I'm trying to make sense of the following example in the package rgl:

 vertices <- c(
     -1.0, -1.0, 0, 1.0,
      1.0, -1.0, 0, 1.0,
      1.0,  1.0, 0, 1.0,
     -1.0,  1.0, 0, 1.0
  )
  indices <- c( 1, 2, 3, 4 )

  open3d()
  wire3d( qmesh3d(vertices,indices) )
One would think that each vertex would have 3 coordinates (x,y,z) what
does the fourth one in the definition of the variable vertices stand
for.

Thanks

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