scripts/nn/layers/max_pool2d.dml

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/*
 * Max Pooling layer.
 */
source("scripts/nn/util.dml") as util

forward = function(matrix[double] X, int C, int Hin, int Win, int Hf, int Wf,
                   int strideh, int stridew, int padh, int padw)
    return (matrix[double] out, int Hout, int Wout) {
  /*
   * Computes the forward pass for a 2D spatial max pooling layer.
   * The input data has N examples, each represented as a 3D volume
   * unrolled into a single vector.
   *
   * This implementation uses `im2col` internally for each image to
   * extract local image regions (patches) of each channel slice into
   * columns, and then performs max pooling over the patches to compute
   * the output maps.
   *
   * Inputs:
   *  - X: Inputs, of shape (N, C*Hin*Win).
   *  - C: Number of input channels (dimensionality of input depth).
   *  - Hin: Input height.
   *  - Win: Input width.
   *  - Hf: Filter height.
   *  - Wf: Filter width.
   *  - strideh: Stride over height.
   *  - stridew: Stride over width.
   *  - padh: Padding for top and bottom sides.
   *      A typical value is 0.
   *  - padw: Padding for left and right sides.
   *      A typical value is 0.
   *
   * Outputs:
   *  - out: Outputs, of shape (N, C*Hout*Wout).
   *  - Hout: Output height.
   *  - Wout: Output width.
   */
  N = nrow(X)
  Hout = as.integer(floor((Hin + 2*padh - Hf)/strideh + 1))
  Wout = as.integer(floor((Win + 2*padw - Wf)/stridew + 1))
  pad_value = -1/0  # in max pooling we pad with -infinity

  # Create output volume
  out = matrix(0, rows=N, cols=C*Hout*Wout)

  # Max pooling - im2col implementation
  parfor (n in 1:N) {  # all examples
    img = matrix(X[n,], rows=C, cols=Hin*Win)  # reshape

    if (padh > 0 | padw > 0) {
      # Pad image to shape (C, (Hin+2*padh)*(Win+2*padw))
      img = util::pad_image(img, Hin, Win, padh, padw, pad_value)
    }

    img_maxes = matrix(0, rows=C, cols=Hout*Wout)  # zeros
    parfor (c in 1:C) {  # all channels
      # Extract local image slice patches into columns with im2col, of shape (Hf*Wf, Hout*Wout)
      img_slice_cols = util::im2col(img[c,], Hin+2*padh, Win+2*padw, Hf, Wf, strideh, stridew)

      # Max pooling on patches
      img_maxes[c,] = colMaxs(img_slice_cols)
    }

    out[n,] = matrix(img_maxes, rows=1, cols=C*Hout*Wout)
  }
}

backward = function(matrix[double] dout, int Hout, int Wout, matrix[double] X,
                    int C, int Hin, int Win, int Hf, int Wf,
                    int strideh, int stridew, int padh, int padw)
    return (matrix[double] dX) {
  /*
   * Computes the backward pass for a 2D spatial max pooling layer.
   * The input data has N examples, each represented as a 3D volume
   * unrolled into a single vector.
   *
   * Inputs:
   *  - dout: Gradient wrt `out` from upstream, of
   *      shape (N, C*Hout*Wout).
   *  - Hout: Output height.
   *  - Wout: Output width.
   *  - X: Input data matrix, of shape (N, C*Hin*Win).
   *  - C: Number of input channels (dimensionality of input depth).
   *  - Hin: Input height.
   *  - Win: Input width.
   *  - Hf: Filter height.
   *  - Wf: Filter width.
   *  - strideh: Stride over height.
   *  - stridew: Stride over width.
   *  - padh: Padding for top and bottom sides.
   *      A typical value is 0.
   *  - padw: Padding for left and right sides.
   *      A typical value is 0.
   *
   * Outputs:
   *  - dX: Gradient wrt `X`, of shape (N, C*Hin*Win).
   */
  N = nrow(X)
  pad_value = -1/0  # in max pooling we pad with -infinity

  # Create gradient volume
  dX = matrix(0, rows=N, cols=C*Hin*Win)

  # Gradient of max pooling
  parfor (n in 1:N, check=0) {  # all examples
    img = matrix(X[n,], rows=C, cols=Hin*Win)
    if (padh > 0 | padw > 0) {
      # Pad image to shape (C, (Hin+2*padh)*(Win+2*padw))
      img = util::pad_image(img, Hin, Win, padh, padw, pad_value)
    }

    dimg = matrix(0, rows=C, cols=(Hin+2*padh)*(Win+2*padw))
    parfor (c in 1:C, check=0) {  # all channels
      img_slice = matrix(img[c,], rows=Hin+2*padh, cols=Win+2*padw)
      dimg_slice = matrix(0, rows=Hin+2*padh, cols=Win+2*padw)
      for (hout in 1:Hout, check=0) {  # all output rows
        hin = (hout-1)*strideh + 1
        for (wout in 1:Wout) {  # all output columns
          win = (wout-1)*stridew + 1
          img_slice_patch = img_slice[hin:hin+Hf-1, win:win+Wf-1]
          max_val_ind = img_slice_patch == max(img_slice_patch)  # max value indicator matrix
          # gradient passes through only for the max value(s) in this patch
          dimg_slice_patch = max_val_ind * dout[n, (c-1)*Hout*Wout + (hout-1)*Wout + wout]
          dimg_slice[hin:hin+Hf-1, win:win+Wf-1] = dimg_slice[hin:hin+Hf-1, win:win+Wf-1]
                                                   + dimg_slice_patch
        }
      }
      dimg[c,] = matrix(dimg_slice, rows=1, cols=(Hin+2*padh)*(Win+2*padw))
    }

    if (padh > 0 | padw > 0) {
      # Unpad image gradient
      dimg = util::unpad_image(dimg, Hin, Win, padh, padw)  # shape (C, (Hin+2*padh)*(Win+2*padw))
    }
    dX[n,] = matrix(dimg, rows=1, cols=C*Hin*Win)
  }
}