Error drawing circles when coord="wcs"

While developing for eteq/astrowidgets#1 , I encountered error when drawing circles in in coord='wcs' mode. Circle radius is set to 20 (I assume it is in pixels but I have no idea if its unit secretly changes when I switch from “data” to “wcs” mode).

Here are the four points I attempted to draw:

RA (deg)	DEC (deg)
3.609821370156458	-30.378452205494682
3.589858963715532	-30.368501816764578
3.5872920549742506	-30.371258703333265
3.58680555297819	-30.366805584777612

What I did was attempting to create a CompoundObject from four individual Circle, each one constructed like this:

Circle(x=ra, y=dec, radius=20, coord='wcs', color='cyan')

This is the error I got from Ginga:

...\ginga\canvas\CanvasObject.py:193: RuntimeWarning: invalid value encountered in sqrt
  radius = np.sqrt(abs(y2 - y1)**2 + abs(x2 - x1)**2)
2018-07-21 12:13:54,733 | E | ImageView.py:1272 (redraw_now) | Error redrawing image: cannot convert float NaN to integer
2018-07-21 12:13:54,747 | E | ImageView.py:1277 (redraw_now) | Traceback:
  File "...\ginga\ImageView.py", line 1258, in redraw_now
    self.redraw_data(whence=whence)
  File "...\ginga\ImageView.py", line 1303, in redraw_data
    self.private_canvas.draw(self)
  File "...\ginga\canvas\DrawingMixin.py", line 723, in draw
    super(DrawingMixin, self).draw(viewer)
  File "...\ginga\canvas\CompoundMixin.py", line 143, in draw
    obj.draw(viewer)
  File "...\ginga\canvas\CompoundMixin.py", line 143, in draw
    obj.draw(viewer)
  File "...\ginga\canvas\types\basic.py", line 1044, in draw
    cr.draw_circle(cx, cy, cradius)
  File "...\ginga\pilw\CanvasRenderPil.py", line 110, in draw_circle
    self.cr.circle((cx, cy), cradius, self.pen, self.brush)
  File "...\ginga\pilw\PilHelp.py", line 124, in circle
    radius = int(radius)

I am using Ginga 2.7.1.dev1944 on Windows 7 64-bit with ipyevents on Jupyter Lab. In case it matters, Astropy is 3.1.dev22186.

I attempted to code dive a bit but got lost at viewer.renderer calls. What am I doing wrong here? Any help would be appreciated. Thanks!

[mwcraig]

How does Ginga compute this conversion internally?

I’m guessing it is derived from the WCS.

Thinking about that just a bit further, it’s probably not too much more work to subclass the shape to make it take an astropy coordinate object and the radius as a value with a units attached.

This might be nice; I can imagine cases where I want to specify the location in world coordinates but I want the mark to have a fixed pixel size (e.g. for aperture photometry).

An alternative might be to allow radius to have a unit (even if x, y are still separate unitless arguments).

[pllim]

Am I correct in assuming that if you have a SkyCoord and you add a unit that is in pixels it does the right conversions?

Currently, for coord='wcs', I pass SkyCoord.ra.deg and SkyCoord.dec.deg into Ginga.

To convert radius from degree to pixels, I would need to know the detector pixel scale (which might or might not be available as metadata in the image). How does Ginga compute this conversion internally?