[BUG] BoxAnnotation doesn't respect fill_color=None

[lucius-messerer]

ALL software version info (bokeh, python, notebook, OS, browser, any other relevant packages)

Bokeh 2.0.0 | Python 3.7 | Firefox 74.0 | Windows 10 build 16299.1747

Description of expected behavior and the observed behavior

Expected behavior: Glyphs generally support turning off fill by supplying a fill_color value of None. Would expect the same from BoxAnnotation.

Observed behavior: BoxAnnotation (and maybe others? haven't tested) instead appears to have a white fill with a low (but nonzero) alpha under these conditions. Setting fill_alpha to zero has the desired effect, but this is not my standard usage pattern for turning off fill, and it should probably be consistent anyway.

Complete, minimal, self-contained example code that reproduces the issue

import bokeh.plotting
import bokeh.models

bokeh.plotting.output_file("box_annotation_test.html")

fig = bokeh.plotting.figure(width=500, height=500, x_range=[0, 3], y_range=[0,3])

fig.circle(x=[1, 1, 2, 2], y=[1, 2, 1, 2], size=100, line_color=None, fill_color="Red")

ba = bokeh.models.BoxAnnotation(bottom=1, top=2, left=1, right=2, line_color="Blue",
                                line_dash="dotted", fill_color=None)
fig.add_layout(ba)

bokeh.plotting.save(fig)

Screenshots or screencasts of the bug in action

The BoxAnnotation is drawn between the centers of the red circles. Notice the "notches" of paler red where the box intersects them.

[bryevdv]

Right, it's currently not explicitly checking for null condition on the JS side, so it is rendering "null" color with a default opacity of 0.4 (whatever canvas defines that to do). For now you can add fill_alpha=0 as a workaround:

Since it should be a simple fix and since there a a workaround I will leave it for now a Good First Issue

[Tekaichi]

We can then assume when fill_color = None, fill_alpha should be set to zero then?
If so, I'm opening a PR with this fix.

[mattpap]

We can then assume when fill_color = None, fill_alpha should be set to zero then?

No, fill_color == None means no rendering should happen. The value of fill_alpha in this situation is irrelevant.

[Tekaichi]

Both the lines and the background should not be rendered then or just the background (which happens if fill_alpha = 0)?

[mattpap]

Line and fill visuals should be considered independently, the same way as glyphs work. Essentially you have to replace:

this.visuals.fill.set_vectorize(ctx, i)
ctx.fill()

with

if (this.visuals.fill.doit) {
  this.visuals.fill.set_vectorize(ctx, i)
  ctx.fill()
}

and similarly change line visuals.

The tricky bit is that annotations have both canvas and CSS modes. This code only handles to former and you will have to figure out the later.

[Tekaichi]

Ok, thanks for the feedback! Fixing that on the PR!