Automatically detect and crop ROI in OpenCV

Question

I have these images to compare with each other. However, there are too many blacks that I think I can crop out to make comparison more effective.

What I want to do is crop Mars. Rectangle or round whichever may yield better results when compared. I was worrying that if the cropping would result to images of different sizes, comparison wouldn't work out as well as expected? Ideas how to do it and sample codes if possible? Thanks in advance

UPDATE: Tried using cvHoughCircles() it won't detect the planet :/

Answer 1

Try to use color detection. You need to find all the colors except black. Here and here are nice explanations of this method.

Answer 2

You can convert these images to gray scale images using cvCvtColor(img,imgGrayScale,CV_BGR2GRAY)

Then threshold them using cvThreshold(imgGrayScale,imgThresh,x,255,CV_THRESH_BINARY). Here, you have to find a good value for x(I think x=50 is ok).

CvMoments *moments = (CvMoments*)malloc(sizeof(CvMoments));
cvMoments(imgThresh, moments, 1);
double moment10 = cvGetSpatialMoment(moments, 1, 0);
double moment01 = cvGetSpatialMoment(moments, 0, 1);
double area = cvGetCentralMoment(moments, 0, 0);
int x = moment10/area;
int y = moment01/area;

Now you know the (x.y) coordinate of the blob. Then you can crop the image using cvSetImageROI(imgThresh, cvRect(x-10, y-10, x+10, y+10)). Here I have assumed that the radius of this blob is less than 10 pixel.

All cropped images are of same size and the white blob (planet) is exactly at the middle of the image.

Then you can compare images using normalized cross-correlation.