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Subject: Re: [office] How to calculate the start and end point of an elliptical arc from given angles.

• From: Thorsten Behrens <tbehrens@suse.com>
• To: Regina Henschel <regina.henschel@libreoffice.org>
• Date: Thu, 4 Apr 2013 12:12:31 +0200

```Hi Regina,

thanks for another detailed analysis here -

you wrote:
> But the specification does not describe, how to calculate the start
> and end point. I have tested it with some applications and found,
> that actually two methods are used.
>
> (1) Used by LibreOffice 4.0 and PowerPoint 2013 Preview
> Draw an angle with the given value (here 30deg and 315deg).
> Calculate the intersection of the leg of the angle with the full
> ellipse (here center (3000|1000) and size (6000|2000)).
>
That at least appears to be the simplest interpretation. Also note
that those enhanced paths are more-or-less a bijection of those binary
ppt autoshape things (see also 20.1.9 in ISO 29500), and keeping that
mapping lossless and/or preventing excessive recalculation seems
desirable.

> I have no proposal, because both methods are reasonable.
>
I wonder how wide-spread those commands are - where did you notice the
difference, I presume there is a bug report somewhere?

> (C) The applications agree in which direction they draw the arc. But
> it looks not like "clockwise", although "clockwise" is specified.
>
That looks like an easy fix then.

> (D) The applications agree use the angle direction as positive from
> positive x-axis to negative y-axis. That is against mathematical
> definition. Again a problem of insufficient angle definitions in the
> specification.
>
Seems uncontroversial to fix, too?

Kind regards,

--

Thorsten Behrens

SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg; GF: Jeff
Hawn, Jennifer Guild, Felix Imendörffer, HRB 16746 (AG Nürnberg)
```

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