From: Rik Cabanier <cabanier@gmail.com>

Date: Sat, 7 Jun 2014 14:32:49 -0700

Message-ID: <CAGN7qDD=a+u_-xVhK3x8AGPRkRmH4jUFt3TzEAyt=_EfSfEo6Q@mail.gmail.com>

To: Kristopher Giesing <kris.giesing@gmail.com>

Cc: Dirk Schulze <dschulze@adobe.com>, FX <public-fx@w3.org>

Date: Sat, 7 Jun 2014 14:32:49 -0700

Message-ID: <CAGN7qDD=a+u_-xVhK3x8AGPRkRmH4jUFt3TzEAyt=_EfSfEo6Q@mail.gmail.com>

To: Kristopher Giesing <kris.giesing@gmail.com>

Cc: Dirk Schulze <dschulze@adobe.com>, FX <public-fx@w3.org>

On Sat, Jun 7, 2014 at 1:17 AM, Kristopher Giesing <kris.giesing@gmail.com> wrote: > There are already plenty of examples where theoretically idempotent > operations aren't idempotent due to rounding error. Off the top of my > head, "Math.sqrt(x)*Math.sqrt(x) === x" generally evaluates to false if x > isn't an integer. > > Therefore, I don't think we need to treat matrix identity in any > particularly special regard here. I would vote to leave isIdentity() in > its most basic and obvious form, that is, testing that the matrix truly is > the identity matrix, and not try to get tricky here about tracking what > transforms have been applied. > > I personally think it is actually valuable for programmers to hit these > kinds of problems so that they understand the implications of finite > precision math. > Are you saying we should provide this API so programmers learn that they shouldn't use it? > On Fri, Jun 6, 2014 at 11:33 PM, Dirk Schulze <dschulze@adobe.com> wrote: > >> Hi, >> >> During the discussion on dev-platform@lists.mozilla.org[1] some >> engineers raised concerns to support isIdentity. >> >> To quote Rik: >> >> “" >> […] as matrices get computed, they are going to jump unpredicably >> between being exactly identity and not. People using isIdentity() to jump >> between code paths are going to get unexpected jumps between code paths >> i.e. typically performance cliffs, or worse if they start asserting that a >> matrix should or should not be exactly identity. For that reason, I would >> remove the isIdentity method. >> “” >> >> So while the following is stable on a matrix that is already identity: >> >> matrix.translateBy(10).translateBy(-10); >> >> Certain functions are likely to cause not identity matrices: >> >> matrix.rotateBy(54.4, 20.5, 20.5).rotateBy(-54.4, 20.5, 20.5); >> >> Note that it rotates by 54.4 degrees and then rotates back by -54.4 >> degrees. While both operations should eliminate themself, it could happen >> that the matrix is not identity anymore. The strange part: on further >> transformations it could be identity again. This is because of different >> computational precision and rounding deviations. This is usually worst if >> trigonometric functions are involved. All rotate functions use sin() or >> cos() and are more likely to cause the described behavior. >> >Received on Saturday, 7 June 2014 21:33:16 UTC

*
This archive was generated by hypermail 2.4.0
: Friday, 17 January 2020 19:49:49 UTC
*