Figure $9\mathrm{.}8$ An example of image flow. A

brighter triangle moves one pixel upward

from time $t_1$ to time $t_2.$ Background intensity

is $3$ while object intensity is $9.$Refer to figure $9\mathrm{.}8$ The intensity function is f $($x; y; t$).$ Consider the topmost pixel at time t$1$ with intensity $7$ within the context $975($its spatial coordinates are x $=$ y $=4).$ Estimate the spatial derivatives of the image function @f $=$@x and @f $=$@y at x $=$ y $=4$; t $=$ t$1$ using a $3$x$3$ neighborhood. Estimate the temporal derivative @f $=$@t at x $=$ y $=4$; t $=$ t$1$: what method is appropriate?