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Extract an ellipse form from an image instead of drawing it inside

asked 2013-12-16 11:35:53 -0500

Mark Freewind gravatar image

updated 2013-12-16 11:38:12 -0500

I have a rectangle that I want to extract the ellipse form of it from a given image. My code looks like:

RotatedRect ellipse ;
float p1 = (float) rect.width/2 + rect.x;
float p2 = (float) rect.height/2 + rect.y;
CvPoint2D32f p = cvPoint2D32f(p1,p2);
ellipse.center = p;
ellipse.angle = 0;
ellipse.size =  cvSize2D32f((float)rect.width, (float)rect.height);
ellipse( image_colored, minEllipse, CV_RGB(255,255,255), 2, 8 );

The input of this code are: an image (image_colored) and CvRect (rect). The last line of code, draws a white ellipse inside the "image_colored", but I am looking to extract the generated ellipse in an other image or just color the rest of the image in black.

Any help, will be highly appreciated. Thank you.

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answered 2013-12-16 23:34:02 -0500

pyro gravatar image

updated 2013-12-16 23:35:01 -0500

There isn't a straightforward way to extract the ellipse. But you may use a mask image to extract the elliptical patch with the rest of the region blacked out.

First, define a mask image which has a white ellipse on a black background. Then use a bitwise and operation to extract the patch.

Python code:

import cv2
import numpy as np
import matplotlib.pyplot as plt

image = cv2.imread('baboon.jpg')
# create a mask image of the same shape as input image, filled with 0s (black color)
mask = np.zeros_like(image)
rows, cols,_ = mask.shape
# create a white filled ellipse
mask=cv2.ellipse(mask, center=(rows/2, cols/2), axes=(50,100), angle=0, startAngle=0, endAngle=360, color=(255,255,255), thickness=-1)
# Bitwise AND operation to black out regions outside the mask
result = np.bitwise_and(image,mask)
# Convert from BGR to RGB for displaying correctly in matplotlib
# Note that you needn't do this for displaying using OpenCV's imshow()
image_rgb = cv2.cvtColor(image, cv2.COLOR_BGR2RGB)
mask_rgb = cv2.cvtColor(mask, cv2.COLOR_BGR2RGB)
result_rgb = cv2.cvtColor(result, cv2.COLOR_BGR2RGB)
# Plotting the results
plt.subplot(131)
plt.imshow(image_rgb)
plt.subplot(132)
plt.imshow(mask_rgb)
plt.subplot(133)
plt.imshow(result_rgb)
plt.show()

Plot:

image description

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Asked: 2013-12-16 11:35:53 -0500

Seen: 4,950 times

Last updated: Dec 16 '13