如何查找由使用matplotlib绘制的图所传递的像素

Question

我使用以下代码绘制函数：

t = np.arange(0., 5., 0.2)
plt.plot(t, (t**2)+10*np.sin(t))
plt.axis('off')

我想知道如何将图另存为0/1数组，如果图通过，则像素值为1，否则为0。

一个后续问题是，如果我以一定的线宽绘制图，我希望像素值只有在图的“中心”线上时才为1，否则为0。我应该怎么做？谢谢！

Answer 1

将图形转换为RGBA数组的方法有很多。最简单的方法可能是将文件另存为PNG，然后使用plt.imread或类似文件再次加载文件。如果您觉得这很round回，您可以使用我在下面使用的plot2img，它抓住画布并将其通过中间表示形式转换为数组作为字符串缓冲区。

之后，仅需对图像进行阈值处理并提取中间轴，就可以使用scikit-image提供的功能轻松完成。

#!/usr/bin/env python
"""
https://stackoverflow.com/q/62014554/2912349
"""

import numpy as np
import matplotlib.pyplot as plt

from matplotlib.backends.backend_agg import FigureCanvasAgg

from skimage.color import rgb2gray
from skimage.filters import threshold_otsu
from skimage.morphology import medial_axis


def plot2img(fig, remove_margins=True):
    # https://stackoverflow.com/a/35362787/2912349
    # https://stackoverflow.com/a/54334430/2912349

    if remove_margins:
        fig.subplots_adjust(left=0, bottom=0, right=1, top=1, wspace=0, hspace=0)

    canvas = FigureCanvasAgg(fig)
    canvas.draw()
    img_as_string, (width, height) = canvas.print_to_buffer()
    return np.fromstring(img_as_string, dtype='uint8').reshape((height, width, 4))


if __name__ == '__main__':

    t = np.arange(0., 5., 0.2)
    y = (t**2)+10*np.sin(t)

    # plot in a large figure such that the resulting image has a high resolution
    fig, ax = plt.subplots(figsize=(20, 20))
    ax.plot(t, y)
    ax.axis('off')

    # convert figure to an RGBA array
    as_rgba = plot2img(fig)

    # close plot made with non-interactive Agg backend so that we can open the other later
    plt.close('all')

    # threshold the image
    as_grayscale = rgb2gray(as_rgba)
    threshold = threshold_otsu(as_grayscale)
    as_bool = as_grayscale < threshold

    # find midline
    midline = medial_axis(as_bool)

    # plot results
    fig, (ax1, ax2) = plt.subplots(1, 2)
    ax1.imshow(as_bool, cmap='gray_r')
    ax2.imshow(midline, cmap='gray_r')
    plt.show()