interpolator.py

import numpy as np
import tensorflow as tf
import tensorflow_hub as hub
from typing import Generator, List, Iterable

"""A wrapper class for running a frame interpolation based on the FILM model on TFHub

Usage:
  interpolator = Interpolator()
  result_batch = interpolator(image_batch_0, image_batch_1, batch_dt)
  Where image_batch_1 and image_batch_2 are numpy tensors with TF standard
  (B,H,W,C) layout, batch_dt is the sub-frame time in range [0..1], (B,) layout.
"""


def _pad_to_align(x, align):
    """Pads image batch x so width and height divide by align.

  Args:
    x: Image batch to align.
    align: Number to align to.

  Returns:
    1) An image padded so width % align == 0 and height % align == 0.
    2) A bounding box that can be fed readily to tf.image.crop_to_bounding_box
      to undo the padding.
  """
    # Input checking.
    assert np.ndim(x) == 4
    assert align > 0, 'align must be a positive number.'

    height, width = x.shape[-3:-1]
    height_to_pad = (align - height % align) if height % align != 0 else 0
    width_to_pad = (align - width % align) if width % align != 0 else 0

    bbox_to_pad = {
        'offset_height': height_to_pad // 2,
        'offset_width': width_to_pad // 2,
        'target_height': height + height_to_pad,
        'target_width': width + width_to_pad
    }
    padded_x = tf.image.pad_to_bounding_box(x, **bbox_to_pad)
    bbox_to_crop = {
        'offset_height': height_to_pad // 2,
        'offset_width': width_to_pad // 2,
        'target_height': height,
        'target_width': width
    }
    return padded_x, bbox_to_crop


class Interpolator:
    """A class for generating interpolated frames between two input frames.

  Uses the Film model from TFHub
  """

    def __init__(self, times_to_interpolate=6, align: int = 64) -> None:
        """Loads a saved model.

    Args:
      align: 'If >1, pad the input size so it divides with this before
        inference.'
    """
        self.times_to_interpolate = times_to_interpolate
        self._model = hub.load("https://tfhub.dev/google/film/1")
        self._align = align

    def __call__(self, x0: np.ndarray, x1: np.ndarray,
                 dt: np.ndarray) -> np.ndarray:
        """Generates an interpolated frame between given two batches of frames.

    All inputs should be np.float32 datatype.

    Args:
      x0: First image batch. Dimensions: (batch_size, height, width, channels)
      x1: Second image batch. Dimensions: (batch_size, height, width, channels)
      dt: Sub-frame time. Range [0,1]. Dimensions: (batch_size,)

    Returns:
      The result with dimensions (batch_size, height, width, channels).
    """
        if self._align is not None:
            x0, bbox_to_crop = _pad_to_align(x0, self._align)
            x1, _ = _pad_to_align(x1, self._align)

        inputs = {'x0': x0, 'x1': x1, 'time': dt[..., np.newaxis]}
        result = self._model(inputs, training=False)
        image = result['image']

        if self._align is not None:
            image = tf.image.crop_to_bounding_box(image, **bbox_to_crop)
        return image.numpy()


def _recursive_generator(
        frame1: np.ndarray, frame2: np.ndarray, num_recursions: int,
        interpolator: Interpolator) -> Generator[np.ndarray, None, None]:
    """Splits halfway to repeatedly generate more frames.

  Args:
    frame1: Input image 1.
    frame2: Input image 2.
    num_recursions: How many times to interpolate the consecutive image pairs.
    interpolator: The frame interpolator instance.

  Yields:
    The interpolated frames, including the first frame (frame1), but excluding
    the final frame2.
  """
    if num_recursions == 0:
        yield frame1
    else:
        # Adds the batch dimension to all inputs before calling the interpolator,
        # and remove it afterwards.
        time = np.full(shape=(1,), fill_value=0.5, dtype=np.float32)
        mid_frame = interpolator(np.expand_dims(frame1, axis=0), np.expand_dims(frame2, axis=0), time)[0]
        yield from _recursive_generator(frame1, mid_frame, num_recursions - 1, interpolator)
        yield from _recursive_generator(mid_frame, frame2, num_recursions - 1, interpolator)


def interpolate_recursively(
        frames: List[np.ndarray], interpolator: Interpolator) -> Iterable[np.ndarray]:
    """Generates interpolated frames by repeatedly interpolating the midpoint.

  Args:
    frames: List of input frames. Expected shape (H, W, 3). The colors should be
      in the range[0, 1] and in gamma space.
    num_recursions: Number of times to do recursive midpoint
      interpolation.
    interpolator: The frame interpolation model to use.

  Yields:
    The interpolated frames (including the inputs).
  """
    times_to_interpolate = interpolator.times_to_interpolate

    n = len(frames)
    for i in range(1, n):
        yield from _recursive_generator(frames[i - 1], frames[i], times_to_interpolate, interpolator)
    # Separately yield the final frame.
    yield frames[-1]