Function inputs

Buildings should be cast to "POLYGON" if they are not already

buildings <- st_cast(buildings,"POLYGON")

Creating the function

A few parameters can be set before running the function. rayno is the number of observer view angles from the viewpoint. More rays are more precise, but decrease processing speed.raydist is the maximum view distance. The function takessfc_POLYGON type and sfc_POINT objects as inputs for buildings abd the viewpoint respectively. If points have a variable view distance the function can be modified by creating a vector of view distance of length(viewpoints) here and then selecting raydist[x] in st_buffer below. Each ray is intersected with building data within its raycast distance, creating one or more ray line segments. The ray line segment closest to the viewpoint is then extracted, and the furthest away vertex of this line segement is taken as a boundary vertex for the isovist. The boundary vertices are joined in a clockwise direction to create an isovist.

st_isovist <- function(
  buildings,
  viewpoint,
  
  # Defaults
  rayno = 20,
  raydist = 100) {
  
  # Warning messages
  if(!class(buildings)[1]=="sfc_POLYGON")     stop('Buildings must be sfc_POLYGON')
  if(!class(viewpoint)[1]=="sfc_POINT") stop('Viewpoint must be sf object')
  
  rayends     <- st_buffer(viewpoint,dist = raydist,nQuadSegs = (rayno-1)/4)
  rayvertices <- st_cast(rayends,"POINT")
  
  # Buildings in raydist
  buildintersections <- st_intersects(buildings,rayends,sparse = FALSE)
  
  # If no buildings block max view, return view
  if (!TRUE %in% buildintersections){
    isovist <- rayends
  }
  
  # Calculate isovist if buildings block view from viewpoint
  if (TRUE %in% buildintersections){
    
    rays <- lapply(X = 1:length(rayvertices), FUN = function(x) {
      pair      <- st_combine(c(rayvertices[x],viewpoint))
      line      <- st_cast(pair, "LINESTRING")
      return(line)
    })
    
    rays <- do.call(c,rays)
    rays <- st_sf(geometry = rays,
                  id = 1:length(rays))
    
    buildsinmaxview <- buildings[buildintersections]
    buildsinmaxview <- st_union(buildsinmaxview)
    raysioutsidebuilding <- st_difference(rays,buildsinmaxview)
    
    # Getting each ray segement closest to viewpoint
    multilines  <- dplyr::filter(raysioutsidebuilding, st_is(geometry, c("MULTILINESTRING")))
    singlelines <- dplyr::filter(raysioutsidebuilding, st_is(geometry, c("LINESTRING")))
    multilines  <- st_cast(multilines,"MULTIPOINT")
    multilines  <- st_cast(multilines,"POINT")
    singlelines <- st_cast(singlelines,"POINT")
    
    # Getting furthest vertex of ray segement closest to view point
    singlelines <- singlelines %>% 
      group_by(id) %>%
      dplyr::slice_tail(n = 2) %>%
      dplyr::slice_head(n = 1) %>%
      summarise(do_union = FALSE,.groups = 'drop') %>%
      st_cast("POINT")
    
    multilines  <- multilines %>% 
      group_by(id) %>%
      dplyr::slice_tail(n = 2) %>%
      dplyr::slice_head(n = 1) %>%
      summarise(do_union = FALSE,.groups = 'drop') %>%
      st_cast("POINT")
    
    # Combining vertices, ordering clockwise by ray angle and casting to polygon
    alllines <- rbind(singlelines,multilines)
    alllines <- alllines[order(alllines$id),] 
    isovist  <- st_cast(st_combine(alllines),"POLYGON")
  }
  isovist
}

Running the function in a loop

It is possible to wrap the function in a loop to get multiple isovists for a multirow sfc_POINT object. There is no need to heed the repeating attributes for all sub-geometries warning as we want that to happen in this case.

isovists   <- lapply(X = 1:length(viewpoints), FUN = function(x) {
  viewpoint   <- viewpoints[x]
  st_isovist(buildings = buildings,
             viewpoint = viewpoint,
             rayno = 41,
             raydist = 100)
})

All isovists are unioned to create a visible area polygon, which can see plotted over the original path, viewpoint and building data below.

isovists <- do.call(c,isovists)
visareapoly <- st_union(isovists) 

ggplot() +
  geom_sf(data = buildings,colour = "transparent",aes(fill = 'Building')) +
  geom_sf(data = footway, aes(color = 'Footway')) +
  geom_sf(data = viewpoints, aes(color = 'Viewpoint')) +
  geom_sf(data = visareapoly,fill="transparent",aes(color = 'Visible area')) +
  scale_fill_manual(values = c("Building" = "grey50"), 
                    guide = guide_legend(override.aes = list(linetype = c("blank"), 
                                         shape = c(NA)))) +
  scale_color_manual(values = c("Footway" = "black", 
                                "Viewpoint" = "red",
                                "Visible area" = "red"),
                     labels = c("Footway", "Viewpoint","Visible area"))+
  guides( color = guide_legend(
    order = 1,
    override.aes = list(
      color = c("black","red","red"),
      fill  = c("transparent","transparent","white"),
      linetype = c("solid","blank", "solid"),
      shape = c(NA,16,NA))))+
  theme_minimal()+
  coord_sf(datum = NA)+
  theme(legend.title=element_blank())