OpenCV#5: Copy a portion from an image

1. set an ROI in the source image

cvSetImageROI( img_src,  cvRect roi );

2. create a new image with the same size as the ROI

IplImage* img_dst = cvCreateImage( cvSize( roi.width, roi.height ), IPL_DEPTH_8U, 1 );  

3. copy the portion from the source image to destination image

cvCopy( img_src, img_dst );

OpenCV#4: Remove small blobs from binary image

// Function to remove small blobs from the binary image
IplImage* remove_small_objects( IplImage* img_in, int size )
{
    IplImage* img_out       = cvCloneImage( img_in );  // return image
    CvMemStorage* storage   = cvCreateMemStorage( 0 );    // container of retrieved contours
    CvSeq* contours         = NULL;
    CvScalar black          = CV_RGB( 0, 0, 0 ); // black color
    CvScalar white          = CV_RGB( 255, 255, 255 );   // white color
    double area;

    // find contours in binary image
    cvFindContours( img_in, storage, &contours, sizeof( CvContour ), CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE );  

    while( contours )   // loop over all the contours
    {
        area = cvContourArea( contours, CV_WHOLE_SEQ );
        if( fabs( area ) <= size )  // if the area of the contour is less than threshold remove it
        {
            // draws the contours into a new image
            cvDrawContours( img_out, contours, black, black, -1, CV_FILLED, 8 ); // removes white dots
        }
/*        else
        {
            cvDrawContours( img_out, contours, white, white, -1, CV_FILLED, 8 ); // fills in holes
        }*/
        contours = contours->h_next;    // jump to the next contour
    }

    cvReleaseMemStorage( &storage );
    return img_out;
}

OpenCV #3: Converting an image to gray scale

int M = img_rgb->width;
int N = img_rgb->height;

// convert to gray scale
IplImage* img_gray = cvCreateImage( cvSize(M, N), IPL_DEPTH_8U, 1 );  
cvCvtColor( img_rgb, img_gray, CV_BGR2GRAY );

DIP using OpenCV #2 : Clearing an image

// clear the image

cvSet( IplImage* img, cvScalar(0,0));

// this will set each and every pixel to zero

Another simple method I recently became aware of:

cvZero( IplImage* img );

Image processing using OpenCV #1: Invert an image

Code to open an image and invert it using OpenCV:

img = cvLoadImage(argv[1]);

// get the image data
height    = img->height;
width     = img->width;
step      = img->widthStep;
channels  = img->nChannels;
data      = (uchar *)img->imageData;

for( i=0;i<height;i++ )
{
    for( j=0;j<width;j++ )
    {
        for( k=0;k<channels;k++ )
            data[i*step+j*channels+k] = 255 - data[i*step+j*channels+k];   
    }
}