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Subject: RE: [office-comment] Orientation problems with angles

• From: "Andreas J. Guelzow" <andreas.guelzow@concordia.ab.ca>
• To: "dennis.hamilton@acm.org" <dennis.hamilton@acm.org>
• Date: Sun, 5 Aug 2012 11:06:42 -0600

```On Sun, 2012-08-05 at 00:35 -0600, Dennis E. Hamilton wrote:
> Help me understand this better,
>
> From the perspective of the observer that you refer to,
>
> Where is the origin, and in which directions are +X and +Y from that
> origin?

I don't know, and I don't care. You do not have to have a given
coordinate system to define a rotation. A rotation is defined by its
centre, the amount and the direction of rotation.

>  And from which axis (+X, +Y, -X, -Y) extending from the origin does
> this clockwise motion originate that opens the angle with respect to
> which the gradient vector is oriented?

I assume you are referring to opacity gradients here. I agree that
draw:opacity is missing a statement what the 0-degree direction is. But
this has absolutely nothing to do with how angles are specified. (Note
that in the case of opacity, the centre of origin need not be specified
since any centre will result in the same gradient.)

Note that in the separate thread I already said that draw:opacity is not
complete, I also said that "draw:rotation(19.209) is completely
specified": "The direction is clockwise, 0 degrees is the vertical
axis." Since any choice of centre  will result in the same lines (note
that for hatches we are a really using a set of parallel lines.
[It could be argued that while the direction of these lines is now
completely specified, and there distance from each other is also given
there are still several hatches that are translated to each other
fitting this description. But this has nothing to do with angles and the
question at hand.]

Andreas

--
Andreas J. Guelzow, PhD, FTICA
Concordia University College of Alberta
```

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