为什么 pyplot.contour() 要求 Z 是一个二维数组? [英] Why does pyplot.contour() require Z to be a 2D array?

问题描述

matplotlib.pyplot.contour() 函数接受 3 个输入数组 X、Y 和 Z.
数组 X 和 Y 指定点的 x 和 y 坐标，而 Z 指定感兴趣的函数在点.

我知道 np.meshgrid() 可以轻松生成用作 contour() 参数的数组:

X = np.arange(0,5,0.01)Y = np.arange(0,3,0.01)X_grid, Y_grid = np.meshgrid(X,Y)Z_grid = X_grid**2 + Y_grid**2plt.contour(X_grid, Y_grid, Z_grid) # 工作正常

这很好用.而且很方便，这也很好用:

plt.contour(X, Y, Z_grid) # 也很好用

但是，为什么 Z 输入 要求 是二维数组?

为什么不允许使用以下内容，即使它指定了所有相同的数据适当对齐?

plt.contour(X_grid.ravel(), Y_grid.ravel(), Z_grid.ravel()) # 不允许

另外，当指定only Z(没有相应的X 和Y)时，语义是什么?

解决方案

看

(这是生成这张图片的代码)

The matplotlib.pyplot.contour() function takes 3 input arrays X, Y and Z.
The arrays X and Y specify the x- and y-coordinates of points, while Z specifies the corresponding value of the function of interest evaluated at the points.

I understand that np.meshgrid() makes it easy to produce arrays which serve as arguments to contour():

X = np.arange(0,5,0.01)
Y = np.arange(0,3,0.01)

X_grid, Y_grid = np.meshgrid(X,Y)
Z_grid = X_grid**2 + Y_grid**2

plt.contour(X_grid, Y_grid, Z_grid)  # Works fine

This works fine. And conveniently, this works fine too:

plt.contour(X, Y, Z_grid)  # Works fine too

However, why is the Z input required to be a 2D-array?

Why is something like the following disallowed, even though it specifies all the same data aligned appropriately?

plt.contour(X_grid.ravel(), Y_grid.ravel(), Z_grid.ravel())  # Disallowed

Also, what are the semantics when only Z is specified (without the corresponding X and Y)?

解决方案

Looking at the documentation of contour one finds that there are a couple of ways to call this function, e.g. contour(Z) or contour(X,Y,Z). So you'll find that it does not require any X or Y values to be present at all.

However in order to plot a contour, the underlying grid must be known to the function. Matplotlib's contour is based on a rectangular grid. But even so, allowing contour(z), with z being a 1D array, would make it impossible to know how the field should be plotted. In the case of contour(Z) where Z is a 2D array, its shape unambiguously sets the grid for the plot.

Once that grid is known, it is rather unimportant whether optional X and Y arrays are flattened or not; which is actually what the documentation tells us:

X and Y must both be 2-D with the same shape as Z, or they must both be 1-D such that len(X) is the number of columns in Z and len(Y) is the number of rows in Z.

It is also pretty obvious that someting like plt.contour(X_grid.ravel(), Y_grid.ravel(), Z_grid.ravel()) cannot produce a contour plot, because all the information about the grid shape is lost and there is no way the contour function could know how to interprete the data. E.g. if len(Z_grid.ravel()) == 12, the underlying grid's shape could be any of (1,12), (2,6), (3,4), (4,3), (6,2), (12,1).

A possible way out could of course be to allow for 1D arrays and introduce an argument shape, like plt.contour(x,y,z, shape=(6,2)). This however is not the case, so you have to live with the fact that Z needs to be 2D.

However, if you are looking for a way to obtain a countour plot with flattened (ravelled) arrays, this is possible using plt.tricontour().

plt.tricontour(X_grid.ravel(), Y_grid.ravel(), Z_grid.ravel()) 

Here a triangular grid will be produced internally using a Delaunay Triangualation. Therefore even completely randomized points will produce a nice result, as can be seen in the following picture, where this is compared to the same random points given to contour.

(Here is the code to produce this picture)

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