图像分割经典算法--《图割》（Graph Cut、Grab Cut-----python实现）

<>1. 算法介绍

Graph Cut（图形切割）应用于计算机视觉领域用来有效的解决各种低级计算机视觉问题，例如图像平滑（image
smoothing）、立体应对问题（stereo correspondence problem）、图像分割（image
segmentation）等等。此类方法把图像分割问题与图的最小割（min cut）问题相关联，在计算机视觉的很多类似的问题中，最小能量（minimum
energy）方案对应解决方案的最大后验估计（maximum posterior estimate）。
友情链接：图像分割经典算法（最小割最大流） <https://blog.csdn.net/mmm_jsw/article/details/83787395>
使用Graph Cut的方法可以精确的解决”二进制问题“（binary
problem），例如对二进制图像进行去噪。可以用两个以上不同的标签（例如立体对应（stereo
correspondence）或灰度图像的去噪）标记像素的问题不能精确解决，但产生的解决方案通常能够接近全局最优的效果。

<>2. GraphCut


GraphCut利用最小割最大流算法进行图像的分割，可以将图像分割为前景和背景。使用该算法时需要在前景和背景处各画几笔作为输入，算法将建立各个像素点与前景背景相似度的赋权图，并通过求解最小切割区分前景和背景。算法效果图如下：
原始图片

标注图片

分割后的图片

相关论文及python实现代码：

PAPER:
1.Fast approximate energy minimization via graph cuts
<http://www.cs.cornell.edu/~rdz/Papers/BVZ-pami01-final.pdf>
2.Graph based algorithms for scene reconstruction from two or more views
<https://dl.acm.org/citation.cfm?id=997606>
3.What energy functions can be minimized via graph cuts?
<http://www.cs.cornell.edu/~rdz/Papers/KZ-PAMI04.pdf>
4.Interactive Graph Cuts for Optimal Boundary & Region Segmentation of Objects
in N-D Images <http://www.csd.uwo.ca/~yuri/Abstracts/iccv01-abs.html>
CODE
https://github.com/cm-jsw/GraphCut <https://github.com/cm-jsw/GraphCut>
参考项目：https://github.com/NathanZabriskie/GraphCut
<https://github.com/NathanZabriskie/GraphCut>

<>3.GrabCut

GrabCut的详细解释参考博客：
图像分割之（三）从Graph Cut到Grab Cut
<https://blog.csdn.net/zouxy09/article/details/8534954>
图像分割之（四）OpenCV的GrabCut函数使用和源码解读
<https://blog.csdn.net/zouxy09/article/details/8535087>

GrabCut
GrabCut是对其的改进版，是迭代的Graph Cut。OpenCV中的GrabCut算法是依据《“GrabCut” - Interactive
Foreground Extraction using Iterated Graph
Cuts》这篇文章来实现的。该算法利用了图像中的纹理（颜色）信息和边界（反差）信息，只要少量的用户交互操作即可得到比较好的分割结果。

GrabCut优点
（1）只需要在目标外面画一个框，把目标框住，它就可以实现良好的分割效果；

（2）增加额外的用户交互（由用户指定一些像素属于目标），对实现的效果进行优化以得到更好的效果；

（3）它的Border Matting技术会使目标分割边界更加自然和完美。


GrabCut同时存在这一些缺点：1.如果背景比较复杂或者背景和目标相似度很大，那分割的效果不太好；2.由于时迭代的GraphCut，所以速度较慢。

GrabCut和GraphCut的不同点
（1）GraphCut的目标和背景的模型是灰度直方图，GrabCut取代为RGB三通道的混合高斯模型GMM；
（2）GraphCut的能量最小化（分割）是一次达到的，而GrabCut取代为一个不断进行分割估计和模型参数学习的交互迭代过程；

（3）GraphCut需要用户指定目标和背景的一些种子点，但是GrabCut只需要提供背景区域的像素集就可以了。也就是说你只需要框选目标，那么在方框外的像素全部当成背景，这时候就可以对GMM进行建模和完成良好的分割了。即GrabCut允许不完全的标注（incomplete
labelling）。

Python实现
代码：
#!/usr/bin/env python '''
===============================================================================
Interactive Image Segmentation using GrabCut algorithm. This application shows
interactive image segmentation using grabcut algorithm. USAGE : python
grabcut.py <filename> README FIRST: Two windows will show up, one for input and
one for output. At first, in input window, draw a rectangle around the object
using mouse right button. Then press 'n' to segment the object (once or a few
times) For any finer touch-ups, you can press any of the keys below and draw
lines on the areas you want. Then again press 'n' for updating the output. Key
'0' - To select areas of sure background Key '1' - To select areas of sure
foreground Key '2' - To select areas of probable background Key '3' - To select
areas of probable foreground Key 'n' - To update the segmentation Key 'r' - To
reset the setup Key 's' - To save the results
===============================================================================
''' import numpy as np import cv2 import sys BLUE = [255,0,0] # rectangle color
RED = [0,0,255] # PR BG GREEN = [0,255,0] # PR FG BLACK = [0,0,0] # sure BG
WHITE = [255,255,255] # sure FG DRAW_BG = {'color' : BLACK, 'val' : 0} DRAW_FG
= {'color' : WHITE, 'val' : 1} DRAW_PR_FG = {'color' : GREEN, 'val' : 3}
DRAW_PR_BG = {'color' : RED, 'val' : 2} # setting up flags rect = (0,0,1,1)
drawing = False # flag for drawing curves rectangle = False # flag for drawing
rect rect_over = False # flag to check if rect drawn rect_or_mask = 100 # flag
for selecting rect or mask mode value = DRAW_FG # drawing initialized to FG
thickness = 3 # brush thickness def onmouse(event,x,y,flags,param): global
img,img2,drawing,value,mask,rectangle,rect,rect_or_mask,ix,iy,rect_over # Draw
Rectangle if event == cv2.EVENT_RBUTTONDOWN: rectangle = True ix,iy = x,y elif
event == cv2.EVENT_MOUSEMOVE: if rectangle == True: img = img2.copy()
cv2.rectangle(img,(ix,iy),(x,y),BLUE,2) rect = (ix,iy,abs(ix-x),abs(iy-y))
rect_or_mask = 0 elif event == cv2.EVENT_RBUTTONUP: rectangle = False rect_over
= True cv2.rectangle(img,(ix,iy),(x,y),BLUE,2) rect =
(ix,iy,abs(ix-x),abs(iy-y)) rect_or_mask = 0 print " Now press the key 'n' a
few times until no further change \n" # draw touchup curves if event ==
cv2.EVENT_LBUTTONDOWN: if rect_over == False: print "first draw rectangle \n"
else: drawing = True cv2.circle(img,(x,y),thickness,value['color'],-1)
cv2.circle(mask,(x,y),thickness,value['val'],-1) elif event ==
cv2.EVENT_MOUSEMOVE: if drawing == True:
cv2.circle(img,(x,y),thickness,value['color'],-1)
cv2.circle(mask,(x,y),thickness,value['val'],-1) elif event ==
cv2.EVENT_LBUTTONUP: if drawing == True: drawing = False
cv2.circle(img,(x,y),thickness,value['color'],-1)
cv2.circle(mask,(x,y),thickness,value['val'],-1) # print documentation print
__doc__ # Loading images if len(sys.argv) == 2: filename = sys.argv[1] # for
drawing purposes else: print "No input image given, so loading default image,
far.jpg \n" print "Correct Usage : python grabcut.py <filename> \n" filename =
'far.jpg' img = cv2.imread(filename) img2 = img.copy() # a copy of original
image mask = np.zeros(img.shape[:2],dtype = np.uint8) # mask initialized to
PR_BG output = np.zeros(img.shape,np.uint8) # output image to be shown # input
and output windows cv2.namedWindow('output') cv2.namedWindow('input')
cv2.setMouseCallback('input',onmouse)
cv2.moveWindow('input',img.shape[1]+10,90) print " Instructions : \n" print "
Draw a rectangle around the object using right mouse button \n" while(1):
cv2.imshow('output',output) cv2.imshow('input',img) k = 0xFF & cv2.waitKey(1) #
key bindings if k == 27: # esc to exit break elif k == ord('0'): # BG drawing
print " mark background regions with left mouse button \n" value = DRAW_BG elif
k == ord('1'): # FG drawing print " mark foreground regions with left mouse
button \n" value = DRAW_FG elif k == ord('2'): # PR_BG drawing value =
DRAW_PR_BG elif k == ord('3'): # PR_FG drawing value = DRAW_PR_FG elif k ==
ord('s'): # save image bar = np.zeros((img.shape[0],5,3),np.uint8) res =
np.hstack((img2,bar,img,bar,output)) cv2.imwrite('grabcut_output.png',output)
cv2.imwrite('grabcut_output_combined.png',res) print " Result saved as image
\n" elif k == ord('r'): # reset everything print "resetting \n" rect =
(0,0,1,1) drawing = False rectangle = False rect_or_mask = 100 rect_over =
False value = DRAW_FG img = img2.copy() mask = np.zeros(img.shape[:2],dtype =
np.uint8) # mask initialized to PR_BG output = np.zeros(img.shape,np.uint8) #
output image to be shown elif k == ord('n'): # segment the image print """ For
finer touchups, mark foreground and background after pressing keys 0-3 and
again press 'n' \n""" if (rect_or_mask == 0): # grabcut with rect bgdmodel =
np.zeros((1,65),np.float64) fgdmodel = np.zeros((1,65),np.float64)
cv2.grabCut(img2,mask,rect,bgdmodel,fgdmodel,1,cv2.GC_INIT_WITH_RECT)
rect_or_mask = 1 elif rect_or_mask == 1: # grabcut with mask bgdmodel =
np.zeros((1,65),np.float64) fgdmodel = np.zeros((1,65),np.float64)
cv2.grabCut(img2,mask,rect,bgdmodel,fgdmodel,1,cv2.GC_INIT_WITH_MASK) mask2 =
np.where((mask==1) + (mask==3),255,0).astype('uint8') output =
cv2.bitwise_and(img2,img2,mask=mask2) cv2.destroyAllWindows()
实现效果：
原始图片、处理过程、处理后的照片分别如下：

GitHub代码：

* https://github.com/downingstreet/GrabCut
<https://github.com/downingstreet/GrabCut>
* https://github.com/Orcuslc/GrabCuthttps://github.com/Orcuslc/GrabCut
<https://github.com/Orcuslc/GrabCuthttps://github.com/Orcuslc/GrabCut>
Grab Cut论文：
“GrabCut”: interactive foreground extraction using iterated graph cuts
<http://wwwpub.zih.tu-dresden.de/~cvweb/publications/papers/2004/siggraph04-grabcut.pdf>

<>4.参考

* 基于GraphCuts图割算法的图像分割----OpenCV代码与实现
<https://blog.csdn.net/wangyaninglm/article/details/44151213>
* Graph cut入门学习 <http://blog.sina.com.cn/s/blog_4cccd8d3010125e6.html>
* 图像分割之（三）从Graph Cut到Grab Cut
<https://blog.csdn.net/zouxy09/article/details/8534954>
* 图像分割之（四）OpenCV的GrabCut函数使用和源码解读
<https://blog.csdn.net/zouxy09/article/details/8535087>
* https://github.com/NathanZabriskie/GraphCut
<https://github.com/NathanZabriskie/GraphCut>
* https://github.com/downingstreet/GrabCut
<https://github.com/downingstreet/GrabCut>
* https://github.com/Orcuslc/GrabCuthttps://github.com/Orcuslc/GrabCut
<https://github.com/Orcuslc/GrabCuthttps://github.com/Orcuslc/GrabCut>

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