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From: Helge Blischke <h.blischke@acm.org>
Newsgroups: comp.lang.postscript
Subject: Re: -100 0 atan ==> 270 or -90?
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Date: Sat, 03 Aug 2013 15:42:18 +0200
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Cecil Westerhof wrote:

> Op zaterdag 3 aug 2013 06:23 CEST schreef luser:
> 
>> I've started writing up a test program for all operators,
>> mostly using the example snippets from the manual.
>>
>> And I've run into a difference between the Red Book (PLRM 1ed)
>> and the GNU C library in the value returned by atan.
>>
>> The Red Book says:
>>
>> -100 0 atan => 270.0
>>
>> But atan2(3) (after suitable conversions) gives me -90.0
>>
>> Is this serious? And, if so, what (if anything) should I do about
>> it?
> 
> It looks like postscript does a modulo 360. Which in certain
> definitions gives a value between 0 and modVal. The funny thing being
> that mod gives a value between -modVal and modVal.
> 
> So I would say it is a bug.
> 
The PLRM (3rd ed.) says:
atan num den atan angle
returns the angle (in degrees between 0 and 360) whose tangent is num 
divided by den.

So the positive value seems to be intentional.

Helge