From: Rik Cabanier <cabanier@gmail.com>

Date: Sun, 11 Nov 2012 14:02:52 -0800

Message-ID: <CAGN7qDB68-Pt1KQ90YqeApuf1GRfwx7a8J1eiOQKA6q3KLxZaw@mail.gmail.com>

To: Boris Zbarsky <bzbarsky@mit.edu>

Cc: www-style@w3.org

Date: Sun, 11 Nov 2012 14:02:52 -0800

Message-ID: <CAGN7qDB68-Pt1KQ90YqeApuf1GRfwx7a8J1eiOQKA6q3KLxZaw@mail.gmail.com>

To: Boris Zbarsky <bzbarsky@mit.edu>

Cc: www-style@w3.org

I think this all depends how you multiply the matrices. Graphics libraries do a pre-concat which is the opposite of its mathematical definition and probably the root of the confusion. On Sun, Nov 11, 2012 at 8:03 AM, Boris Zbarsky <bzbarsky@mit.edu> wrote: > On 11/11/12 12:56 AM, Simon Sapin wrote: > >> Le 11/11/2012 05:13, Rik Cabanier a écrit : >> >>> It's always left to right so: >>> >>> >>> accumulator = accumulator * to_matrix(function) >>> >> >> Thanks for answering. For some reason I settled on the opposite in my >> code. >> > > That's because Rik is mostly wrong. ;) > > This whole thing is bit of a mess and continues to be in spite of repeated > requests to fix it up. :( See http://lists.w3.org/Archives/** > Public/www-style/2011May/0633.**html<http://lists.w3.org/Archives/Public/www-style/2011May/0633.html>and > http://lists.w3.org/Archives/**Public/www-style/2011Feb/0531.**html<http://lists.w3.org/Archives/Public/www-style/2011Feb/0531.html>for some previous issues that were raised... Unfortunately, they were only > partially addressed. http://dev.w3.org/csswg/css3-** > transforms/#mathematical-**description<http://dev.w3.org/csswg/css3-transforms/#mathematical-description>now defines what matrix each transform function corresponds to, but not how > to apply the matrix. Note that the matrices defined in this section will > act as expected if they are treated as matrices acting on column vectors by > left-multiplication. > > Rik is thinking of this in terms of what seems to be the usual graphics > programming convention in which matrices act on row vectors by > right-multiplication. So for _him_ the multiplication puts the new matrix > to the right of the accumulator. > > If you're thinking of your matrices as acting on column vectors by > left-multiplication, then your multiplication will need to put the new > matrix to the left of the accumulator. > see also http://en.wikipedia.org/wiki/Transformation_matrix#Composing_and_inverting_transformations > > These situations are completely equivalent (because just taking the > transpose of everything will go between the two of them). The main places > where confusion can arise are the definition of the "transformation matrix" > and the actual definition of what matrices matrix() produces. The latter > has been dealt with, but the former has not as you note. > > What I think we should do is define in section 21 that the matrices it > defines act on the column vector [x,y,z,1] by left-multiplication. And > then we should clarify section 6 by either describing what the > multiplication order for the matrices is is or by saying that the > transformations are applied in order from left to right and that the > "transformation matrix" is the matrix of the resulting transformation. That > avoids the problem, since transformation application, while > non-commutative, does not have the issue of "well, but _how_ do I apply the > transformation". Yes, if we write it out, it will be less confusing. > > > Similar questions arise with nested "transformed" stacking contexts >>> >> > See links above. :( > > > Is your confusion with preserver-3d or with regular 2d transforms? >>> In the case of 2d, the outer transform happens first (= to the left) >>> >> > See, this is part of the problem. The way the spec is phrased right now, > "first" is "to the _right_". And the fact that not everyone is on board > with that is not helping. :( > > -Boris > >Received on Sunday, 11 November 2012 22:03:20 UTC

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