The answer is yes and no. There are people who have done remarkably well by learning yoga on their own through books, videos…. Naturally a book and a video is a teacher too. Self learning requires a mind that is very introspective on the physical as well as subtle level where is the question of deep breathing techniques and meditations. It is a wisdom from the yore that a guru doesn’t need to present to impart the knowledge but it is always safer for a beginner to start with a competent teacher who has a patience, knowledge and skills to guide a student on the path of Yoga.