Merge background volume and overlay to new colors.

Usage

vol.merge(
  volume,
  overlay_colors,
  bbox_threshold = 0L,
  forced_overlay_color = NULL
)

volume: 3D array, can be numeric (gray-scale intensity values) or color strings. If numeric, the intensity values must be in range [0, 1].
overlay_colors: 3D array of color strings, values which are not part of the overlay (and should display background in the result) must have NA instead of a color string. Must have same dimensions as the volume.
bbox_threshold: numerical, the threshold intensity used to separate background and foreground. All voxels with intensity values greater than this value in the background volume will be considered foreground voxels. Background-only slices at the borders of the volume will be discarded (in the merged, final image). Pass NULL to use the full image without applying any bounding box.
forced_overlay_color: NULL or an rgb color string, like '#FF0000' for red. If NULL, the activation colors will be used as foreground colors. Otherwise, the given color will be for all foreground vertices.

3D array of color strings, the merged colors