vcg_detect_collision: Detect collisions between two geometries

In ravetools: Signal and Image Processing Toolbox for Analyzing Intracranial Electroencephalography Data

Detect collisions between two geometries

Description

Reports, for every element of y, whether it comes within radius of x, together with the exact minimum distance. Each side may be a point cloud, a set of connected line segments (for example diffusion streamlines or an electrode shaft), or a triangular mesh, so the function covers all nine pairings with one call.

The test is exact: it produces neither false negatives nor false positives, and distance is the true minimum distance rather than a vertex-sampled approximation.

Usage

vcg_detect_collision(
  x,
  y,
  mode_x = c("auto", "points", "segments", "mesh"),
  mode_y = c("auto", "points", "segments", "mesh"),
  radius = 0,
  test_level = c("element", "unit", "whole"),
  include_interior = FALSE
)

Arguments

x	the geometry that gets indexed; a matrix with n rows and 2 or 3 columns, or a surface (see ensure_mesh3d)
y	the geometry that gets queried; results are aligned to this side. Same accepted types as x
mode_x, mode_y	how to interpret x and y: 'points' treats each row as an isolated point, 'segments' chains consecutive rows together (p1-p2, p2-p3, ...), and 'mesh' uses the triangular faces. The default 'auto' resolves to 'mesh' for a surface that has faces and 'points' otherwise, so a matrix of connected line segments must ask for 'segments' explicitly
radius	distance tolerance; a collision is reported when the minimum distance is at most radius. Defaults to 0, meaning literal contact; a positive tolerance is usually what is wanted
test_level	how exhaustively y is scanned. Answers are always reported per unit of y: a point when mode_y = 'points', a chain when 'segments', a face when 'mesh'. 'element' (the default) measures every element of every unit and reports the closest one; 'unit' stops at the first hit inside each unit and reports that one; 'whole' stops at the first hit anywhere and answers only whether the two geometries collide. Because a point and a face each are their own unit, 'element' and 'unit' differ only when mode_y = 'segments'
include_interior	whether geometry lying strictly inside a closed x surface counts as a collision even when it never comes within radius of the surface itself; requires mode_x = 'mesh'. Default is FALSE

Details

x is indexed once into a uniform spatial grid and y is streamed against it, so put the larger or repeatedly-reused geometry in x and the geometry you want per-unit answers about in y.

Several chains share one matrix and are delimited by rows of NA; any row that is not fully finite is treated as a separator. This lets a whole bundle of streamlines be passed as a single matrix:

  rbind(
    c(0, 0, 0), c(1, 0, 0), c(2, 0, 0),   # first streamline
    c(NA, NA, NA),                        # separator
    c(0, 5, 0), c(1, 5, 0)                # second streamline
  )

Those separators define the units that results are reported against: with mode_y = 'segments' the matrix above is two units, so hit_unit has two entries whatever the test_level. With mode_y = 'points' every row is its own unit, separators included, so hit_unit stays aligned with the rows of y and reports NA at each separator.

Separators split the matrix into slots, and every slot is a chain, empty ones included: two adjacent separators enclose an empty chain, which reports NA. The two ends are deliberately not symmetric. A separator in the first row opens an empty leading chain, but a separator in the last row closes the chain before it rather than opening a trailing empty one:

matrix	chains
A	1: [A]
A NA B	2: [A] [B]
A NA NA B	3: [A] [] [B]
NA A	2: [] [A]
A NA	1: [A]

That rule exists so a bundle assembled by appending a separator to every chain gives exactly one unit per chain, whichever of them happen to be empty:

  do.call("rbind", lapply(tracts, function(line) rbind(line, NA)))

Since mode_x plays no part in that grouping, the index of a segment is always counted from the start of its own chain rather than from the top of the matrix. The unit column gives the chain.

Value

A list. collide is always present, so a plain yes-or-no question can be asked at any test_level:

collide

a length-one logical: whether x and y collide at all. FALSE for empty input

hit_unit

logical, one entry per unit of y, omitted when test_level = 'whole'. NA marks a unit with nothing to test, and is deliberately not FALSE; see Why hit_unit reports NA below

representation

a data frame with one row per hit unit, so nrow equals sum(hit_unit, na.rm = TRUE) and a collision-free query gives zero rows. When test_level = 'whole' it holds at most one row

summary

what the query actually was, so a stored result can be read back without the call that made it

summary holds radius, test_level, include_interior, and one entry per side. Each side reports its mode as resolved (never 'auto'), the matching unit_type of 'point', 'line', or 'face', and n_units: one per row for 'points' with separator rows included, one per chain for 'segments', one per face for 'mesh'. So summary$y$n_units always equals length(hit_unit), and remains the only record of the size of y when test_level = 'whole' drops that vector.

The columns of representation name the collision on both sides:

unit

1-based unit of y: the row itself when mode_y = 'points', the position of the chain when 'segments', the face column when 'mesh'

index

1-based element within that unit: always 1 for a point, the position of the segment counted from the start of its own chain for 'segments', and the face vertex nearest x (1, 2, or 3) for 'mesh'

distance

the exact distance from that element to x. Distances beyond radius are never computed

x_index

1-based element of x that was hit: a row when mode_x is 'points', the row where the segment starts when 'segments', or a face column when 'mesh'. NA for a hit found by include_interior rather than by proximity

Why hit_unit reports NA

NA means there was no geometry here to test. FALSE means geometry was here and it missed. Three situations produce the first:

mode_y	the unit reports NA when
'points'	the row is a separator
'segments'	the chain is empty (two adjacent separators)
'segments'	the chain holds one vertex, so carries no segment

mode_y = 'mesh' therefore never reports NA: a face is always testable. (The scan does skip faces flagged as deleted, but a surface is rebuilt from its vertex and face matrices on every call, so no face arrives flagged.)

