesn't it mean that we just have to calculate the luminance by
r * 0.2125 + g * 0.7154 + b * 0.0721 and set the alpha to this
luminance?
If that is the case, Opera, librsvg, Incscape are right. Can someone
complain if/why I'am wrong?

-Dirk