triangulate_quads {quadmesh}

Triangles from quads

Description

Convert quad index to triangles, this converts the 'rgl mesh3d (ib)' quad index to the complementary triangle index '(it)'.

Usage

triangulate_quads(quad_index, clockwise = FALSE)

Arguments

quad_index

the 'ib' index of quads from 'quadmesh'

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.

Details

Triangle pairs from each quad are interleaved in the result, so that neighbour triangles from a single quad are together.

Value

matrix of triangle indices

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)