Datashade support for VectorField

[brey]

Is your feature request related to a problem? Please describe.

In a case of very large datasets for e.g. wind data visualising quivers without datashade will be suboptimal. Is there a timeline or implementation path for this feature?

Describe the solution you'd like

Support Datashader as anticipated here

Describe alternatives you've considered

None

Additional context

None

[TheoMathurin]

Hi,

I also use HoloViews to visualise wind data.

It's likely that you don't need every single arrow shown on screen. If so, a workaround is to create a sparse vector field based on local average, dynamically adapting to the current viewport to keep a constant, user-determined number of vectors on screen.

Something along these lines:

            import datashader as ds

            def make_vector_field(x_range, y_range):
                cvs = ds.Canvas(plot_width=n_arrows_along_x,
                                plot_height=n_arrows_along_y,
                                x_range=x_range, y_range=y_range)
                angle = cvs.raster(data['angle'])
                magnitude = cvs.raster(data['magnitude'])
                vector_field = hv.VectorField((angle['longitude'], angle['latitude'], np.pi/2 - np.pi/180*angle[:, :], magnitude[:, :])).opts(magnitude='Magnitude')
                return vector_field


            xy_range = hv.streams.RangeXY()
            dmap = hv.DynamicMap(make_vector_field, streams=[xy_range])

Here is what my implementation looks like. It's on a remote server so there's some latency.

[brey]

Thanks @TheoMathurin

Looks interesting.

However, my use case is 12M nodes in unstructured mesh, so subsetting becomes more complicated.

[jbednar]

I may not be imagining the use case correctly, but I would think that people will have difficulting seeing the directions if a very large number of arrows are packed into a single plot without subsetting.

@TheoMathurin 's suggestion is precisely what I was coming here to suggest, i.e. using Datashader to render the unstructured mesh into a couple of structured meshs (representing the vector components), which will always have a fixed number of arrows to display and will thus be renderable in a readable form.