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. Benoit > > Nick > > _______________________________________________ > dev-platform mailing list > dev-platform@lists.mozilla.org > https://lists.mozilla.org/listinfo/dev-platform > _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
