мое решение такое:
int x = currentContour.MCvContour.rect.X;
int y = currentContour.MCvContour.rect.Y;
int width = currentContour.MCvContour.rect.Width;
int height = currentContour.MCvContour.rect.Height;
Rectangle rect = new Rectangle(x, y, width, height);
CvInvoke.cvSetImageROI(colorImage, rect);