[cairo] Shouldn't Cairo use/offer degrees rather than radians?

David Kastrup dak at gnu.org
Tue Jun 27 09:37:07 UTC 2017

Lawrence D'Oliveiro <ldo at geek-central.gen.nz> writes:

> On Tue, 27 Jun 2017 10:24:18 +0200, Guillermo Rodriguez wrote:
>> 2017-06-27 9:57 GMT+02:00 Lawrence D'Oliveiro:
>>> On Tue, 27 Jun 2017 08:03:36 +0200, David Kastrup wrote:
>>>> PostScript uses degrees.  PDF uses degrees.  
>>> No!  
>> Actually, yes..
> Trig calculations are usually easier in radians.

Nobody uses Taylor series for actual trig calculations.  The Cordic
algorithms work from full revolutions.

If I write

#include "math.h"
#include "stdio.h"

main ()
  printf ("%lg\n", cos (M_PI/2));
  return 0;

and run it, the output is 6.12323e-17 .  So apparently radians do not
make things easy enough for computer/user to work well in graphics

If you want to turn something by exactly 90 degrees or 180 degrees (not
exactly a rare event in graphics), you cannot do so by using a Cairo
function accepting angle arguments but have to specify the transform
matrix manually.

That is one important reason that graphics formats like SVG, PostScript
and PDF specify angles in degrees: that way right angles are actually

David Kastrup

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