If you have the codebook with you, you can represent an image as a bag of (visual) words, using this function. This gives the histogram of the codebook entries (called codewords), in the extracted features of th eimage. You need to pass the extracted features from the image also as input to the function. This function can work with any type of features. One important note is, you MUST have a working version of 'yael' library of INRIA on your machine. The output histogram can be normalized (third argument).