Interfacing with lidR

Working with LAS objects

The workhorse of lidR is the LAS data object, an S4 class that emulates the structure of a .las/.laz point cloud file. lacunr does not include any example data in .las format, but we can generate a quick and dirty approximation by converting the built-in glassfire dataset to a LAS object:

# convert 'glassfire' to LAS format
las_glassfire <- lidR::LAS(data = glassfire)
#> Creation of a LAS object from data but without a header:
#> Scale factors were set to 0.001 and XYZ coordinates were quantized to fit the scale factors.

This triggers a message because we have not supplied all of the required metadata found in a proper .las file, but the new LAS object will suffice for the purposes of the following examples.

Now that glassfire has been converted, the point cloud data.table is now stored in an S4 slot called @data. If we try to feed the whole LAS object to lacunr’s voxelize() function, it will throw an error:

# the wrong way to call voxelize() for a 'LAS' object:
vox <- voxelize(las_glassfire, edge_length = c(0.5, 0.5, 0.5))
#> Error in voxelize(las_glassfire, edge_length = c(0.5, 0.5, 0.5)): Input must be a data.frame or data.table

Instead, we need to extract the @data slot and supply that to voxelize():

# voxelize the LAS point cloud, taking care to input the correct S4 slot
vox <- voxelize(las_glassfire@data, edge_length = c(0.5, 0.5, 0.5))

Voxelization using lidR

lidR offers its own extremely versatile voxelization function, voxel_metrics(). This provides a useful alternative to voxelize(), although it is important to note that both functions utilize different algorithms and will not produce identical results (see the following section for more details).

voxel_metrics() returns a lasmetrics3d object. lacunr’s bounding_box() function can accept this as an input, but it also requires that it contain a column named N, recording the number of points in each voxel. This column can be generated by voxel_metrics() using the following:

# voxelize at 1m resolution, creating a column N containing the number of points
vox <- lidR::voxel_metrics(las_glassfire, ~list(N = length(Z)), res = 1)
# convert to array
box <- bounding_box(vox)

Failing to include this column will cause bounding_box() to throw an error.

lacunr::voxelize() vs lidR::voxel_metrics()

voxelize() is adapted from the function voxels(), originally written by J. Antonio Guzmán Q. for the package rTLS. It is intended as a complement rather than a replacement for lidR’s more elaborate voxel_metrics(). Each function has a different underlying algorithm and will produce distinct results from the same input data. The chief advantages of voxelize() over voxel_metrics() are:

1. It allows — and in fact requires — the user to specify all three dimensions of the desired voxel resolution independently. This makes it possible to completely customize the shape of the voxels, in the rare instance that one wishes to divide up a point cloud into a non-cubic voxel grid. voxel_metrics() permits at most two dimensions.
2. The point cloud can be divided into an even number of voxel bins. For example, if you have a point cloud that spans 12 meters in the X dimension, and voxelize it at a resolution of 1 meter, the resulting data will be binned into 12 1-meter voxels along the X axis. The same point cloud will be binned into 13 voxels by voxel_metrics(). This is due to differences in how each function aligns the point cloud data within the voxel grid.