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

Lawrence D'Oliveiro ldo at geek-central.gen.nz
Tue Jun 27 23:37:49 UTC 2017

On Tue, 27 Jun 2017 11:37:07 +0200, David Kastrup wrote:

> 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 .

Which, with any reasonable pixel resolution that Cairo might be
expected to cope with, is indistinguishable from zero.

> 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.

On the contrary: specifying angles of ±90/±180 or ±0.5π/±π works just
fine. You can try it out for yourself with my “Matrix Basics”
interactive notebook <https://github.com/ldo/qahirah_notebooks>.

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