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.

[dcherian]

I have a use-case where I'd like a single color in a colormap to be transparent ("background value") but there seems to be no way to specify this at the moment. I am doing this for "continuous" variables, not "categorical".

[jbednar]

For Datashader right now, NaN values in are transparent for float arrays and zero values are transparent for unsigned integer arrays. I.e., transparency is determined by the data, not by the colormap. You can file a feature request to support colormaps with transparent values. Alternatively, I believe Bokeh supports colormaps with transparent values, and so if you display the output in Bokeh I'd assume this should work already.

[dcherian]

transparency is determined by the data, not by the colormap. You can file a feature request to support colormaps with transparent values.

isn't that this issue?!

[jbednar]

Oh, is it? :-) The OP mentions "dynamic alpha", and I wouldn't think of a colormap as providing dynamic anything, but if all this issue needs is respecting the colormap's alpha value, then I agree with my earlier self that it shouldn't be a big lift, and I'm happy to accept a PR that does that.

In any case, @dcherian , I guess Bokeh's support for this doesn't work for your use case since you are rendering directly to tiles? In that case small changes would be needed to datashader/transfer_functions/init.py to respect the colormap's alpha.

Some thought would be needed about what to do with the alpha argument for shading, since if supplied it should be overriding the colormap's values as documented. I'd be ok if we changed alpha to have a default of None instead of 255 and to use the colormap's alpha in that case, though this would be a (minor) breaking change.