More on spatial file rendering

Following from my post yesterday I had a colleague ask me: “So what happens when you force client-side rendering at the detailed level? Does it blow up with too much data?”

Interesting question. Let’s find out.

(For those of you who don’t want to read the rest of the post… the answer is “no – we’re too clever for that!”)

As outlined in the documentation here, you can adjust the complexity threshold where we switch between client-side and server-side rendering, or you can force either using “:render=true” or “:render=false” on the URL. So here is my workbook from yesterday with the “:render=true” flag set at the raw level – notice that the map is being rendered in the browser:

Clearly it didn’t blow up. This is because Tableau Server does dynamic simplification of the polygons based on the zoom level of the viewer. We reduce the complexity of the data we deliver to the browser, removing data points that can’t be seen. Clever, huh? This means that for the above map we only ship 7.2MB of data to the browser (see the bootstrap package in the debug window) so there is no danger of the runtime exploding due to too many points.

Performance was acceptable for initial view but I’m local on GB Ethernet to my server. As previously documented, it would be slower on a low bandwidth connection, but once the map is rendered responsiveness is great! You can test it yourself here:

https://tab.databender.net/views/ShapefileRenderingTest/GCCMap?:embed=y&:render=true&:jsdebug=true

The simplification of the polygons is done on the server and this becomes apparent when you zoom. From the above map, I zoomed right in to Port Melbourne – a much closer view. Initially the map renders with the outline of the polygons fuzzy – it’s just zoomed the existing client-side rendered image:

However, the server is busy in the background computing the new model. Watching the trace, the pane-zoom event sits in “pending” state for a few seconds, then it starts to stream down the more granular polygon definitions and the boundaries sharpen up:

Additionally, to manage the data we ship to the browser, we also clip the data to the viewport so we only transfer the vertices we need to render. You can see the pane-pan-server event in pending state after I pan:

The viewport is updated once the data has arrived:

So – you can safely force client-side rendering even with very complex shapefiles however there are performance trade-offs as the server must compute simplified polygons for each visual model. The upside is that you can zoom in a long way and still have detailed boundaries.

Kudos to our product and development teams for developing this feature in such a robust way!

About Alan Eldridge

Hi. I'm Alan. By day I manage the sales consulting team for Tableau Software in Australia/New Zealand. By night, I'm a caped crusader. Or sleeping. Most often it's sleeping.
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