"""
=========================================
Adapting gray-scale filters to RGB images
=========================================

There are many filters that are designed work with gray-scale images but not
color images. To simplify the process of creating functions that can adapt to
RGB images, scikit-image provides the ``adapt_rgb`` decorator.

To actually use the ``adapt_rgb`` decorator, you have to decide how you want to
adapt the RGB image for use with the gray-scale filter. There are two
pre-defined handlers:

``each_channel``
    Pass each of the RGB channels to the filter one-by-one, and stitch the
    results back into an RGB image.
``hsv_value``
    Convert the RGB image to HSV and pass the value channel to the filter.
    The filtered result is inserted back into the HSV image and converted
    back to RGB.

Below, we demonstrate the use of ``adapt_rgb`` on a couple of gray-scale
filters:
"""
from skimage.color.adapt_rgb import adapt_rgb, each_channel, hsv_value
from skimage import filter

@adapt_rgb(each_channel)
def sobel_each(image):
    return filter.sobel(image)

@adapt_rgb(hsv_value)
def sobel_hsv(image):
    return filter.sobel(image)

"""
We can use these functions as we would normally use them, but now they work
with both gray-scale and color images. Let's plot the results with a color
image:
"""

from skimage import data
import matplotlib.pyplot as plt

image = data.lena()

fig, (ax_each, ax_hsv) = plt.subplots(ncols=2)
ax_each.imshow(sobel_each(image))
ax_hsv.imshow(sobel_hsv(image))

"""
.. image:: PLOT2RST.current_figure

Notice that the result for the value-filtered image preserves the color of the
original image, but channel filtered image combines in a more surprising way.
In other common cases, smoothing for example, the channel filtered image will
produce a better result than the value-filtered image.

You can also create your own handler functions for ``adapt_rgb``. To do so,
just create a function with the following signature::

    def handler(image_filter, image, *args, **kwargs):
        # Manipulate RGB image here...
        image = image_filter(image, *args, **kwargs)
        # Manipulate filtered image here...
        return image

Note that ``adapt_rgb`` handlers are written for filters where the image is the
first argument.

As a very simple example, we can just convert any RGB image to grayscale and
then return the filtered result:

"""
from skimage.color import rgb2gray

def as_gray(image_filter, image, *args, **kwargs):
    gray_image = rgb2gray(image)
    return image_filter(gray_image, *args, **kwargs)

"""
It's important to create a signature that uses ``*args`` and ``**kwargs`` to
pass arguments along to the filter so that the decorated function is allowed to
have any number of positional and keyword arguments.

Finally, we can use this handler with ``adapt_rgb`` just as before:
"""

@adapt_rgb(as_gray)
def sobel_gray(image):
    return filter.sobel(image)

fig, ax = plt.subplots()
ax.imshow(sobel_gray(image))

plt.show()

"""
.. image:: PLOT2RST.current_figure

.. note::

    A very simple check of the array shape is used for detecting RGB images, so
    ``adapt_rgb`` is not recommended for functions that support 3D volumes or
    color images in non-RGB spaces.

"""

Add example for `adapt_rgb`