英文:
Matploltib quiver plot: argument order
问题
给定一个二维数组，其中包含方位值（如由高程模型的方位导数表示的罗盘方位），我正在使用matplotlib创建一个箭头图。这个箭头图被放置在一个彩色的方位值矩阵上，以充当检查。
问题: 尽管matplotlib.pyplot.quiver()期望的参数顺序是quiver([X, Y], U, V, [C], **kw)，但为什么我的代码只在使用quiver([X, Y], V, U)（即U和V的顺序相反）时才得到预期的答案？
顺便说一下，在绘图时，我已将plt.imshow的原点移动到lower（如这里所讨论）。我认为问题与我的索引等相关。
以下是Python 3.5和matplotlib v3.x的代码：
import numpy as npimport matplotlib.pyplot as pltdef compassBearing_to_standardPosition__degrees_counterClockwise(bearing_deg):"""Vector magnitude and direction calculations assume angle is relative to the x axisi.e. 0 degrees north is at 3 o'clockAdjust compass bearings to be relative to standard position"""std_pos = (450 - bearing_deg) % 360return std_posdef calculate_U_and_V__vector_magnitude_and_direction(angle_degrees, magnitude=1):"""Calculates the components of a vector given in magnitude (U) and direction (V) formangle: Expected that angles are in standard positioni.e. relative to the x axis or where 3 o'clock is zero and not the compass bearingwhere 12 o'clock is 0magnitude: defaults to 1"""angle_rad = np.deg2rad(angle_degrees)x = magnitude * np.cos(angle_rad) # change in x == Uy = magnitude * np.sin(angle_rad) # change in y == Vreturn x, ydef array_indices(arr, indexing='xy'):"""Calculates index positions of each cell in arrayThese can be used to map to e.g. when creating a quiver plotindexing: Giving the string 'ij' returns a meshgrid withmatrix indexing, while 'xy' returns a meshgrid with Cartesian indexing.In the 2-D case with inputs of length M and N, the outputs are of shape(N, M) for 'xy' indexing and (M, N) for 'ij' indexing."""nrows, ncols = arr.shapenx = 1ny = 1x = np.linspace(0, ncols-1, ncols)y = np.linspace(0, nrows-1, nrows)xi, yi = np.meshgrid(x, y, indexing=indexing)return xi, yi# 创建一个方位角度网格（北方为0度）aspect_grid = np.array([[216, 226, 151],[74, 323, 268],[177, 204, 84]])# 获取数组索引x, y = array_indices(aspect_grid, indexing='xy')# 获取U和Vx_change, y_change = calculate_U_and_V__vector_magnitude_and_direction(aspect_grid.flatten())# 在imshow上绘制quivercmap = 'twilight_shifted' # 这需要matplotlib v3.xplt.imshow(np.floor(aspect_grid), cmap=cmap, origin='lower')plt.colorbar(label="Aspect (degrees N)")plt.quiver(x, y, y_change, x_change, pivot='middle') # <<< 为什么不是x、y、x_change、y_change？plt.title("Surface aspect values")plt.show()
英文:
Given a 2D array of aspect values (compass bearings e.g. as represented by the aspect derivative of an elevation model), I'm creating a quiver plot using matplotlib. This is being placed over a coloured matrix of the aspect values to act as a check.
The code is working in that it creates what I want but only where the arguments are opposite to what I expect. I'm making a simple mistake but can't spot it.
QUESTION: Although matplotlib.pyplot.quiver() expects quiver([X, Y], U, V, [C], **kw), why does my code only give the expected answer where quiver([X, Y], V, U) (i.e. U and V are the other way around) is used?
Incidentally, when plotting, I've shifted the origin of plt.imshow to lower (as discussed here). I think the problem lies somewhere related to my indexing etc.
