Compared to image features extracted from VGG-Net (Simonyan and Zisserman, 2014) and ResNet , this type of features from Faster R-CNN has achieved state-of-the-art performance in both image captioning and VQA  tasks.
the image features F I for a raw image I are represented by: where M = 36 is the number of detected objects in an image 1 , and n f = 2048 is the dimension of the feature vector.