how to find spot on single line

i want to find white spot on black line and black spot on white surface. anyone can give me its code for my projects

[Image Analyst]

Read this http://www.mathworks.com/matlabcentral/answers/6200-tutorial-how-to-ask-a-question-on-answers-and-get-a-fast-answer and then improve your question so that we can help you.