On Mon, Jun 2, 2014 at 10:56 AM, Nick Alexander <nalexan...@mozilla.com> wrote:
> 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? > It looks like an oversight. I'll ask Dirk to update it. > 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. > > Nick > _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
