code: def mock_shader_func(agg, span=None):
      img = tf.shade(agg, cmap=viridis, span=span, how="log")
      img = tf.set_background(img, "black")
      return img
  
file_path: datashader/datashader/tests/test_tiles.py
imports: from __future__ import annotations
  import datashader
  import datashader.transfer_functions
  from datashader.colors import viridis
  from datashader.tiles import render_tiles
  from datashader.tiles import gen_super_tiles
  from datashader.tiles import _get_super_tile_min_max
  from datashader.tiles import calculate_zoom_level_stats
  from datashader.tiles import MercatorTileDefinition
  import numpy
  import pandas
label_desc: Use image node_id 3 for calling a global function with example usage: mock_shader_func(agg, span) and returns: img