sclass IIPureQuad extends Meta implements IIntegralImage {

  int imageSizeInBits; // e.g. 8 = 256*256

  int[] pixelSumsByChannel; // average color of whole image stored as pixel sum

  IIntegralImage[] quadrants = new[4]; // in scan line order

  int imageSize() { ret 1 << imageSizeInBits; }

  int halfSize() { ret 1 << (imageSizeInBits-1); }

  public int getWidth() { ret imageSize(); }

  public int getHeight() { ret imageSize(); }

  int pixelCount() { ret 1 << (imageSizeInBits*2); }

  public double getIntegralValue(int x, int y, int channel) {

    int quadX = x >> (imageSizeInBits-1);  // 0 or 1

    int quadY = y >> (imageSizeInBits-1);  // 0 or 1

    int quadrant = quadX | (quadY << 1); // 0 to 3

    int xLo = x & ~halfSize(); // clear topmost bit

    int yLo = y & ~halfSize(); // clear topmost bit

    // first query quadrant we landed in

    double pixelSum = quadrants[quadrant].getIntegralValue(xLo, yLo, channel);

    // add quadrant to the left

    if (quadX != 0)

      pixelSum += quadrants[quadrant-1].getIntegralValue(halfSize(), yLo, channel);

    // add quadrant above

    if (quadY != 0)

      pixelSum += quadrants[quadrant-2].getIntegralValue(xLo, halfSize(), channel);

    // add full top left quadrant

    if (quadrant == 3)

      pixelSum += quadrants[0].getIntegralValue(halfSize(), halfSize(), channel);

    ret pixelSum;

  }

}