Image Inpainting {#tutorial_py_inpainting}
================

Goal
----

In this chapter,
    -   We will learn how to remove small noises, strokes etc in old photographs by a method called
        inpainting
    -   We will see inpainting functionalities in OpenCV.

Basics
------

Most of you will have some old degraded photos at your home with some black spots, some strokes etc
on it. Have you ever thought of restoring it back? We can't simply erase them in a paint tool
because it is will simply replace black structures with white structures which is of no use. In
these cases, a technique called image inpainting is used. The basic idea is simple: Replace those
bad marks with its neighbouring pixels so that it looks like the neigbourhood. Consider the image
shown below (taken from [Wikipedia](http://en.wikipedia.org/wiki/Inpainting)):

![image](images/inpaint_basics.jpg)

Several algorithms were designed for this purpose and OpenCV provides two of them. Both can be
accessed by the same function, **cv2.inpaint()**

First algorithm is based on the paper **"An Image Inpainting Technique Based on the Fast Marching
Method"** by Alexandru Telea in 2004. It is based on Fast Marching Method. Consider a region in the
image to be inpainted. Algorithm starts from the boundary of this region and goes inside the region
gradually filling everything in the boundary first. It takes a small neighbourhood around the pixel
on the neigbourhood to be inpainted. This pixel is replaced by normalized weighted sum of all the
known pixels in the neigbourhood. Selection of the weights is an important matter. More weightage is
given to those pixels lying near to the point, near to the normal of the boundary and those lying on
the boundary contours. Once a pixel is inpainted, it moves to next nearest pixel using Fast Marching
Method. FMM ensures those pixels near the known pixels are inpainted first, so that it just works
like a manual heuristic operation. This algorithm is enabled by using the flag, cv2.INPAINT_TELEA.

Second algorithm is based on the paper **"Navier-Stokes, Fluid Dynamics, and Image and Video
Inpainting"** by Bertalmio, Marcelo, Andrea L. Bertozzi, and Guillermo Sapiro in 2001. This
algorithm is based on fluid dynamics and utilizes partial differential equations. Basic principle is
heurisitic. It first travels along the edges from known regions to unknown regions (because edges
are meant to be continuous). It continues isophotes (lines joining points with same intensity, just
like contours joins points with same elevation) while matching gradient vectors at the boundary of
the inpainting region. For this, some methods from fluid dynamics are used. Once they are obtained,
color is filled to reduce minimum variance in that area. This algorithm is enabled by using the
flag, cv2.INPAINT_NS.

Code
----

We need to create a mask of same size as that of input image, where non-zero pixels corresponds to
the area which is to be inpainted. Everything else is simple. My image is degraded with some black
strokes (I added manually). I created a corresponding strokes with Paint tool.
@code{.py}
import numpy as np
import cv2

img = cv2.imread('messi_2.jpg')
mask = cv2.imread('mask2.png',0)

dst = cv2.inpaint(img,mask,3,cv2.INPAINT_TELEA)

cv2.imshow('dst',dst)
cv2.waitKey(0)
cv2.destroyAllWindows()
@endcode
See the result below. First image shows degraded input. Second image is the mask. Third image is the
result of first algorithm and last image is the result of second algorithm.

![image](images/inpaint_result.jpg)

Additional Resources
--------------------

-#  Bertalmio, Marcelo, Andrea L. Bertozzi, and Guillermo Sapiro. "Navier-stokes, fluid dynamics,
    and image and video inpainting." In Computer Vision and Pattern Recognition, 2001. CVPR 2001.
    Proceedings of the 2001 IEEE Computer Society Conference on, vol. 1, pp. I-355. IEEE, 2001.
2.  Telea, Alexandru. "An image inpainting technique based on the fast marching method." Journal of
    graphics tools 9.1 (2004): 23-34.

Exercises
---------

-#  OpenCV comes with an interactive sample on inpainting, samples/python/inpaint.py, try it.
2.  A few months ago, I watched a video on [Content-Aware
    Fill](http://www.youtube.com/watch?v=ZtoUiplKa2A), an advanced inpainting technique used in
    Adobe Photoshop. On further search, I was able to find that same technique is already there in
    GIMP with different name, "Resynthesizer" (You need to install separate plugin). I am sure you
    will enjoy the technique.
