Ipelet that measures the area of a polygon

measure.lua

label = "Measure"

about = [[
Measures areas of polygons
]]

function measure(model, num)
  local p = model.doc[model.pno]

  if p:primarySelection() == nil then
    model.ui:explain('No primary selection')
    return
  end
  local obj = p[p:primarySelection()]

  if obj:type() ~= 'path' then
    model.ui:explain('Selection is not a path object')
    return
  end

  shape = obj:shape()
  if #shape ~= 1 then
    model.ui:explain('Selection contains more than one subpath')
    return
  end

  curve = shape[1]
  if curve.type ~= 'curve' then
    model.ui:explain('Selection contains something which is not a line segment')
    return
  end
  
  local area = 0
  for i, segment in ipairs(curve) do
    if segment.type ~= 'segment' then
      model.ui:explain('Selection contains something which is not a line segment')
      return
    end
    startPoint = segment[1]
    endPoint = segment[2]
    x1 = startPoint.x
    y1 = startPoint.y
    x2 = endPoint.x
    y2 = endPoint.y
    print('segment ' .. x1 .. ' ' .. y1 .. ' ' .. x2 .. ' ' .. y2)
    area = area + x1 * y2 - x2 * y1
    print('area now ' .. area)
  end

  -- still need to add the last segment
  startPoint = curve[#curve][2]
  endPoint = curve[1][1]
  x1 = startPoint.x
  y1 = startPoint.y
  x2 = endPoint.x
  y2 = endPoint.y
  print('last segment ' .. x1 .. ' ' .. y1 .. ' ' .. x2 .. ' ' .. y2)
  area = area + x1 * y2 - x2 * y1
  print('final area ' .. area)

  model.ui:explain('Area = ' .. (0.5 * area))
end

----------------------------------------------------------------------

methods = {
   { label = "Measure", run=measure }
}