function Heatmap(params) {
    Transform.call(this, null, params);
  }
Render a heatmap image for input raster grid data.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): *} [params.field] - The field with raster grid
  data. If unspecified, the tuple itself is interpreted as a raster grid.
@param {string} [params.color] - A constant color value or function for
  individual pixel color. If a function, it will be invoked with an input
  object that includes $x, $y, $value, and $max fields for the grid.
@param {number} [params.opacity] - A constant opacity value or function for
  individual pixel opacity. If a function, it will be invoked with an input
  object that includes $x, $y, $value, and $max fields for the grid.
@param {string} [params.resolve] - The method for resolving maximum values
  across multiple input grids. If 'independent' (the default), maximum
  calculation will be performed separately for each grid. If 'shared',
  a single global maximum will be used for all input grids.
@param {string} [params.as='image'] - The output field in which to store
  the generated bitmap canvas images (default 'image').