函数定义：
hog(image, orientations=9, pixels_per_cell=(8, 8), cells_per_block=(3, 3),
block_norm='L2-Hys', visualize=False, transform_sqrt=False,
feature_vector=True, multichannel=None, *, channel_axis=None)
参数解释：
Extract Histogram of Oriented Gradients (HOG) for a given image.
Compute a Histogram of Oriented Gradients (HOG) by
1. (optional) global image normalization
2. computing the gradient image in `row` and `col`
3. computing gradient histograms
4. normalizing across blocks
5. flattening into a feature vector
Parameters
----------
image : (M, N[, C]) ndarray
Input image.
orientations : int, optional
Number of orientation bins.
pixels_per_cell : 2-tuple (int, int), optional
Size (in pixels) of a cell.
cells_per_block : 2-tuple (int, int), optional
Number of cells in each block.
block_norm : str {'L1', 'L1-sqrt', 'L2', 'L2-Hys'}, optional
Block normalization method:
``L1``
Normalization using L1-norm.
``L1-sqrt``
Normalization using L1-norm, followed by square root.
``L2``
Normalization using L2-norm.
``L2-Hys``
Normalization using L2-norm, followed by limiting the
maximum values to 0.2 (`Hys` stands for `hysteresis`) and
renormalization using L2-norm. (default)
For details, see [3]_, [4]_.
visualize : bool, optional
Also return an image of the HOG. For each cell and orientation bin,
the image contains a line segment that is centered at the cell center,
is perpendicular to the midpoint of the range of angles spanned by the
orientation bin, and has intensity proportional to the corresponding
histogram value.
transform_sqrt : bool, optional
Apply power law compression to normalize the image before
processing. DO NOT use this if the image contains negative
values. Also see `notes` section below.
feature_vector : bool, optional
Return the data as a feature vector by calling .ravel() on the result
just before returning.
multichannel : boolean, optional
If True, the last `image` dimension is considered as a color channel,
otherwise as spatial. This argument is deprecated: specify
`channel_axis` instead.
channel_axis : int or None, optional
If None, the image is assumed to be a grayscale (single channel) image.
Otherwise, this parameter indicates which axis of the array corresponds
to channels.
.. versionadded:: 0.19
`channel_axis` was added in 0.19.
Returns
-------
out : (n_blocks_row, n_blocks_col, n_cells_row, n_cells_col, n_orient) ndarray
HOG descriptor for the image. If `feature_vector` is True, a 1D
(flattened) array is returned.
hog_image : (M, N) ndarray, optional
A visualisation of the HOG image. only provided if `visualize` is True.
