FPN_Anchor_Box_Generator.ipynb

fpn_anchor_box_generator.ipynb

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  "metadata": {
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      "name": "FPN_Anchor_Box_Generator.ipynb",
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      "authorship_tag": "ABX9TyPrh/bZ3xPUEp0sBAQS5QZ4",
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      "name": "python3",
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  "cells": [
    {
      "cell_type": "markdown",
      "metadata": {
        "id": "view-in-github",
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      "source": [
        "<a href=\"https://colab.research.google.com/gist/Hamptonjc/c688c522cdcafd59c26d6268061fe692/fpn_anchor_box_generator.ipynb\" target=\"_parent\"><img src=\"https://colab.research.google.com/assets/colab-badge.svg\" alt=\"Open In Colab\"/></a>"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {
        "id": "CD9PC5y2594l"
      },
      "source": [
        "# Anchor Box Generator for Feature Pyramid Network\n",
        "***For use with RPN***"
      ]
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "mhRdJnGF55nB"
      },
      "source": [
        "# Imports\n",
        "import numpy as np\n",
        "from typing import List, NamedTuple"
      ],
      "execution_count": 31,
      "outputs": []
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "t0UsahSIDGps"
      },
      "source": [
        "# Define some NamedTuple classes for clean, readable code :)\n",
        "\n",
        "class AnchorBox(NamedTuple):\n",
        "  x_1:float\n",
        "  y_1:float \n",
        "  x_2:float\n",
        "  y_2:float\n",
        "\n",
        "class PyramidAnchorBoxes(NamedTuple):\n",
        "  p2:List[AnchorBox]\n",
        "  p3:List[AnchorBox]\n",
        "  p4:List[AnchorBox]\n",
        "  p5:List[AnchorBox]\n",
        "  p6:List[AnchorBox]"
      ],
      "execution_count": 32,
      "outputs": []
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "r3s5jdOp6JEg"
      },
      "source": [
        "def generate_anchor_boxes(ratios: List[float], anchor_box_area: int) -> List[AnchorBox]:\n",
        "  \"\"\"\n",
        "  ratios: list of aspect ratios (Example: [0.5,1,2])\n",
        "  anchor_box_area: area of the anchor box.\n",
        "  Returns: list of AnchorBox tuples, one for each aspect ratio.\n",
        "  Note: Anchor boxes are in x1,y1,x2,y2 coordinates,\n",
        "                      where the center of the box = 0\n",
        "  \"\"\"\n",
        "  anchor_boxes = []\n",
        "  for ratio in ratios:\n",
        "    height = np.sqrt(anchor_box_area/ratio)\n",
        "    width = anchor_box_area/height\n",
        "    x_1, y_1 = (-width/2, height/2)\n",
        "    x_2, y_2 = (width/2, -height/2)\n",
        "    anchor_boxes.append(AnchorBox(x_1,y_1,x_2,y_2))\n",
        "  return anchor_boxes"
      ],
      "execution_count": 39,
      "outputs": []
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "2T2wYvWfCsBq"
      },
      "source": [
        "def generate_pyramid_anchor_boxes(ratios: List[float], anchor_box_areas: List[int]):\n",
        "  \"\"\"\n",
        "  ratios: list of aspect ratios to use. (Ex: [0.5,1,2])\n",
        "  anchor_box_areas: list of areas where each area corresponds to a level in the feature pyramid (following the order [p2,p3,p4,p5,p6]).\n",
        "  Returns: PyramidAnchorBoxes tuple, where each item in tuple is a list of anchor boxes for that feature map size (p2,p3,etc.)\n",
        "  \"\"\"\n",
        "  pyramid_anchor_boxes = []\n",
        "  for area in anchor_box_areas:\n",
        "    anchor_boxes = generate_anchor_boxes(ratios, area)\n",
        "    pyramid_anchor_boxes.append(anchor_boxes)\n",
        "  pyramid_anchor_boxes =  PyramidAnchorBoxes(p2=pyramid_anchor_boxes[0], p3=pyramid_anchor_boxes[1],\n",
        "                                             p4=pyramid_anchor_boxes[2], p5=pyramid_anchor_boxes[3],\n",
        "                                             p6=pyramid_anchor_boxes[4])\n",
        "  return pyramid_anchor_boxes"
      ],
      "execution_count": 40,
      "outputs": []
    },
    {
      "cell_type": "markdown",
      "metadata": {
        "id": "lAXInFnnLPAn"
      },
      "source": [
        "# Example Usage"
      ]
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "b3Eq2IjjG6Vz"
      },
      "source": [
        "# Define ratios and anchor box areas. These are the values used in the FPN paper.\n",
        "ratios = [0.5,1.,2.]\n",
        "anchor_box_areas = [32**2, 64**2, 128**2, 256**2, 512**2]\n",
        "pyramid_anchor_boxes = generate_pyramid_anchor_boxes(ratios, anchor_box_areas)"
      ],
      "execution_count": 41,
      "outputs": []
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "ziq4yQ0CHi6q",
        "outputId": "48d1a7b2-165f-485e-9553-cf00cd8ef818",
        "colab": {
          "base_uri": "https://localhost:8080/",
          "height": 68
        }
      },
      "source": [
        "# p2 feature map list of anchor boxes\n",
        "pyramid_anchor_boxes.p2"
      ],
      "execution_count": 42,
      "outputs": [
        {
          "output_type": "execute_result",
          "data": {
            "text/plain": [
              "[AnchorBox(x_1=-11.31370849898476, y_1=22.627416997969522, x_2=11.31370849898476, y_2=-22.627416997969522),\n",
              " AnchorBox(x_1=-16.0, y_1=16.0, x_2=16.0, y_2=-16.0),\n",
              " AnchorBox(x_1=-22.62741699796952, y_1=11.313708498984761, x_2=22.62741699796952, y_2=-11.313708498984761)]"
            ]
          },
          "metadata": {
            "tags": []
          },
          "execution_count": 42
        }
      ]
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "7gzalasMHkhq",
        "outputId": "dcb88583-540c-4615-9592-c2c897d9f457",
        "colab": {
          "base_uri": "https://localhost:8080/",
          "height": 34
        }
      },
      "source": [
        "# the second anchor box of feature map p2.\n",
        "pyramid_anchor_boxes.p2[1]"
      ],
      "execution_count": 43,
      "outputs": [
        {
          "output_type": "execute_result",
          "data": {
            "text/plain": [
              "AnchorBox(x_1=-16.0, y_1=16.0, x_2=16.0, y_2=-16.0)"
            ]
          },
          "metadata": {
            "tags": []
          },
          "execution_count": 43
        }
      ]
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "Zd0FqBJxH0in",
        "outputId": "0aa2e93e-f778-4cc9-f054-b054ac242617",
        "colab": {
          "base_uri": "https://localhost:8080/",
          "height": 34
        }
      },
      "source": [
        "# the x_1 coordinate of the anchor box.\n",
        "pyramid_anchor_boxes.p2[1].x_1"
      ],
      "execution_count": 44,
      "outputs": [
        {
          "output_type": "execute_result",
          "data": {
            "text/plain": [
              "-16.0"
            ]
          },
          "metadata": {
            "tags": []
          },
          "execution_count": 44
        }
      ]
    }
  ]
}