tmap-pixel-bug.R

require(tmap)
require(dplyr)
require(stars)
require(shiny)

server = shinyServer(function(input, output, session) {
  
  tmap_mode("view")
  
  output$map = renderTmap({
    
    set.seed(round(input$seed))
    d = data.frame(lon = rnorm(1e5, mean = -95.5, sd=4) |> pmin(179) |> pmax(-179),
                   lat = rnorm(1e5, mean = 37.5, sd=4) |> pmin(89) |> pmax(-89)) |> 
      group_by(lat = round(lat), lon = round(lon)) |> 
      summarise(n = length(lat), .groups = 'drop') |> 
      st_as_sf(coords = c('lon','lat'), crs = 'WGS84') |>
      st_rasterize()
    
    set.seed(42) # to prove bug is data related
    
    tm_basemap("Stamen.TonerLite") +
      tm_shape(d) +
      tm_raster("n", palette = c('white','red'), alpha=0.5)
    
  })
  
})

ui = fluidPage(
  
  absolutePanel(bottom = 100, left = 50, style = 'z-index: 100;',
    numericInput('seed', 'Seed', value = 54)
  ),
  
  tmapOutput('map', width = '98vw', height = '93vh')
  
)

shinyApp(ui, server, options = list(launch.browser = FALSE))