matplotlib之多邊形選區(PolygonSelector)的使用

多邊形選區概述

多邊形選區是一種常見的對象選擇方式,在一個子圖中,單擊鼠標左鍵即構建一個多邊形的端點,最後一個端點與第一個端點重合即完成多邊形選區,選區即為多個端點構成的多邊形。在matplotlib中的多邊形選區屬於部件(widgets),matplotlib中的部件都是中性(neutral )的,即與具體後端實現無關。

多邊形選區具體實現定義為matplotlib.widgets.PolygonSelector類,繼承關系為:Widget->AxesWidget->_SelectorWidget->PolygonSelector。

PolygonSelector類的簽名為class matplotlib.widgets.PolygonSelector(ax, onselect, useblit=False, lineprops=None, markerprops=None, vertex_select_radius=15)

PolygonSelector類構造函數的參數為:

  • ax:多邊形選區生效的子圖,類型為matplotlib.axes.Axes的實例。
  • onselect:多邊形選區完成後執行的回調函數,函數簽名為def onselect( vertices),vertices數據類型為列表,列表元素格式為(xdata,ydata)元組。
  • drawtype:多邊形選區的外觀,取值范圍為{“box”, “line”, “none”},”box”為多邊形框,”line”為多邊形選區對角線,”none”無外觀,類型為字符串,默認值為”box”。
  • lineprops:多邊形選區線條的屬性,默認值為dict(color=’k’, linestyle=’-‘, linewidth=2, alpha=0.5)。
  • markerprops:多邊形選區端點的屬性,默認值為dict(marker=’o’, markersize=7, mec=’k’, mfc=’k’, alpha=0.5)。
  • vertex_select_radius:多邊形端點的選擇半徑,浮點數,默認值為15,用於端點選擇或者多邊形閉合。

PolygonSelector類中的state_modifier_keys公有變量 state_modifier_keys定義瞭操作快捷鍵,類型為字典。

  • “move_all”: 移動已存在的選區,默認為”shift”。
  • “clear”:清除現有選區,默認為 “escape”,即esc鍵。
  • “move_vertex”:正方形選區,默認為”control”。

PolygonSelector類中的verts特性返回多邊形選區中的多有端點,類型為列表,元素為(x,y)元組,即端點的坐標元組。

案例

官方案例,https://matplotlib.org/gallery/widgets/polygon_selector_demo.html

案例說明

單擊鼠標左鍵創建端點,最終點擊初始端點閉合多邊形,形成多邊形選區。選區外的數據元素顏色變淡,選區內數據顏色保持不變。

按esc鍵取消選區。按shift鍵鼠標可以移動多邊形選區位置,按ctrl鍵鼠標可以移動多邊形選區某個端點的位置。退出程序時,控制臺輸出選區內數據元素的坐標。

控制臺輸出:

Selected points:
[[2.0 2.0]
 [1.0 3.0]
 [2.0 3.0]]

案例代碼

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])

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