Colormap alpha support

[kevinjohncutler]

Looks like transfer_functions.shade() only allows dynamic alpha when the cmap is a single color. That is, if an arbitrary RGBA colormap is passed as cmap, the alpha value of that colormap is ignored and a global alpha value is applied to the RGB values pulled from the colormap. Is this the intended behavior?

[amaloney]

Hi @kevinjohncutler would you mind adding a minimal reproducible example to the issue, if you are still seeing the problem?

Having one generally helps out everyone involved, as well as future searchers with similar (if not the same) problem.

[jbednar]

I don't recall anyone discussing this topic in early Datashader development. Definitely open to seeing details about how this could be used and what it should do. It doesn't seem like a big lift.