# calculate part of solid circle in 2D array

Roy Smith roy at panix.com
Mon Nov 25 15:46:16 CET 2013

```In article <1fc9a269-4847-4d29-a35e-5cf91731e295 at googlegroups.com>,
Robert Voigtländer <r.voigtlaender at gmail.com> wrote:

> Thanks a lot for the links.
>
> I don't need it to be drawn. I need the fields within the arc for some
> statistical calculations for an occupancy map.
> So the target is a 2D array, not a picture.
>
> Robert

If you go with one of the suggestions to use a graphics package to draw
the arc, you can then take the resulting bitmap image and iterate over
it to see which pixels are black and which are white.

expensive".  Trig is cheaper than you think, because it's all done down
in the C libraries, and I wouldn't be surprised if it's not pushed all
the way down to the hardware on most modern machines.  Compared to your
Python application code, I suspect the amount of trig needed for this
problem isn't going to be a major factor in timing.

Are you guessing that trig is too slow, or have you actually done some
profiling to measure whether it is or not?

What's the resolution required?  Let's assume a 1k x 1k image with the
sensor in the center and you need 1 degree angular resolution.  There's
about 2200 pixels in each 1-degree sector.  You could pre-compute those
and store 360 sets of 2200 pixels each (about 800k points total).  For
any given 30 degree sector, just "or" together the 30 1-degree slices
and you've got your set of pixels for that sector.

Will it be fast enough?  Beats me, but it's easy to test.

```