R/primitives.R

In quadmesh: Quadrangle Mesh

#' Triangles from quads
#'
#' Convert quad index to triangles, this converts the 'rgl mesh3d (ib)' quad
#' index to the complementary triangle index '(it)'.
#'
#' Triangle pairs from each quad are interleaved in the result, so that neighbour
#' triangles from a single quad are together.
#' @param quad_index the 'ib' index of quads from 'quadmesh'
#' @param clockwise if true triangles are wound clockwise, if false
#'   anticlockwise. This affects which faces rendering engines consider to be
#'   the 'front' and 'back' of the triangle. If your mesh appears 'inside out',
#'   try the alternative setting.
#' @return matrix of triangle indices
#' @export
#' @examples
#' triangulate_quads(cbind(c(1, 2, 4, 3), c(3, 4, 6, 5)))
#'
#' qm <- quadmesh(raster::crop(etopo, raster::extent(140, 160, -50, -30)))
#' tri <- triangulate_quads(qm$ib)
#' plot(t(qm$vb))
#' tri_avg <- colMeans(matrix(qm$vb[3, tri], nrow = 3), na.rm = TRUE)
#' scl <- function(x) (x - min(x))/diff(range(x))
#' tri_col <- grey(seq(0, 1, length = 100))[scl(tri_avg) * 99 + 1]
#' ## tri is qm$ib converted to triangles for the same vertex set
#' polygon(t(qm$vb)[rbind(tri, NA), ])
#' polygon(t(qm$vb)[rbind(tri, NA), ], col = tri_col)
triangulate_quads <- function(quad_index, clockwise = FALSE) {

  if (clockwise){
    matrix(rbind(quad_index[c(1L, 2L, 4L), ], quad_index[c(2L, 3L, 4L), ]), 3L)
  } else {
    matrix(rbind(quad_index[c(1L, 4L, 2L), ], quad_index[c(4L, 3L, 2L), ]), 3L)
  }
}