Clip mesh or RGL object to general region

Modifies a mesh3d object so that values of a function are bounded.

clipMesh3d(mesh, fn, bound = 0, greater = TRUE, 
           minVertices = 0, plot = FALSE, keepValues = FALSE,
           keepTags = FALSE)
clipObj3d(ids = tagged3d(tags), fn, bound = 0, greater = TRUE,
           minVertices = 0,
           replace = TRUE, tags)

Arguments

mesh: A mesh3d object.
fn: A function used to determine clipping, or a vector of values from such a function, with one value per vertex.
bound: The value(s) of fn on the clipping boundary.
greater: Logical; whether to keep fn >= bound or not.
minVertices: See Details below.
plot: Logical; whether or not to plot the mesh.
keepValues: Logical; whether to save the function values at each vertex when plot = FALSE.
keepTags: Whether to keep the "tags" component of the result; see details below.
ids: The RGL id value(s) of objects to clip.
tags: Object tags; an alternate way to specify ids. Ignored if ids is given.
replace: Should the ids objects be deleted after the clipped ones are drawn?

Details

These functions transform a mesh3d object or other RGL objects by removing parts where fn violates the bound.

For clipMesh3d the fn argument can be any of the following:

a character vector naming a function (with special names "x", "y", and "z" corresponding to functions returning those coordinates)
a function
a numeric vector with one value per vertex
NULL, indicating that the numeric values are saved in mesh$values

For clipObj3d any of the above except NULL may be used.

If fn is a numeric vector, with one value per vertex, those values will be used in the test. If it is a function with formal arguments x, y and z, it will receive the coordinates of vertices in those arguments, otherwise it will receive the coordinates in a single n x 3 matrix. The function should be vectorized and return one value per vertex, to check against the bound.

These operations are performed on the mesh:

First, all quads are converted to triangles.

Next, each vertex is checked against the condition.

Modifications to triangles depend on how many of the vertices satisfy the condition (fn >= bound or fn <= bound, depending on greater) for inclusion.

If no vertices in a triangle satisfy the condition, the triangle is omitted.
If one vertex satisfies the condition, the other two vertices in that triangle are shrunk towards it by assuming fn is locally linear.
If two vertices satisfy the condition, the third vertex is shrunk along each edge towards each other vertex, forming a quadrilateral made of two new triangles.
If all vertices satisfy the condition, they are included with no modifications.

Modifications to line segments are similar: the segment will be shortened if it crosses the boundary, or omitted if it is entirely out of bounds. Points, spheres, text and sprites will just be kept or rejected.

The minVertices argument is used to improve the approximation to the boundary when fn is a non-linear function. In that case, the interpolation described above can be inaccurate. If minVertices is set to a positive number (e.g. 10000), then each object is modified by subdivision to have at least that number of vertices, so that pieces are smaller and the linear interpolation is more accurate. In the clipObj3d function, minVertices can be a vector, with entries corresponding to each of the entries in ids.

The `keepTags` argument

If keepTags = TRUE, a "tags" element will be added to the result. It will be a vector with one entry per point, segment, triangle and quad in the output mesh. (These tags are not related to the tags used to identify rgl objects.) The mesh tags may be used to show the correspondence between the parts of the input mesh and output mesh. By default, the tags are constructed as a numerical sequence over points, segments, triangles and quads in the input mesh, in that order, starting from one. This is the same order used for colours when shading with meshColor == "faces".

For example, start with a mesh with one point, two segments, three triangles and four quads, but no tags member. It would implicitly tag the parts from one to ten as

c(1, # the point 2:3, # the two segments 4:6, # the three triangles 7:10) # the four quads

If clipping deleted the segments and the first triangle, the output would contain the seven element result

mesh$tags <- c(1, # the point remains # no segments now 5:6, # the two remaining triangles # were previously items 5 and 6 7:10) # the four quads

The tags output may contain repetitions. For example, when a triangle is partially clipped and replaced by several smaller triangles, entries for all of them will contain the value corresponding to the original triangle.

The mesh$tags component may be supplied as part of the input mesh as any type of vector; the output will propagate values to the new mesh. The input length must match the total number of points, segments, triangles and quads in the input mesh or an error will be raised.

Value

If plot = FALSE, clipMesh3d returns new mesh3d object in which all vertices (approximately) satisfy the clipping condition. Note that the order of vertices will likely differ from the original order, and new vertices will be added near the boundary (and if minVertices > 0, in the interior). If in addition keepValues = TRUE, a component named "values" will be added to the mesh containing the values for each vertex. If keepTags = TRUE, the tags component described below will be added to the output mesh.

If plot = TRUE, the result will be plotted with shade3d and its result returned.

clipObj3d is called for the side effect of modifying the scene. It returns a list of new RGL id values corresponding to the ids passed as arguments.

Author

Duncan Murdoch

References

See https://stackoverflow.com/q/56242470/2554330 and https://laustep.github.io/stlahblog/posts/MeshClipping.html for a motivating example.

Examples


# Show the problem that minVertices solves:

cube <- cube3d(col = rainbow(6), meshColor = "faces")

# This function only has one argument, so it will 
# be passed x, y and z in columns of a matrix
vecnorm <- function(vals) apply(vals, 1, function(row) sqrt(sum(row^2)))

open3d()
mfrow3d(2, 2, sharedMouse = TRUE)
id1 <- shade3d(cube)
# All vertices have norm sqrt(3), so this clips nothing:
clipObj3d(id1, fn = vecnorm, bound = sqrt(2))
next3d()
id2 <- wire3d(cube, lit = FALSE)
clipObj3d(id2, fn = vecnorm, bound = sqrt(2))

# This subdivides the cube, and does proper clipping:
next3d()
id3 <- shade3d(cube)
clipObj3d(id3, fn = vecnorm, bound = sqrt(2), minVertices = 200)
next3d()
id4 <- wire3d(cube, lit = FALSE)
clipObj3d(id4, fn = vecnorm, bound = sqrt(2), minVertices = 200)
3D plot