Using lidR to calculate null voxels

Question

Is it possible to return null voxels (those without any points) using voxel_metrics()?

As the description is written, voxels are only returned when they contain at least one point. I am currently attempting to look at ratios of empty and not empty voxels in various height layers such as discussed in Pearse et al. (2019).

Reference:

Pearse, G. D., Watt, M. S., Dash, J. P., Stone, C., & Caccamo, G. (2019). Comparison of models describing forest inventory attributes using standard and voxel-based lidar predictors across a range of pulse densities. International Journal of Applied Earth Observation and Geoinformation, 78, 341-351.

Answer 1

You cannot do that with voxel_metrics() but you can generate the missing voxels yourself

library(lidR)
LASfile <- system.file("extdata", "Megaplot.laz", package="lidR")
las = readLAS(LASfile)

# Compute a metric for each real voxel
res = 2
vox = voxel_metrics(las, ~mean(Intensity), res)

# Generate all the possible voxels
x = seq(min(vox$X), max(vox$X), res)
y = seq(min(vox$Y), max(vox$Y), res)
z = seq(min(vox$Z), max(vox$Z), res)
all_vox = expand.grid(X = x, Y = y, Z = z)
data.table::setDT(all_vox)

# Merge both unsing a join
fullvox = vox[all_vox, on = c("X", "Y", "Z")]

# 3D rendering. It is hard to see anything in fullvox
# because most of the voxels are NA and are displayed gray.
plot(vox, color = "V1", size = 0.5, trim = 50)
plot(fullvox, color = "V1", size = 0.5, trim = 50)