From: Brendan Kenny <bckenny@gmail.com>

Date: Thu, 18 Nov 2010 19:45:08 -0800

Message-ID: <AANLkTikjdOFBPVktQsNQDUDbkN=82naTD2CVMRydcBv-@mail.gmail.com>

To: Boris Zbarsky <bzbarsky@mit.edu>

Cc: "www-style@w3.org" <www-style@w3.org>

Date: Thu, 18 Nov 2010 19:45:08 -0800

Message-ID: <AANLkTikjdOFBPVktQsNQDUDbkN=82naTD2CVMRydcBv-@mail.gmail.com>

To: Boris Zbarsky <bzbarsky@mit.edu>

Cc: "www-style@w3.org" <www-style@w3.org>

On Thu, Nov 18, 2010 at 7:08 PM, Boris Zbarsky <bzbarsky@mit.edu> wrote: > On 11/18/10 9:36 PM, Brendan Kenny wrote: >> >> Absolutely, but I think in almost all cases the translate function is >> a better option for that. > > Sure, but various CSSOM apis (e.g. computed style on a transitioning > element) will spit out matrix(). > >> Most matrix transform functions will >> probably be generated from a calculation of some sort, and it's not >> clear to me how one would even calculate, for instance, a rotation >> applied to a translation specified in one of the available units >> without knowing the scale factor between the two at calculation time. > > I'm not sure what the issue there is, actually... Applying a rotation to a > translation Just Works. Here's an example: > > ( cos(x) -sin(x) 0 ) ( 1 0 2em ) > ( sin(x) cos(x) 0 ) ( 0 1 3ex ) > ( 0 0 1 ) ( 0 0 1 ) > > That's applying a rotation by x to a (2em, 3ex) translation. Just doing out > the multiplication gives us: > > ( cos(x) -sin(x) 2em*cos(x) - 3ex*sin(x) ) > ( sin(x) cos(x) 2em*sin(x) + 3ex*cos(x) ) > ( 0 0 1 ) ouch, yeah, I was thinking of the general case, and not even the right way around. > [snip] > I don't follow this last part. How can it possibly make sense to make the > linear part of the matrix have length units, when it's being multiplied by > lengths (whereas the translation part is being _added_ to lengths)? Unit > analysis failure, sorry. ;) yeah... >> I'm sure such authors will be a tiny minority. On the other hand, if >> anyone copies and pastes a matrix from any kind of graphics text, it's >> all but certain that units won't be included. > > True; all such texts assume a predefined coordinate system. CSS doesn't > have one. > CSS has no concept of user units (or even of a current coordinate system, > which is necessary to define user units in the first place), so the only way > to handle the e and f values of the transform matrix is to do one of the > following: > 1) Have the spec assume an arbitrary unit for them and specify what it > is (which it doesn't). I assume webkit makes just such an arbitrary > unit choice; which one? This makes the entries _look_ like numbers > to web developers, even though they're no such thing. > 2) Have these be specified with length units in the matrix (what Gecko > does). This makes the fact that they're lengths explicit. > 3) Go ahead and try to actually define something akin to user units and > try to graft it onto CSS somehow. Apart from conceptual purity and > a feeling of well-being and contentment, this is fundamentally the > same as option 1, as far as I can see. I'll have to get some sleep and then think about this, but it does seem that I am aiming for option 3 which would indeed be a de facto option 1. Conceding the point for the moment, any ideas on matrix3d(), since, as was pointed out[1], some kind of inverse length unit would be required? [1] http://lists.w3.org/Archives/Public/www-style/2009Oct/0360.html (again)Received on Friday, 19 November 2010 03:45:41 GMT

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