On Mon, Jun 2, 2014 at 3:03 PM, Benoit Jacob <jacob.benoi...@gmail.com> wrote:
> > > > 2014-06-02 17:13 GMT-04:00 Rik Cabanier <caban...@gmail.com>: > > >> >> >> On Mon, Jun 2, 2014 at 11:08 AM, Benoit Jacob <jacob.benoi...@gmail.com> >> wrote: >> >>> >>> >>> >>> 2014-06-02 14:06 GMT-04:00 Benoit Jacob <jacob.benoi...@gmail.com>: >>> >>> >>>> >>>> >>>> 2014-06-02 13:56 GMT-04:00 Nick Alexander <nalexan...@mozilla.com>: >>>> >>>> On 2014-06-02, 9:59 AM, Rik Cabanier wrote: >>>>> >>>>>> >>>>>> >>>>>> >>>>>> On Mon, Jun 2, 2014 at 9:05 AM, Nick Alexander < >>>>>> nalexan...@mozilla.com >>>>>> <mailto:nalexan...@mozilla.com>> wrote: >>>>>> >>>>>> On 2014-06-02, 4:59 AM, Robert O'Callahan wrote: >>>>>> >>>>>> On Mon, Jun 2, 2014 at 3:19 PM, Rik Cabanier < >>>>>> caban...@gmail.com >>>>>> <mailto:caban...@gmail.com>> wrote: >>>>>> >>>>>> isIdentity() indeed suffers from rounding errors but since >>>>>> it's useful, I'm >>>>>> hesitant to remove it. >>>>>> In our rendering libraries at Adobe, we check if a matrix >>>>>> is >>>>>> *almost* >>>>>> identity. Maybe we can do the same here? >>>>>> >>>>>> >>>>>> One option would be to make "isIdentity" and "is2D" state bits >>>>>> in the >>>>>> object rather than predicates on the matrix coefficients. Then >>>>>> for each >>>>>> matrix operation, we would define how it affects the >>>>>> isIdentity >>>>>> and is2D >>>>>> bits. For example we could say translate(tx, ty, tz)'s result >>>>>> isIdentity if >>>>>> and only if the source matrix isIdentity and tx, ty and tz are >>>>>> all exactly >>>>>> 0.0, and the result is2D if and only if the source matrix is2D >>>>>> and tz is >>>>>> exactly 0.0. >>>>>> >>>>>> With that approach, isIdentity and is2D would be much less >>>>>> sensitive to >>>>>> precision issues. In particular they'd be independent of the >>>>>> precision used >>>>>> to compute and store store matrix elements, which would be >>>>>> helpful I think. >>>>>> >>>>>> >>>>>> I agree that most mathematical ways of determining a matrix (as a >>>>>> rotation, or a translation, etc) come with isIdentity for free; >>>>>> but >>>>>> are most matrices derived from some underlying transformation, or >>>>>> are they given as a list of coefficients? >>>>>> >>>>>> >>>>>> You can do it either way. Here are the constructors: >>>>>> http://dev.w3.org/fxtf/geometry/#dom-dommatrix-dommatrix >>>>>> >>>>>> So you can do: >>>>>> >>>>>> var m = new DOMMatrix(); // identity = true, 2d = true >>>>>> var m = new DOMMatrix("translate(20 20) scale(4 4) skewX"); // >>>>>> identity = depends, 2d = depends >>>>>> var m = new DOMMatrix(otherdommatrix; // identity = inherited, >>>>>> 2d = >>>>>> inherited >>>>>> var m = new DOMMatrix([a b c d e f]); // identity = depends, 2d = >>>>>> true >>>>>> var m = new DOMMatrix([m11 m12... m44]); // identity = depends, >>>>>> 2d = >>>>>> depends >>>>>> >>>>>> If the latter, the isIdentity flag needs to be determined by the >>>>>> constructor, or fed as a parameter. Exactly how does the >>>>>> constructor determine the parameter? Exactly how does the user? >>>>>> >>>>>> >>>>>> The constructor would check the incoming parameters as defined: >>>>>> >>>>>> http://dev.w3.org/fxtf/geometry/#dom-dommatrixreadonly-is2d >>>>>> http://dev.w3.org/fxtf/geometry/#dom-dommatrixreadonly-isidentity >>>>>> >>>>> >>>>> Thanks for providing these references. As an aside -- it worries me >>>>> that these are defined rather differently: is2d says "are equal to 0", >>>>> while isIdentity says "are '0'". Is this a syntactic or a semantic >>>>> difference? >>>>> >>>>> But, to the point, the idea of "carrying around the isIdentity flag" >>>>> is looking bad, because we either have that A*A.inverse() will never have >>>>> isIdentity() == true; or we promote the idiom that to check for identity, >>>>> one always creates a new DOMMatrix, so that the constructor determines >>>>> isIdentity, and then we query it. This is no better than just having >>>>> isIdentity do the (badly-rounded) check. >>>>> >>>> >>>> The way that propagating an "is identity" flag is better than >>>> determining that from the matrix coefficients, is that it's predictable. >>>> People are going to have matrices that are the result of various arithmetic >>>> operations, that are close to identity but most of the time not exactly >>>> identity. On these matrices, I would like isIdentity() to consistently >>>> return false, instead of returning false 99.99% of the time and then >>>> suddenly accidentally returning true when a little miracle happens and a >>>> matrix happens to be exactly identity. >>>> >>> >>> ...but, to not lose sight of what I really want: I am still not >>> convinced that we should have a isIdentity() method at all, and by default >>> I would prefer no such method to exist. I was only saying the above _if_ we >>> must have a isIdentity method. >>> >> >> Scanning through the mozilla codebase, IsIdentity is used to make >> decisions if objects were transformed. This seems to match how we use >> Identity() internally. >> Since this seems useful for native applications, there's no reason why >> this wouldn't be the case for the web platform (aka blink's "rational web >> platform principle"). If for some reason the author *really* wants to know >> if the matrix is identity, he can calculate it manually. >> >> I would be fine with keeping this as an internal flag and defining this >> behavior normative. >> > > Gecko's existing code really isn't authoritative on matrix design issues!! > Please don't use it as an argument either way in this discussion --- just > because it uses isIdentity() doesn't imply even that isIdentity is actually > useful to it. It could just be shooting itself in the feet. > > (not to pick on any code in particular --- I haven't looked. Just > generically saying I've seen more bad code than good code in this > department). > Sure. My argument was that if this is useful for applications and they can use it successfully, why not expose it to the web platform? I looked at the WebKit and Blink codebases and they also use this API a lot. _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
