Optical flow estimation is a low level phase of motion recovery problem. Its output is a 2D projection of the velocity field on the image plane

which is analyzed further to get high level motion descriptions. A number of approaches have been developed for the optical flow estimation. The gradient based approach for the optical flow estimation is one of the importance kinds

it assumes brightness conservation under motion. This provides one constraint for the two unknown local components of the velocity vector. It is ill determined system of equation. A color image has some significant advantages over black and white image: it can give more optical information. Therefore

the colors images may be improve the solution of the optical flow estimation problem.The paper makes an introductory review on the optical flow estimation including the last achievement in this field. First

give the general concepts and fundamental principles of optical flow. Then

some methods for optical flow estimation are listed

black and white image and color image. Finally

make a discussion on some open problems and point out possible directions for further research.