多边形选区概述
多边形选区是一种常见的对象选择方式，在一个子图中，单击鼠标左键即构建一个多边形的端点，最后一个端点与第一个端点重合即完成多边形选区，选区即为多个端点构成的多边形。在matplotlib中的多边形选区属于部件（widgets），matplotlib中的部件都是中性（neutral ）的，即与具体后端实现无关。
多边形选区具体实现定义为matplotlib.widgets.PolygonSelector类，继承关系为：Widget->AxesWidget->_SelectorWidget->PolygonSelector。
PolygonSelector类的签名为class matplotlib.widgets.PolygonSelector(ax, onselect, useblit=False, lineprops=None, markerprops=None, vertex_select_radius=15)
PolygonSelector类构造函数的参数为：
- ax：多边形选区生效的子图，类型为matplotlib.axes.Axes的实例。
- onselect：多边形选区完成后执行的回调函数，函数签名为def onselect( vertices)，vertices数据类型为列表，列表元素格式为(xdata,ydata)元组。
- drawtype：多边形选区的外观，取值范围为{"box", "line", "none"}，"box"为多边形框，"line"为多边形选区对角线，"none"无外观，类型为字符串，默认值为"box"。
- lineprops：多边形选区线条的属性，默认值为dict(color='k', linestyle='-', linewidth=2, alpha=0.5)。
- markerprops：多边形选区端点的属性，默认值为dict(marker='o', markersize=7, mec='k', mfc='k', alpha=0.5)。
- vertex_select_radius：多边形端点的选择半径，浮点数，默认值为15，用于端点选择或者多边形闭合。
PolygonSelector类中的state_modifier_keys公有变量 state_modifier_keys定义了操作快捷键，类型为字典。
- “move_all”： 移动已存在的选区，默认为"shift"。
- “clear”：清除现有选区，默认为 "escape"，即esc键。
- “move_vertex”：正方形选区，默认为"control"。
PolygonSelector类中的verts特性返回多边形选区中的多有端点，类型为列表，元素为(x,y)元组，即端点的坐标元组。
案例
官方案例，https://matplotlib.org/gallery/widgets/polygon_selector_demo.html
案例说明
单击鼠标左键创建端点，最终点击初始端点闭合多边形，形成多边形选区。选区外的数据元素颜色变淡，选区内数据颜色保持不变。
按esc键取消选区。按shift键鼠标可以移动多边形选区位置，按ctrl键鼠标可以移动多边形选区某个端点的位置。退出程序时，控制台输出选区内数据元素的坐标。
控制台输出：
Selected points:
[[2.0 2.0]
[1.0 3.0]
[2.0 3.0]]
案例代码
import numpy as np
from matplotlib.widgets import PolygonSelector
from matplotlib.path import Path
class SelectFromCollection:
"""
Select indices from a matplotlib collection using `PolygonSelector`.
Selected indices are saved in the `ind` attribute. This tool fades out the
points that are not part of the selection (i.e., reduces their alpha
values). If your collection has alpha < 1, this tool will permanently
alter the alpha values.
Note that this tool selects collection objects based on their *origins*
(i.e., `offsets`).
Parameters
----------
ax : `~matplotlib.axes.Axes`
Axes to interact with.
collection : `matplotlib.collections.Collection` subclass
Collection you want to select from.
alpha_other : 0 <= float <= 1
To highlight a selection, this tool sets all selected points to an
alpha value of 1 and non-selected points to *alpha_other*.
"""
def __init__(self, ax, collection, alpha_other=0.3):
self.canvas = ax.figure.canvas
self.collection = collection
self.alpha_other = alpha_other
self.xys = collection.get_offsets()
self.Npts = len(self.xys)
# Ensure that we have separate colors for each object
self.fc = collection.get_facecolors()
if len(self.fc) == 0:
raise ValueError('Collection must have a facecolor')
elif len(self.fc) == 1:
self.fc = np.tile(self.fc, (self.Npts, 1))
self.poly = PolygonSelector(ax, self.onselect)
self.ind = []
def onselect(self, verts):
path = Path(verts)
self.ind = np.nonzero(path.contains_points(self.xys))[0]
self.fc[:, -1] = self.alpha_other
self.fc[self.ind, -1] = 1
self.collection.set_facecolors(self.fc)
self.canvas.draw_idle()
def disconnect(self):
self.poly.disconnect_events()
self.fc[:, -1] = 1
self.collection.set_facecolors(self.fc)
self.canvas.draw_idle()
if __name__ == '__main__':
import matplotlib.pyplot as plt
fig, ax = plt.subplots()
grid_size = 5
grid_x = np.tile(np.arange(grid_size), grid_size)
grid_y = np.repeat(np.arange(grid_size), grid_size)
pts = ax.scatter(grid_x, grid_y)
selector = SelectFromCollection(ax, pts)
print("Select points in the figure by enclosing them within a polygon.")
print("Press the 'esc' key to start a new polygon.")
print("Try holding the 'shift' key to move all of the vertices.")
print("Try holding the 'ctrl' key to move a single vertex.")
plt.show()
selector.disconnect()
# After figure is closed print the coordinates of the selected points
print('\nSelected points:')
print(selector.xys[selector.ind])