Displaying custom data on hover for RGBA image

It would be nice to be possible to show some sort of information on RGBA images using Bokeh (https://stackoverflow.com/questions/54753032/hover-image-rgba-bokeh/54753614#54753614). For instance, I’m working on a project in which we provide an RGBA image where the alpha represents some sort of confidence. We would like, when hovering an image pixel, to be able to show this confidence (in this case, the rgba[:,:,3] value).

[maximlt]

As can be seen in the Bokeh 2.4.3 docs hovering over an RGBA image shows the 32-bit RGBA int value.

It seems like a better default would be to display the RGBA tuple, as asked on Discourse (we also get this request on hvPlot/HoloViews).

I’m writing first in this old issue to make sure that it should not be re-opened, I’m not entirely sure the PR that closed it effectively fulfilled the original request. Happy to open a new feature request otherwise, if it’s a good idea!

[bryevdv]

My original (offhand) proposal was to expose new special fields $red, $green, $blue and $alpha that users could use to extract color sub-components from any value. This approach:

• would work for any color column, whether it is used for ImagRGBA or not
• affords a simple mechanism for users to arrange the components exactly how they want (e.g in a tuple in one line? Or separate rows for every channel) without inventing some baroque color-formatting DSL
• Is immediately understandable by anyone who already knows what RGBA channels are (i.e. everyone who would want to be able to)
• works well with existing formatters: want hex values? No problem, used “printf” format with $red, done!

I still (personally) think this is the bst option that trades a minuscule amount of verbosity for 1) much more flexibility for users, 2) much less maintenance burden for developers. We can cerainly continue to debate/discuss that further but at this point I really must insist on a fresh issue.