face-api/src/ssdMobilenetv1/outputLayer.ts

import * as tf from '@tensorflow/tfjs/dist/tf.es2017.js';

import { OutputLayerParams } from './types';


function getCenterCoordinatesAndSizesLayer(x: tf.Tensor2D) {
  const vec = tf.unstack(tf.transpose(x, [1, 0]))

  const sizes = [
    tf.sub(vec[2], vec[0]),
    tf.sub(vec[3], vec[1])
  ]

  const centers = [
    tf.add(vec[0], tf.div(sizes[0], tf.scalar(2))),
    tf.add(vec[1], tf.div(sizes[1], tf.scalar(2)))
  ]

  return {
    sizes,
    centers
  }
}

function decodeBoxesLayer(x0: tf.Tensor2D, x1: tf.Tensor2D) {
  const {
    sizes,
    centers
  } = getCenterCoordinatesAndSizesLayer(x0)

  const vec = tf.unstack(tf.transpose(x1, [1, 0]))

  const div0_out = tf.div(tf.mul(tf.exp(tf.div(vec[2], tf.scalar(5))), sizes[0]), tf.scalar(2))
  const add0_out = tf.add(tf.mul(tf.div(vec[0], tf.scalar(10)), sizes[0]), centers[0])

  const div1_out = tf.div(tf.mul(tf.exp(tf.div(vec[3], tf.scalar(5))), sizes[1]), tf.scalar(2))
  const add1_out = tf.add(tf.mul(tf.div(vec[1], tf.scalar(10)), sizes[1]), centers[1])

  return tf.transpose(
    tf.stack([
      tf.sub(add0_out, div0_out),
      tf.sub(add1_out, div1_out),
      tf.add(add0_out, div0_out),
      tf.add(add1_out, div1_out)
    ]),
    [1, 0]
  )
}

export function outputLayer(
  boxPredictions: tf.Tensor4D,
  classPredictions: tf.Tensor4D,
  params: OutputLayerParams
) {
  return tf.tidy(() => {

    const batchSize = boxPredictions.shape[0]

    let boxes = decodeBoxesLayer(
      tf.reshape(tf.tile(params.extra_dim, [batchSize, 1, 1]), [-1, 4]) as tf.Tensor2D,
      tf.reshape(boxPredictions, [-1, 4]) as tf.Tensor2D
    )
    boxes = tf.reshape(
      boxes,
      [batchSize, (boxes.shape[0] / batchSize), 4]
    )

    const scoresAndClasses = tf.sigmoid(tf.slice(classPredictions, [0, 0, 1], [-1, -1, -1]))
    let scores = tf.slice(scoresAndClasses, [0, 0, 0], [-1, -1, 1]) as tf.Tensor

    scores = tf.reshape(
      scores,
      [batchSize, scores.shape[1] as number]
    )

    const boxesByBatch = tf.unstack(boxes) as tf.Tensor2D[]
    const scoresByBatch = tf.unstack(scores) as tf.Tensor1D[]

    return {
      boxes: boxesByBatch,
      scores: scoresByBatch
    }

  })
}