KDE2D Transform
The kde2d transform ≥ 5.8 performs two-dimensional kernel density estimation over an input data stream and returns the results as one or more raster grids (matrices) of density estimates. The output raster grids can be used downstream to create isocontour or heatmap visualizations. For a complete example, see the contour plot example visualization.
Transform Parameters
| Property | Type | Description |
|---|---|---|
| size | Number[ ] | Required. The spatial dimensions [width, height] over which to perform density estimation. |
| x | Field | Required. The x-coordinate field for density estimation. |
| y | Field | Required. The y-coordinate field for density estimation. |
| weight | Field | The data point weight field for density estimation. If unspecified, all data points are assumed to have a weight of 1. |
| groupby | Field[ ] | The data fields to group by. If not specified, a single group containing all data objects will be used. |
| cellSize | Number | Contour density calculation cell size. This parameter determines the level of spatial approximation. For example, the default value of 4 results in 2x reductions to the width and height. A value of 1 results in an output raster grid with base dimensions matching the size parameter. |
| bandwidth | Number[ ] | The KDE kernel bandwidths. The input can be a two-element array specifying separate x and y bandwidths, or a single-element array specifying both. If the values are unspecified or less than zero, the bandwidth will be automatically determined. |
| counts | Boolean | A boolean flag indicating if the output values should be probability estimates (false, default) or smoothed counts (true). |
| as | String | The output field at which to write a generated raster grid. The default is "grid". |
Usage
This example generates 10 levels of contours for the 2D kernel density estimate of a source data stream. The kde2d transform draws pre-computed pixel (x, y) coordinates from the x_value and y_value data fields, and uses the width and height signals to configure the area over which densities. A subsequent isocontour transform then generates level sets for the output raster grid of density estimates.
{
"type": "kde2d",
"x": "x_value",
"y": "y_value",
"size": [{"signal": "width"}, {"signal": "height"}],
"as": "grid"
},
{
"type": "isocontour",
"field": "grid",
"levels": 10
}