Image gradient

In graphics programs such as Photoshop, the term gradient is used for a gradual blend of colour which can be considered as an even gradation from low to high values, as used from white to black in the images to the right. If one views an image as a function, with darker pixels corresponding to higher function values, then the arrows showing the directions along which the image grows from white to dark are just the vector gradient of that function at the points in the image.