TL;DR: Bokeh and VisPy are both awesome projects. However, I wonder whether we need to change where things are currently going. While Bokeh is great at 2D and the browser, 3D is not supported. While Vispy is super-fast and good at 3D and custom visualizations, it's support for the browser is poor. I don't want to tell scientists that they need two or three visualization libraries. I want it all in one library.

Pride and discomfort

A short while ago, I was watching the talks from SciPy 2015 about Bokeh by Bryan Van de Ven and VisPy by Luke Campagnola. Being involved in both these projects, I felt very proud at seeing how both these projects are progressing. Both projects are awesome in their own way and really change the way that scientists can do visualization in Python.

At the same time, however, I felt a certain sense of discomfort. At first I was not sure why this was, but it was in part triggered by a question from the audience at the end of the VisPy talk about whether the VisPy and Bokeh projects can be combined in some way.

I don't think they can.

Bokeh is very much aimed at 2D plotting and does all its rendering in JavaScript. VisPy has strong support to allow 3D visualization, but does not have good browser support. Combining the two projects would essentially mean a rewrite.

And this is the thing that bothers me; both projects offer something great, but both also lack a fundamental feature that is essential to becoming the visualization library.

Bokeh

Bokeh was written from the start to render in the browser. The drawing system is implemented in CoffeeScript (which is compiled to JavaScript), which means that interactions with the plot can work without a Python (server) process.

I think this is essential for a visualization library of the future. Scientists want to share their results using interactive visualizations, or even small (dashboard) apps. HTML is the obvious medium to achieve this.

Bokeh uses the 2D canvas for its drawing, which is pretty fast, but not nearly as fast as WebGL. WebGL is currently being added to Bokeh, but I'm not sure if we can reach the performance that WebGL might ideally provide, because Bokeh was not designed to target WebGL from the start.

There have also been discussions (triggered by user requests) about adding support for 3D. I have mixed feelings about this, because 3D is very much an afterthought. Sure, we could add support for a few specific plot types, but users are going to ask for more. And 3D is always going to suck, as it does in Matplotlib, because 3D was not a primary target to start with. For proper 3D support, you need a good scene graph and first class support for lighting, cameras, etc. You can build a good 2D plotting API on top of a 3D visualization system (if taken into account from the start), but not the other way around.

VisPy

VisPy was written from the start to be a very fast and flexible visualization system, to support 2D, 3D and any special visualization that a user could think of.

I think this is essential for a visualization library of the future. Scientists sometimes have weird data that needs to be visualized in special ways, and 3D is nowadays not a rarity.

VisPy uses OpenGL for its drawing. We did target OpenGL ES 2.0 from the start, to allow rendering in WebGL and on mobile devices, so we did have the web in mind to some extend. However, the browser support of VisPy is implemented at a low level and goes more or less like this: the Python process sends OpenGL commands (via a custom format) to the browser to make the visualization appear. User input (mouse, keyboard) is captured and send to the Python process, where VisPy's event system takes care of translating and zooming camera's, etc., which causes an update, and thus new commands send to the browser. This all works pretty well, except that the visualization relies on the Python process. As a consequence, VisPy in the browser is not as snappy as it should be, and it won't work in an exported HTML document.

Cyrille Rossant has thought of some ideas on how to incorporate e.g. camera models in JavaScript, but these solutions are complex and do not scale well to user-defined interactions.

I want it all

Don't get me wrong: I think both Bokeh and VisPy are awesome. However, the way things are going now, it looks like we'll be having one library that's good at interactive 2D plotting via HTML, but sucks at 3D, and one library that's good at 2D/3D/other visualizations, but sucks on the web.

And this worries me.

Further, although both Bokeh and VisPy have plans to support svg/eps export, for the time being Matplotlib is the way to go for publication quality static images.

Imagine a scientist asking which visualization library she should use. "It depends ..." What if she has 3D data, she wants to share visualizations with her peers, and wants publication quality figures? These are not unreasonable or rare requests, and we should be able to answer: "Sure, just use this one library!"

Here is my list of the utopian visualization library:

  • Great at 2D, 3D and flexible enough for custom visualizations.
  • Leverages the GPU to allow large datasets and still render in real time.
  • Targets (or can target) the browser; visualizations in static HTML should be fully functional and interactive.
  • Also works great on the desktop (though this can be achieved via Xul/Electron/NW.js).
  • Supports export to eps and svg.

(Let me know if you think more points should be added.)

EDIT: In my next post I explain that this is actually a bad idea, and that there are other ways to achieve an easier workflow for our users.

Can we have it all?

I'm certain that we can have it all, but I won't pretend it's easy. To realize such a system, all requirements need to be incorporated from the start. Modifying either Bokeh or VisPy to support these will essentially be a rewrite.

One requirement would be that the complete drawing system runs in JavaScript. That does not sound like a fun implementation task, since the scene graph, transformations, and camera/interaction models are complex enough as it is. Maybe we can use PyScript to write such a system in JavaScript using a Python syntax. That way we should be able to reuse some of the code from VisPy.

A compromise from the point of view of Bokeh is that WebGL is not as widely supported as the 2D canvas, although this is getting much better.

A compromise from the point of view of VisPy is that WebGL is slightly slower than desktop GL and lacks certain features that are currently supported in VisPy, such as integration with OpenCL. In theory, the code could be written in a way to allows running it both as Python and PyScript, which might solve these issues.

Finally, implementing such a system is a major undertaking. Are there enough developers on the current projects that are interested in this? Are there financial resources to get this off the ground? Should we even try to create a single holistic visualization library?

I don't know. But I think we should at least discuss this.

Side notes

Khronos (the group responsible for the OpenGL standard) is working on a new API to target the GPU: Vulcan. This API is lower level and simpler, and intended for both visualization and compute. Cyrille Rossant wrote a blog post about targeting it in VisPy. It is not clear yet whether Vulkan will be available in the browser. If it will, I think we should probably use that instead of WebGl.

Final words

I know that what I am proposing is a bit insane. Yet, if we do not take some form of action, we'll be moving towards a future that is different from what we need to serve scientists with a proper unified visualization solution.


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