I have struggled with this a bit. I've come to the conclusion that video applications typically treat the rows of matrices as X and columns as the Y dimension. So that means when you plot your image using imagesc, the X direction is top to bottom and Y direction is left to right. So when you use quiver to plot the arrows, you will need to do quiver(flow.Vy, flow.Vx). In general, it is advisable to watch videos and the flows, plot the directions (rose(flow.Orientation(chosenPixel))) and check how directions map to what flow.Orientation tells you.