Saliency map is one common way to explain the behaviours of image networks. It takes the absolute value of the partial derivative of the input image with respect to the target output element. (Here it is the argmax element in the last token of the generated sequence) To show the saliency for better visualization, we take the square root of the partial derivative with blurring effects. Eq. 3 shows the saliency map S as a function of input image x, and L is the current sequence length.