vshesh

vshesh

Intersection over union calculation

Hi,

I am trying to calculate the intersection over union for two oriented rectangles, like this picture;

I know there are lots of algorithms for this so I am curious if anyone has seen an implementation. Rectangles are stored as either four corners in counterclockwise order or {{center x, center y}, area, aspect, orientation} tuples.

Otherwise, any thoughts on how to accomplish this in elixir? I will be computing IoU for all pairwise combinations of two sets of ~100 rectangles for a soft-realtime application.

So something like:


defmodule Boxes do
  defstruct center: {0,0}, area: 0, aspect: 1.0, orientation: 0
  @type t :: %__MODULE__{center: {number(), number()}, area: number(), aspect: number(), orientation: number()}
  
  @doc """
  Computes the intersection over union score for two oriented rectangles.
  """
  def iou(box1, box2) do
    
  end
  

  def iou_batch(boxes1, boxes2) do
    Enum.map(boxes1, fn box1 -> Enum.map(boxes2, fn box2 -> iou(box1, box2) end) end)
  end

  @spec corners(__MODULE__.t()) :: list(number())
  @doc """
  Computes the corners of the box given a state vector
  """
  def corners(box) do
    [
      ... # you can assume this function works
    ]
  end
end

First Post!

mindok

mindok

I haven’t seen an Elixir implementation. I suspect you will just have to code it up.

There is a topology library (search hex for “topo” - GitHub repo is here: GitHub - pkinney/topo: A Geometry library for Elixir that calculates spatial relationships between two geometries · GitHub) that has functions to detect whether polylines intersect or overlap, but it doesn’t calculate the amount of overlap. You could take a look at that for some code examples.

I wouldn’t worry too much about performance - 100 x 99 x some basic arithmetic won’t take long at all on GHz (billions of calcs / second) CPUs.

The general approach would be:

  1. Convert all representations of rectangles into a common format
  2. Compute the polygon resulting from the intersection of the two rectangles - stack overflow has a couple of algorithms https://stackoverflow.com/questions/44797713/calculate-the-area-of-intersection-of-two-rotated-rectangles-in-python/45268241 and https://stackoverflow.com/questions/11670028/area-of-intersection-of-two-rotated-rectangles/11672022#11672022 - another library from pkinney will give you intersections of lines - GitHub - pkinney/segseg_ex: Segment-segment intersection classifier and calculator for Elixir · GitHub
  3. Compute the area of the resulting polygon. That would be the intersection. The union would be the area of the two rectangles minus the intersection.

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