Bokeh: `alpha` silently not working on `image`

[darribas]

I'm running the following piece of bokeh code:

import numpy as np
from bokeh.plotting import figure, show, output_file
from bokeh.tile_providers import STAMEN_TERRAIN

bbox = [-10910125.412422614,
        3520337.5607719547,
        -10861114.121738775,
        3567832.0546971313]

N = 10
x = np.linspace(bbox[0], bbox[2], N)
y = np.linspace(bbox[1], bbox[3], N)
xx, yy = np.meshgrid(x, y)
d = np.sin(xx)*np.cos(yy)

p = figure(x_range=(bbox[0], bbox[2]), y_range=(bbox[1], bbox[3]))
p.add_tile(STAMEN_TERRAIN)

p.image(image=[d], x=bbox[0], y=bbox[1], \
        dw=bbox[2]-bbox[0], dh=bbox[3]-bbox[1], \
        alpha=0.5, palette='Blues9')

output_file('image.html', title='test')
show(p)

which, when you open the html file created, looks like:

As can be seen, even though I have included alpha=0.5, the overlaid image is completely opaque, not allowing to see through it. According to the bokeh documentation, alpha is allowed, but it does not seem to make any effect.

[bryevdv]

Oh I see. The issue is that the alpha and color keywords are conveniences to set line_alpha/fill_alpha and line_color/fill_color more easily. That is the useful thing for almost eveyr other glyph, but it does not do anything useful for image. Options:

• try to make it meaning for image as well
• explicitly remove it from image and other glyphs it does not apply to

Either way it probably makes sense to add a global_alpha property to Image since it does not currently have one.

[bryevdv]

Classifying as bug since docs are misleading, at a minimum.

[bryevdv]

this was duped by #7474 (not noticed) and that issue has been completed and closed.