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Numerical gradient
FX = gradient(F)
[FX,FY] = gradient(F)
[FX,FY,FZ,...] = gradient(F)
[...] = gradient(F,h)
[...] = gradient(F,h1,h2,...)
The gradient of a function of two variables, F(x,y), is defined as
$$\nabla F=\frac{\partial F}{\partial x}\widehat{i}+\frac{\partial F}{\partial y}\widehat{j}$$
and can be thought of as a collection of vectors pointing in the direction of increasing values of F. In MATLAB^{®} software, numerical gradients (differences) can be computed for functions with any number of variables. For a function of N variables, F(x,y,z, ...),
$$\nabla F=\frac{\partial F}{\partial x}\widehat{i}+\frac{\partial F}{\partial y}\widehat{j}+\frac{\partial F}{\partial z}\widehat{k}+\mathrm{...}$$
FX = gradient(F), where F is a vector, returns the one-dimensional numerical gradient of F. Here FX corresponds to ∂F/∂x, the differences in x (horizontal) direction.
[FX,FY] = gradient(F), where F is a matrix, returns the x and y components of the two-dimensional numerical gradient. FX corresponds to ∂F/∂x, the differences in x (horizontal) direction. FY corresponds to ∂F/∂y, the differences in the y (vertical) direction. The spacing between points in each direction is assumed to be one.
[FX,FY,FZ,...] = gradient(F), where F has N dimensions, returns the N components of the gradient of F. There are two ways to control the spacing between values in F:
A single spacing value, h, specifies the spacing between points in every direction.
N spacing values (h1,h2,...) specifies the spacing for each dimension of F. Scalar spacing parameters specify a constant spacing for each dimension. Vector parameters specify the coordinates of the values along corresponding dimensions of F. In this case, the length of the vector must match the size of the corresponding dimension.
Note The first output FX is always the gradient along the 2nd dimension of F, going across columns. The second output FY is always the gradient along the 1st dimension of F, going across rows. For the third output FZ and the outputs that follow, the Nth output is the gradient along the Nth dimension of F. |
[...] = gradient(F,h), where h is a scalar, uses h as the spacing between points in each direction.
[...] = gradient(F,h1,h2,...) with N spacing parameters specifies the spacing for each dimension of F.
Calculate the 2-D gradient of on a grid.
v = -2:0.2:2; [x,y] = meshgrid(v); z = x .* exp(-x.^2 - y.^2); [px,py] = gradient(z,.2,.2);
Plot the contour lines and vectors in the same figure.
contour(v,v,z) hold on quiver(v,v,px,py) hold off
Create a 3-D array.
F(:,:,1) = magic(3); F(:,:,2) = pascal(3);
The command,
gradient(F)
takes dx = dy = dz = 1. However, the command,
[PX,PY,PZ] = gradient(F,0.2,0.1,0.2)
takes dx = 0.2, dy = 0.1, and dz = 0.2.