Finding largest blob in image - opencv

Question

I am having some issues extracting a blob from an image using EmguCV. Everything I see online uses the Contours object, but I guess that was removed from EmguCV3.0? I get an exception every time I try to use it. I haven't found many recent/relevant SO topics that aren't out of date.
Basically, I have a picture of a leaf. The background might be white, green, black, etc. I want to essentially remove the background so that I can perform operations on the leaf without interference with the background. I'm just not sure where I'm going wrong here:
Image<Bgr, Byte> Original = Core.CurrentLeaf.GetImageBGR;
Image<Gray, Byte> imgBinary = Original.Convert<Gray, Byte>();
imgBinary.PyrDown().PyrUp(); // Smoothen a little bit
imgBinary = imgBinary.ThresholdBinaryInv(new Gray(100), new Gray(255)); // Apply inverse suppression
// Now, copy pixels from original image that are black in the mask, to a new Mat. Then scan?
Image<Gray, Byte> imgMask;
imgMask = imgBinary.Copy(imgBinary);
CvInvoke.cvCopy(Original, imgMask, imgBinary);
VectorOfVectorOfPoint contoursDetected = new VectorOfVectorOfPoint();
CvInvoke.FindContours(imgBinary, contoursDetected, null, Emgu.CV.CvEnum.RetrType.List, Emgu.CV.CvEnum.ChainApproxMethod.ChainApproxSimple);
var contoursArray = new List<VectorOfPoint>();
int count = contoursDetected.Size;
for (int i = 0; i < count; i++)
{
using (VectorOfPoint currContour = contoursDetected[i])
{
contoursArray.Add(currContour);
}
}
With this, I get a black image with a tiny bit of white lines. I've racked my brain back and forth and haven't been able to come up with something. Any pointers would be much appreciated!

Answer 1

I think that you need to find which one is the largest area using ContourArea on each one of the contours.
After you find the largest contour you need to fill it (because the contour is just the putline of the blob and not all the pixel in it) using FillPoly and create a mask that as the leaf pixels with value 1 and the everything else with 0.
In the end use the mask to extract the leaf pixels from the original image
I am not so proficient in c# so i attach a code in python with opencv to give you some help.
The resulted image:
Hope this will be helpful enough.
import cv2
import numpy as np
# Read image
Irgb = cv2.imread('leaf.jpg')
R,G,B = cv2.split(Irgb)
# Do some denosiong on the red chnnale (The red channel gave better result than the gray because it is has more contrast
Rfilter = cv2.bilateralFilter(R,25,25,10)
# Threshold image
ret, Ithres = cv2.threshold(Rfilter,0,255,cv2.THRESH_BINARY_INV+cv2.THRESH_OTSU)
# Find the largest contour and extract it
im, contours, hierarchy = cv2.findContours(Ithres,cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_NONE )
maxContour = 0
for contour in contours:
contourSize = cv2.contourArea(contour)
if contourSize > maxContour:
maxContour = contourSize
maxContourData = contour
# Create a mask from the largest contour
mask = np.zeros_like(Ithres)
cv2.fillPoly(mask,[maxContourData],1)
# Use mask to crop data from original image
finalImage = np.zeros_like(Irgb)
finalImage[:,:,0] = np.multiply(R,mask)
finalImage[:,:,1] = np.multiply(G,mask)
finalImage[:,:,2] = np.multiply(B,mask)
cv2.imshow('final',finalImage)

Answer 2

I recommend you look into Otsu thresholding. It gives you a threshold which you can use to divide the image into two classes (background and foreground). using OpenCV's threshold method you can then create a mask if necessary.