Transform a Point Cloud Using Another Stage

This stage uses another stage to modify the point cloud in the pipeline. When used with a Delaunay triangulation or a raster and operator = "-" or "+", the interpolated value is combined with the point Z coordinate (typically to build a normalization stage). When used with operator = "=", the interpolated value is written as-is into store_in_attribute without any Z arithmetic - useful for attaching per-point labels such as a tree-segmentation ID. The interpolation method is linear in the triangle mesh and bilinear in the raster.
When used with a 4x4 Rotation-Translation Matrix, it multiplies the coordinates of the points to apply the rigid transformation described by the matrix. This stage modifies the point cloud in the pipeline but does not produce any output.

Usage

transform_with(stage, operator = "-", store_in_attribute = "", bilinear = TRUE)

Arguments

stage: A stage that produces a triangulation, raster, or Rotation-Translation Matrix (RTM), sometimes also referred to as an "Affine Transformation Matrix". Can also be a 4x4 RTM matrix.
operator: A string. '-', '+' and '=' are supported (only with a triangulation or a raster). With '=' the raster (or TIN) value is stored as-is into store_in_attribute without any Z arithmetic; this is the typical way to attach a per-point label such as a tree-segmentation ID. '=' requires store_in_attribute to be set.
store_in_attribute: A string. Use an extra byte attribute to store the result (only with a triangulation or a raster). Required when operator = "=".
bilinear: bool. If the stage is a raster stage, the Z values are interpolated with a bilinear interpolation. FALSE to desactivate it.

Value

This stage transforms the point cloud in the pipeline. It consequently returns nothing.

RTM Matriz

Warning: lasR uses bounding boxes oriented along the XY axes of each processed chunk to manage data location and the buffer properly. Transforming the point cloud with a rotation matrix affects its bounding box and how lasR handles the buffer. When used with a matrix that has a rotational component, it is not safe to add stages after the transformation unless the user is certain that there is no buffer involved.

Examples

f <- system.file("extdata", "Topography.las", package="lasR")

# with a triangulation
mesh  <- triangulate(filter = keep_ground())
trans <- transform_with(mesh)
pipeline <- mesh + trans + write_las()
ans <- exec(pipeline, on = f)

# with a matrix
a = 20 * pi / 180
m <- matrix(c(
  cos(a), -sin(a), 0, 1000,
  sin(a), cos(a), 0, 0,
  0, 0, 1, 0,
  0, 0, 0, 1), nrow = 4, byrow = TRUE)

pipeline = transform_with(m) + write_las()
exec(pipeline, on = f)
#> $load_matrix
#>           [,1]       [,2] [,3] [,4]
#> [1,] 0.9396926 -0.3420201    0 1000
#> [2,] 0.3420201  0.9396926    0    0
#> [3,] 0.0000000  0.0000000    1    0
#> [4,] 0.0000000  0.0000000    0    1
#> 
#> $write_las
#> [1] "/tmp/RtmpZ9kGxN/Topography.las"
#> 

# store a raster value as-is into an extra byte attribute (operator = "=").
# Here we attach a tree-segmentation ID from region_growing() to each point.
# Use add_extrabytes() with a wide enough type (tree IDs can exceed 255, so
# 'UserData' - a 1-byte 0-255 LAS field - is not suitable here).
g <- system.file("extdata", "MixedConifer.las", package="lasR")
chm  <- rasterize(1, "max", filter = keep_first())
lmx  <- local_maximum_raster(chm, 5)
tree <- region_growing(chm, lmx, max_cr = 10)
eb   <- add_extrabytes("int", "tree_id", "Tree segmentation ID")
# bilinear = FALSE so the integer label is taken from the nearest pixel as-is.
attr <- transform_with(tree, operator = "=", store_in_attribute = "tree_id", bilinear = FALSE)
pipeline <- chm + lmx + tree + eb + attr + write_las()
ans <- exec(pipeline, on = g)