Code below (using python 3.5 and matplotlib v3.x):
import numpy as npimport matplotlib.pyplot as pltdef compassBearing_to_standardPosition__degrees_counterClockwise(bearing_deg):"""Vector magnitude and direction calculations assume angle is relative to the x axisi.e. 0 degrees north is at 3 o'clockAdjust compass bearings to be relative to standard position"""std_pos=(450 - bearing_deg) % 360return(std_pos)def calculate_U_and_V__vector_magnitude_and_direction(angle_degrees, magnitude=1):"""Calculates the components of a vector given in magnitude (U) and direction (V) formangle: Expected that angles are in standard positioni.e. relative to the x axis or where 3 o'clock is zero and not the compass bearingwhere 12 o'clock is 0magnitude: defaults to 1"""angle_rad=np.deg2rad(angle_degrees)x = magnitude * np.cos(angle_rad) # change in x == Uy = magnitude * np.sin(angle_rad) # change in y == Vreturn(x,y)def array_indices(arr, indexing='xy'):"""Calculates index positions of each cell in arrayThese can be used to map to e.g. when creating a quiver plotindexing: Giving the string 'ij' returns a meshgrid withmatrix indexing, while 'xy' returns a meshgrid with Cartesian indexing.In the 2-D case with inputs of length M and N, the outputs are of shape(N, M) for 'xy' indexing and (M, N) for 'ij' indexing."""nrows, ncols = arr.shapenx = 1ny = 1x = np.linspace(0, ncols-1, ncols)y = np.linspace(0, nrows-1, nrows)#y = np.linspace(nrows-1, 0, nrows) # note that the largest vlue is firstxi, yi = np.meshgrid(x, y, indexing=indexing)return(xi, yi)#Create a toy aspect grid (degrees North)aspect_grid=np.array([[ 216, 226, 151],[ 74, 323, 268],[ 177, 204, 84]])#Get the array indicesx,y=array_indices(aspect_grid, indexing='xy')#Get U and Vx_change,y_change=calculate_U_and_V__vector_magnitude_and_direction(aspect_grid.flatten())#Plot quiver over imshowcmap = 'twilight_shifted' # this will expect matplotlib v3.xplt.imshow(np.floor(aspect_grid), cmap=cmap, origin='lower')plt.colorbar(label="Aspect (degrees N)")plt.quiver(x, y, y_change, x_change, pivot='middle') # <<< why not x,y,x_change,y_change?plt.title("Surface aspect values")plt.show()
答案1
得分: 1
When you pass your aspect_grid array into calculate_U_and_V__vector_magnitude_and_direction, you aren't converting them from absolute bearing to counterclockwise degrees since compassBearing_to_standardPosition__degrees_counterClockwise is not being called in calculate_U_and_V__vector_magnitude_and_direction. Due to the 90-degree misalignment of the two conventions, this leads cos(angle) to correspond to the y component and sin(angle) to correspond to the x component (due to the property cos(x - pi/2) == sin(x)). In order to correct this, you simply need to use the conversion you have set up (which does correctly convert from bearing to standard position) by doing something like:
#...angle_degrees = compassBearing_to_standardPosition__degrees_counterClockwise(angle_degrees)angle_rad = np.deg2rad(angle_degrees)#...
in calculate_U_and_V__vector_magnitude_and_direction. This will then allow you to use:
plt.quiver(x, y, x_change, y_change, pivot='middle')
and get the correct result.
英文:
When you pass your aspect_grid array into calculate_U_and_V__vector_magnitude_and_direction you aren't converting them from absolute bearing to counterclockwise degrees since compassBearing_to_standardPosition__degrees_counterClockwise is not being called in calculate_U_and_V__vector_magnitude_and_direction. Due to the 90-degree misalignment of the two conventions this leads cos(angle) to correspond to the y component and sin(angle) to correspond to the x component (due to the property cos(x - pi/2) == sin(x)). In order to correct this you simply need to use the conversion you have set up (which does correctly convert from bearing to standard position) by doing something like
#...angle_degrees = compassBearing_to_standardPosition__degrees_counterClockwise(angle_degrees)angle_rad=np.deg2rad(angle_degrees)#...
in calculate_U_and_V__vector_magnitude_and_direction. This will then allow you to use
plt.quiver(x, y, x_change, y_change, pivot='middle')
and get the correct result:
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