From: Brendan Kenny <bckenny@gmail.com>

Date: Wed, 2 Dec 2009 15:42:39 -0600

Message-ID: <ab96c3ef0912021342r7196eb57h2dfa4bdf32b2538@mail.gmail.com>

To: "Dr. Olaf Hoffmann" <Dr.O.Hoffmann@gmx.de>

Cc: www-style@w3.org

Date: Wed, 2 Dec 2009 15:42:39 -0600

Message-ID: <ab96c3ef0912021342r7196eb57h2dfa4bdf32b2538@mail.gmail.com>

To: "Dr. Olaf Hoffmann" <Dr.O.Hoffmann@gmx.de>

Cc: www-style@w3.org

I hadn't noticed this, but it's a very good point. For those a little rusty on their transforms, a determinant of 1 is important because it means that the area of the figure being transformed in preserved. This is true for skewX() and skewY(), but not for skew(<angle>,<angle>) as written. As far as I know this function doesn't really correspond to anything in widespread use and it gives results that aren't necessarily what the user wants. I think the best interpretation could be a skew in one direction followed by a skew in the other, but since that operation would be order dependent, it still could lead to relatively surprising results. Is there a compelling reason to include it? 2009/12/2 Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>: > Simon Fraser: >>skew(a, b) results in the this matrix: [1 tan(b) tan(a) 1 0 0]. > > Zack Weinberg: >> >> For what it's worth, this is the matrix that Gecko produces for >> >> skew(ThetaX, ThetaY): >> | 1 tan(ThetaX) 0| >> | tan(ThetaY) 1 0| >> | 0 0 1| >> >> skewx(theta) is treated as identical to skew(theta, 0) and >> skewy(theta) is treated as identical to skew(0, theta). >> >> I have no position on whether this is useful to authors or >> mathematically meaningful. >> >> zw > > Interesting ... > Then the naming is misleading. > Because the determinant is not 1 for arbitrary angles, > this is not really skewing, it is a mixture of skewing and scaling > or something even more general... > > skewX and skewY are defined to be the same as in SVG, therefore those > are simple and really related to skewing/shearing, because the determinant > is 1. > SVG has no skew without X or Y, therefore currently the problem occurs > only with the undefined property in the CSS3 proposal. > > Maybe more useful to have a vector and an angle as value > to be able to share along the vector instead of having this surprising > transformation not related to skew or shear. > > Or with two angles alpha and beta one could define it as > something like > rotate(-alpha) skewX(beta) rotate(alpha) > to get something meaningful > > > > Olaf > > > http://mathworld.wolfram.com/ShearMatrix.html > > > > >Received on Wednesday, 2 December 2009 21:43:14 GMT

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