How can i find the centroid of the largest white(actually yellow) area using OpenCV with C++2008?

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When you apply threshold on image (at least the one implemented in OpenCV) you get image with one channel. If you are talking about yellow, than you have multi-channel image. How this happened?

Step 1: Use findContours function to find all objects in image (single channel binary image).

Step 2: Use contourArea function on each contour found in step 1 to find the largest one.

Step 3: Use moments function to find centroid of your contour.

You can use cvBlobsLib add on package. This will allow you to find blobs based on a given criterion, Area in your case. The index of the blob can be used to find other properties including centroid. The package performs very well though the documentation is sparse.