Optionally floodfill empty space, allow choice of inside of mesh being free or occupied

[helenol]

Changes

• New parameter for SDF builder: fill_inside, which determines if (for a watertight mesh) the inside is supposed to be free or occupied. Examples:
  • Stanford bunny: inside is occupied, since it's a single object that you're looking at from outside.
  • Scan of an office: inside is NOT occupied, since the mesh represents the walls of the entire office and you're looking from inside.
• New option to fill any free space in the TSDF with a fixed value. This basically performs a flood-fill within the axis aligned bounding box (AABB) of the entire allocated TSDF, setting all unallocated/unobserved voxels with any free neighbors to a fixed distance, 4*voxel size by default.
• Make slice height in the visualization a ROS param

Should hopefully fix https://github.com/ethz-asl/voxblox_ground_truth/issues/3

Examples on Office Mesh

Watertight mesh of 3D reconstruction of an office (MeshLab with xray shader so you can actually see the office-y bits inside):

Behavior before (fill_inside = true floodfill_unoccupied = false):

With fill_inside = false and floodfill_unoccupied = false

With fill_inside = false and floodfill_unoccupied = true

Stanford bunny with floodfill (note the error on the edge... it's a result of how we iterate over the neighbors in a lazy way rather than proper floodfill :/ ):

Let me know if this feature is useful then I'll fix the floodfilling edge.

[helenol]

Looks great Helen! Thank you for implementing this.

Regarding the flood fill, is the current problem due to the order in which you iterate over the voxels (i.e. with a triple for loop instead of propagating a wavefront from a seed point)?

Yes absolutely. Again the "lazy" way of doing it. ;) But actually can be easily fixed since you actually know that if there are no neighbors = you're outside the mesh structure, so fill_inside will tell you the value. I'll implement that before I merge.

FYI I also updated the CMakeLists to expose the targets properly for linking (using this as a library within antoher thing...)

[helenol]

Here's the new/fixed version, note the lack of cut-off corner: