Python+OpenCV：直方图反向投影(Histogram Backprojection)

Algorithm in Numpy

1. First we need to calculate the color histogram of both the object we need to find (let it be 'M') and the image where we are going to search (let it be 'I').

import numpy as np
import cv2 as cvfrom matplotlib import pyplot as plt
#roi is the object or region of object we need to find
roi = cv.imread('rose_red.png')
hsv = cv.cvtColor(roi,cv.COLOR_BGR2HSV)
#target is the image we search in
target = cv.imread('rose.png')
hsvt = cv.cvtColor(target,cv.COLOR_BGR2HSV)
# Find the histograms using calcHist. Can be done with np.histogram2d also
M = cv.calcHist([hsv],[0, 1], None, [180, 256], [0, 180, 0, 256] )
I = cv.calcHist([hsvt],[0, 1], None, [180, 256], [0, 180, 0, 256] )

2. Find the ratio . Then backproject R, ie use R as palette and create a new image with every pixel as its corresponding probability of being target. ie B(x,y) = R[h(x,y),s(x,y)] where h is hue and s is saturation of the pixel at (x,y).

After that apply the condition B(x,y)=min[B(x,y),1].

h,s,v = cv.split(hsvt)
B = R[h.ravel(),s.ravel()]
B = np.minimum(B,1)
B = B.reshape(hsvt.shape[:2])

3. Now apply a convolution with a circular disc, B=D∗B, where D is the disc kernel.

disc = cv.getStructuringElement(cv.MORPH_ELLIPSE,(5,5))
cv.filter2D(B,-1,disc,B)
B = np.uint8(B)
cv.normalize(B,B,0,255,cv.NORM_MINMAX)

4. Now the location of maximum intensity gives us the location of object. If we are expecting a region in the image, thresholding for a suitable value gives a nice result.

ret,thresh = cv.threshold(B,50,255,0)

That's it !!

Backprojection in OpenCV

####################################################################################################
# 图像直方图反向投影(Histogram Backprojection)
def lmc_cv_image_histogram_backprojection():"""函数功能: 图像直方图反向投影(Histogram Backprojection)。"""# 图像直方图反向投影(Histogram Backprojection)# 读取图像roi = lmc_cv.imread('D:/99-Research/Python/Image/messi_roi.jpg')hsv_roi = lmc_cv.cvtColor(roi, lmc_cv.COLOR_BGR2HSV)target = lmc_cv.imread('D:/99-Research/Python/Image/messi.jpg')hsv_target = lmc_cv.cvtColor(target, lmc_cv.COLOR_BGR2HSV)target = lmc_cv.cvtColor(target, lmc_cv.COLOR_BGR2RGB)# calculating object histogramroihist = lmc_cv.calcHist([hsv_roi], [0, 1], None, [180, 256], [0, 180, 0, 256])# normalize histogram and apply backprojectionlmc_cv.normalize(roihist, roihist, 0, 255, lmc_cv.NORM_MINMAX)dst = lmc_cv.calcBackProject([hsv_target], [0, 1], roihist, [0, 180, 0, 256], 1)# Now convolute with circular discdisc = lmc_cv.getStructuringElement(lmc_cv.MORPH_ELLIPSE, (5, 5))lmc_cv.filter2D(dst, -1, disc, dst)# threshold and binary ANDret, thresh = lmc_cv.threshold(dst, 50, 255, 0)thresh = lmc_cv.merge((thresh, thresh, thresh))res = lmc_cv.bitwise_and(target, thresh)res = np.hstack((target, thresh, res))pyplot.figure('Image Display 1')pyplot.imshow(res, cmap='gray')pyplot.xticks([])pyplot.yticks([])pyplot.title('Histogram Backprojection')pyplot.show()# 根据用户输入保存图像if ord("q") == (lmc_cv.waitKey(0) & 0xFF):# 销毁窗口pyplot.close()return