Is it okay to contact potential advisor if you have no specific topic in mind about your dissertation? I wanted to pursue graphics for masters, its a bit difficult narrowing it down. Is it okay to ask them for a specific topic?
I'm a professor in Germany and I throw any master's students out who come asking me for a topic. It needs to be a question they want answered or some computing question they want to explore. Bachelor's I do a bit of hand-holding on.
You should research the interests of the professor and see if he or she is likely to be interested in your question. And do check if the professor is male or female before writing your letter. All emails addressing me as Herr Professor get piped to /dev/null.
In my experience, you don't need to have a specific topic yet. I think a good way to approach it would be to talk about some paper or papers this person wrote that got you really excited about his/her group. You can also talk about some project that you did that you really liked. In this letter, you want to give the person a bit of a chance to get to know you. I think a good length would be no more than two-thirds of a page in MS Word. You could think of this as a cover letter when sending a CV to apply for a job.
Finding the topic is probably going to involve a lot of work on your part, but you can use your advisor as a sounding board, and your advisor can suggest some things to read.
Try to attend lots of seminars. They can be quite stimulating.