Vector serialization of Geopandas data

[paddymul]

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Display (is this related to visual display of a value), Performance, Other (please describe in Issue Description)

Enhancement Description

Buckaroo should serialize GeoPandas geometry columns for the frontend in a way that allows a specially crafted displayer to render vector graphics for that column.

Pseudo Code Implementation

Here is some raw code

Note that this should run in GeopandasBuckarooWidget._df_to_obj

def serialize_geom(geom):
    # Shapely interface to extract coordinates is... not uniform
    # For simplicity, implement it for Polygons and later for other objects...
    if geom.geom_type == 'Polygon':
      return numpy.array(geom.exterior.coords.xy).T.tolist()
    elif geom.geom_type == 'MultiPolygon':
      return [serialize_geom(g) for g in geom.geoms]
    else:
      raise NotImplementedError("No serialization for %s" % self.geom_type)

def serialize_to_dict(gdf):
  """
  Serialize GeoDataFrame
  """
  import numpy

  # Let's work with 'index' output structure, it's closer to what we want
  dct = gdf.to_dict(orient='index')

  items = []
  for indx, fields in dct.items():
    # Create feature, with "id" value from GDF index
    item = { 'id': indx }

    # Transform 'geometry' object into list of coordinates
    geom = fields.pop('geometry')
    item['geometry'] = serialize_geom(geom)

    # Add all the remaining fields to output feature/item
    item.update(fields)
    items.append(item)

  return items

Ideally the transform will be compatible with a future move to arrow/parquet serialization

Prior Art

#224

[m-richards]

@paddymul you may already be aware of this but if not https://github.com/developmentseed/lonboard may be a useful reference here when looking to make this more performant - it exposes and interface to visualise GeoDataFrames in deck.gl where geoarrow is used as the transfer format betwen python and the browser (but you would then need a different way to render the geometries, rather than using the shapely svg (as I understand is what's currently happening from the linked issue above). I'm afraid I'm not across the finer details myself, but if you've got questions I'm happy to try and answer