[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"
int
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
applications.
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
representable.
--
David Kastrup
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