问题
I'm trying to understand how the quiver function in the NumPy module works. Supposedly it allows to visualize graphically the values of two arrays, for example horizontal and vertical velocities. I have the following very simple example, but I show it just to see if you can help me to find out what I'm not doing well:
x = np.linspace(0,1,11)
y = np.linspace(1,0,11)
u = v = np.zeros((11,11))
u[5,5] = 0.2
plt.quiver(x, y, u, v)
The code produces the following figure:
As you can see, the arrow is not an arrow, but a line and it is longer than 0.2. My intention is to get an arrow of length 0.2 and I thought I could do it using quiver. Is it possible? Or should I better use another command?
回答1:
matplotlib quiver does auto scaling. Set the scale to 1 to get your 0.2 units in x an y:
x = np.linspace(0,1,11)
y = np.linspace(1,0,11)
u = v = np.zeros((11,11))
u[5,5] = 0.2
plt.quiver(x, y, u, v, scale=1)
If you don't set scale, matplotlib uses an auto scaling algorithm based on the average vector length and the number of vectors. Since you only have one vector with a length greater zero, it becomes really big. Adding more vectors makes the arrows successively smaller.
To have equal x and y extensions of your arrow a few more adjustments are needed:
x = np.linspace(0,1,11)
y = np.linspace(1,0,11)
u = v = np.zeros((11,11))
u[5,5] = 0.2
plt.axis('equal')
plt.quiver(x, y, u, v, scale=1, units='xy')
Both axes need to be equal and the units need to be set to xy.
回答2:
Not related to your problem, but interesting to mention: Funny thing is that by writing u[5, 5] = 0.2 you are implyingv[5, 5] = 0.2 as well (as shown in your diagonal arrow), since before you wrote u = v = np.zeros((11, 11)). You could avoid that by writing
u = np.zeros((11, 11))
v = np.zeros((11, 11))
to make u and v independent from each other.
回答3:
The quiver function visualizes a vector field, like this:
(from the examples page). In this type of plot, the vector at a point represents the magnitude of the field vector at that point. For example, if you're visualizing velocity of a fluid, the length of the arrow represents the speed of the fluid.
You can think of a vector field as a function mapping input (position) to output (the vector, e.g. velocity). The output values have no relation to the input positions; in particular, they may not even be measured in the same units! So a quiver plot can only show the relative magnitude of the field at different points - only the relative lengths of the arrows are meaningful, not their absolute lengths.
In other words, you shouldn't expect a field value of 0.2 to be represented by an arrow of length 0.2 in data units.
However, matplotlib provides an option you can specify to do that: the scale_units option to quiver. According to the documentation, you can give scale_units = 'xy' and it will render the arrow lengths using the same units as the axes.
来源：https://stackoverflow.com/questions/34375345/how-does-pythons-matplotlib-pyplot-quiver-exactly-work