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Re: Proposal: transformation matrix interface

From: David Sheets <kosmo.zb@gmail.com>
Date: Wed, 13 Mar 2013 16:21:56 -0700
Message-ID: <CAAWM5TxbxHkDL8=kyr-TgY+tqR1ggmd9d74GK2CSpCuFF_RK6w@mail.gmail.com>
To: Dirk Schulze <dschulze@adobe.com>
Cc: "www-svg@w3.org list" <www-svg@w3.org>, www-style list <www-style@w3.org>, "public-canvas-api@w3.org" <public-canvas-api@w3.org>
On Sat, Mar 9, 2013 at 5:50 AM, Dirk Schulze <dschulze@adobe.com> wrote:
> On Feb 14, 2013, at 3:21 PM, David Sheets <kosmo.zb@gmail.com> wrote:
>> 2. Is a DOMException necessary for the error conditions? Are
>> DOMExceptions generateable by user code? I would like to be able to
>> provide user-space implementations of Matrix that internally leverage
>> invariants for better performance/behavior. For example, if I know
>> that I am only dealing with rotation matrices, I might use an
>> implementation of Matrix that internally uses only a quaternion
>> representation. Similarly, if I know that I am only going to use
>> special orthogonal transformations (no scale, no skew, det=1), I might
>> use a dual quaternion representation internally. It would be nice to
>> be able to pass these constructions into APIs that expect Matrix
>> objects without building a native Matrix first.
>
> There is a DOMException interface to generate DOMExceptions. You can prototype Matrix with your own methods and attributes that can throw your generated exceptions. Is that sufficient for you?

Does that provide identical semantics? Has something changed since
Marcos' SO answer
<http://stackoverflow.com/questions/5136727/manually-artificially-throwing-a-domexception-with-javascript#answer-9856490>
and the thread on public-script-coord
<http://lists.w3.org/Archives/Public/public-script-coord/2012JanMar/0458.html>?

Is there some way to make instanceof behave identically? Or do Matrix
users need to wrap a native Matrix to poke for exceptions? What
happens when a script in a JS environment without DOM (e.g. node.js)
wants to use a Matrix?

>> 4. In DecomposedMatrix, "quaternions of type sequence<double> Is an
>> sequence of four double items representing the two quaternions for the
>> rotation of the matrix." In the pseudocode for the decomposition, I
>> only see a single quaternion generated. The other quaternion that
>> describes the rotation is simply the negation of this quaternion as H
>> double covers SO(3). I believe that the decomposition should always
>> return the quaternion that represents the shortest rotation which
>> corresponds to the quaternion with a positive scalar component
>> (quaternion[3]?). This matters when an API user wants to take the
>> logarithm of a quaternion and retrieve consistent bivector angles.
>
> At the moment Matrix follows CSS3 Transforms and the implementation behavior of browsers. The definition seems bogus, since we just have one quaternion with 4 arguments, not two. I will correct the specification text. In the meantime, can you go into more details how you suggest this operation should look like?

I think the provided pseudocode implements what I am proposing the
prose describe. Specifically, I believe that the prose should indicate
that a single quaternion is returned which has a non-zero scalar part
and L_2 magnitude of 1 ("unit quaternion"). It might also mention that
this implies that the returned quaternion represents the geodesic
(shortest path) for interpolators. To produce the complementary arc,
the quaternion's elements simply need scaling by -1. This requirement
is quite cheap for implementors, nicely uniform for users, and a good
invariant.

Thanks for your work.

Hope this helps,

David
Received on Wednesday, 13 March 2013 23:22:49 GMT

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