An empty chain cannot collide with anything, so reporting FALSE would not be wrong in the set-theoretic sense. It is avoided for two reasons. In 'points' mode hit_unit is aligned to the rows of y so that it can be indexed straight back into the input matrix, which means separator rows occupy slots; calling those FALSE asserts something about a point that does not exist. And sum(is.na(hit_unit)) counts the degenerate units that were passed in – empty or single-vertex streamlines in a tract file, deleted faces in a surface – which is usually a data-quality signal worth seeing rather than silently folding into "missed". Once flattened to FALSE that count cannot be recovered.

Most uses never have to deal with it. which already drops NA, so which(hit_unit) gives the colliding units directly, and representation$unit is that same vector without any coercion. NA only intrudes on sum / any / if without na.rm, and on logical subsetting such as tracts[hit_unit], which yields NULL entries. Where a plain mask really is wanted:

  mask <- res$hit_unit %in% TRUE     # NA becomes FALSE

test_level = 'whole' reports a colliding unit, not necessarily the first one in order: the scan stops as soon as any thread finds a collision. collide itself is deterministic, as are the two lower levels, which measure each unit in row order.

Both geometries must already share one coordinate space. Surface coordinates, volume ⁠IJK⁠ indices, and scanner coordinates are all different spaces, and no transform is applied here.

include_interior relies on ray casting and therefore needs x to be watertight with coherently oriented faces; see vcg_fix_defects to repair a surface that is not.

Multi-threading follows ravetools_threads. The interior test always runs single-threaded.

Coercing Surface Inputs

The surface objects are converted to 'mesh3d' object before applying further calculations.

When surface is a surface ieegio object, the returned mesh3d$vb contains vertices that have been left-multiplied by surface$geometry$transforms[[1]] (the first transform stored in the geometry, typically the ScannerAnat or voxel-to-world transform).

Breaking change: Earlier versions (before 0.2.6) of ravetools returned the raw surface$geometry$vertices without applying any transform, so downstream code often multiplied by surface$geometry$transforms[[1]] (or an equivalent) manually before working in world space. Such code will now double apply the transform and produce incorrect coordinates. If you previously applied a transform from surface$geometry$transforms by hand after calling a ravetools mesh function on an 'ieegio_surface', remove that manual step.

Surfaces with an empty or missing geometry$transforms list (for example, surfaces produced by ieegio's volume_to_surface, which stores an identity transform) are unaffected.

If geometry$transforms contains multiple transforms targeting different coordinate spaces, only the first one is used. Callers that need a specific target space should select and apply that transform themselves before calling ravetools mesh functions.

Examples

library(ravetools)

# A spherical region of interest
roi <- vcg_sphere()

# Two streamlines in one matrix, separated by an NA row: the first passes
# through the sphere, the second stays well clear of it
streamlines <- rbind(
  cbind(seq(-3, 3, by = 0.5), 0, 0),
  c(NA, NA, NA),
  cbind(seq(-3, 3, by = 0.5), 5, 0)
)

result <- vcg_detect_collision(
  x = roi,
  y = streamlines,
  mode_y = "segments",
  radius = 0.1
)

# Which streamlines touch the sphere, and where does each come closest?
result$hit_unit
result$representation

# What was actually asked, including what `mode = "auto"` resolved to
result$summary

# A bundle assembled by appending a separator to every streamline gives one
# unit per streamline, even where a streamline is empty: here the middle one
# contributes only its own separator, and reports NA rather than vanishing
tracts <- list(
  cbind(seq(-3, 3, by = 0.5), 0, 0),
  matrix(numeric(0), ncol = 3),
  cbind(seq(-3, 3, by = 0.5), 5, 0)
)
bundle <- do.call("rbind", lapply(tracts, function(line) rbind(line, NA)))
vcg_detect_collision(roi, bundle, mode_y = "segments",
                     radius = 0.1)$hit_unit

# The same question, giving up on a streamline once it is known to collide.
# `hit_unit` is unchanged; `index` names the first colliding segment instead
# of the closest one, which here happens to be the same segment
vcg_detect_collision(roi, streamlines, mode_y = "segments", radius = 0.1,
                     test_level = "unit")$representation

# Does the bundle touch the sphere at all?
vcg_detect_collision(roi, streamlines, mode_y = "segments", radius = 0.1,
                     test_level = "whole")$collide

# A point cloud against the same region, asking only for proximity
points <- rbind(c(0, 0, 0), c(1.05, 0, 0), c(10, 10, 10))
vcg_detect_collision(roi, points, radius = 0.1)$hit_unit

# The centre of the sphere is far from its surface, so it only counts as a
# collision when the interior is included
vcg_detect_collision(roi, rbind(c(0, 0, 0)), radius = 0.1)$collide
vcg_detect_collision(roi, rbind(c(0, 0, 0)), radius = 0.1,
                     include_interior = TRUE)$collide

ravetools documentation built on Aug. 31, 2026, 5:07 p.m.