matplotlib in python - how to extracting data from contour lines-CodePudding

i want to get data from a contour of different iso-line. the question matplotlib - extracting data from contour lines gives an example

import matplotlib.pyplot as plt
x = [1,2,3,4]
y = [1,2,3,4]
m = [[15,14,13,12],[14,12,10,8],[13,10,7,4],[12,8,4,0]]
cs = plt.contour(x,y,m, [9.5])
plt.show()
coord = cs.collections[0].get_paths()

it get the coordinates of line with value euqal to 9.5.

now, i need to get the coordinate of multi-isoline from one contour, so i need to change the value to represent different line, but when i use loop, it means python needs to construct the contour at each loop, how can i construct the contour once and then change the value to represent different line?

CodePudding user response：

You can plot several contours at once with plt.contour by giving a list of the values you wish to contour. Then, you can access them all from the returned ContourSet using cs.allsegs or by using get_paths on each item in the cs.collections list.

For example:

import matplotlib.pyplot as plt
x = [1,2,3,4]
y = [1,2,3,4]
m = [[15,14,13,12],[14,12,10,8],[13,10,7,4],[12,8,4,0]]
cs = plt.contour(x,y,m, [9.5, 10.5, 11.5])
plt.show()

# Option 1: use allsegs
all_coords = cs.allsegs
print(all_coords)

# Option 2: use cs.collections[X].get_paths()
coords1 = cs.collections[0].get_paths()
coords2 = cs.collections[1].get_paths()
coords3 = cs.collections[2].get_paths()
print(coords1)
print(coords2)
print(coords3)

Where the printed coords are then:

Option 1 (allsegs):

[[array([[4.        , 1.625     ],
       [3.25      , 2.        ],
       [3.        , 2.16666667],
       [2.16666667, 3.        ],
       [2.        , 3.25      ],
       [1.625     , 4.        ]])],
 [array([[4.        , 1.375     ],
       [3.        , 1.83333333],
       [2.75      , 2.        ],
       [2.        , 2.75      ],
       [1.83333333, 3.        ],
       [1.375     , 4.        ]])],
 [array([[4.   , 1.125],
       [3.   , 1.5  ],
       [2.25 , 2.   ],
       [2.   , 2.25 ],
       [1.5  , 3.   ],
       [1.125, 4.   ]])]]

Option 2 (get_paths()):


[Path(array([[4.        , 1.625     ],
       [3.25      , 2.        ],
       [3.        , 2.16666667],
       [2.16666667, 3.        ],
       [2.        , 3.25      ],
       [1.625     , 4.        ]]), array([1, 2, 2, 2, 2, 2], dtype=uint8))]
[Path(array([[4.        , 1.375     ],
       [3.        , 1.83333333],
       [2.75      , 2.        ],
       [2.        , 2.75      ],
       [1.83333333, 3.        ],
       [1.375     , 4.        ]]), array([1, 2, 2, 2, 2, 2], dtype=uint8))]
[Path(array([[4.   , 1.125],
       [3.   , 1.5  ],
       [2.25 , 2.   ],
       [2.   , 2.25 ],
       [1.5  , 3.   ],
       [1.125, 4.   ]]), array([1, 2, 2, 2, 2, 2], dtype=uint8))]