Re: [NTG-context] atan2 function

Are you using MP in scaled integer or in doubleprecision mode? I have not looked into the MP angle operator code, but my suspicion is that it is based on some super-clever John Hobby scheme optimized for scaled integer calculations. Hans has integrated the entire math library (and more) into lmtx. On 13/10/22 13/10/22, 13:51, Thangalin via ntg-context wrote:

Thank you, Max.

The angle function doesn't appear to provide the same calculation as my atantwo in all cases.

https://pdfhost.io/v/Oqj7XmibJ_scaled

The shorter line segment should be directed towards the vertex closest to the longer line segment. I tried using both:

theta := angle( dx, dy ); theta := angle( dy, dx );

Neither made a difference, in some cases the angle differs from what I'd expect.

My implementation is based on the first version given here:

https://en.wikipedia.org/wiki/Atan2#Definition_and_computation

I have something that works, so this is more of a curiosity as to there being a difference between my implementation of atantwo and the angle function.

Here's an example with many lines and the angle function: https://pdfhost.io/v/1T4jgBnxh_scaled

On Wed, Oct 12, 2022 at 11:42 PM Max Chernoff wrote:

...

Hi,

...

How is atan2 called? I rolled my own as follows:

...

Is atan with two parameters supposed to behave like atan2?

At mp-math.mpxl:167 there is:

vardef atan primary x = angle(1,x) enddef ;

The MetaPost manual says:

The angle operator takes a pair and computes the two-argument arctangent; i.e., angle is the inverse of the dir operator

So it looks like "angle" is the function that you want for "atan2".

Thanks, -- Max

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