Headless static (svg, png) image generation

[abastardi]

Enable generation of PNG images directly on the server, similar to the Vega.js headless mode (https://github.com/trifacta/vega/wiki/Headless-Mode).

[bryevdv]

@mattpap do you have any thoughts about what it would take to expose the phantomjs PNG generation we do with the test script to users?

[damianavila]

But using the phantom approach we have an image with the buttons and uncentered... so probably we need to pre-process the plot to render properly... just thoughts....

[bryevdv]

Right it won't work as-is. I can think of a few possible ways to use phantoms:

• allow automation of the existing preview/save tool to save the image buffer directly to disk without having the user have to navigate a file save dialog.
• putting the plot in a different template/div layout for headless image generation, that does not have the toolbar, etc., and using the current "screenshot" approach that the test script uses to save images.

There may be others too.

[bryevdv]

Update: it does look as though node-canvas may work for this, I am going to start some preliminary experimentation.

[bryevdv]

Initial experiments on OS X are encouraging that node-canvas should be suitable, even for plots with image data. However, installing node-canvas and its dependencies is not trivial and will take some work to make it as simple and easy as possible for users.

[bryevdv]

Getting BokehJS working in node.js is a first prerequisite, I have made a separate issue for that task,

[hhuuggoo]

we should rename this issue - there is no reason to involve the server at all here right?

[abastardi]

@hhuuggoo, the idea is to be able to produce plots on the server rather than having them rendered in the browser. This can be useful when you need to generate plots to be used in some context other than display in the browser (e.g., generation of a PDF report).

[bryevdv]

@abastardi yes but in general you will be able to generate images from python, whether that's in the server, or in an interactive session or IPython notebook makes no difference.