使用实时相机预览更新matplotlib中的帧

Question

我是Python和Matplotlib的新手。我的计算机连接到两台USB摄像机，我打算使用matplotlib中的子图（1,2,1）和子图（1,2,2）按时间顺序绘制两台摄像机的帧。当我使用我的代码执行此操作时，我要么只绘制一个帧，要么在绘图区域中获得黑屏。

我的代码如下所示

#import
import cv2
import matplotlib.pyplot as plt

#Initiate the two cameras
cap1 = cv2.VideoCapture(0)
cap2 = cv2.VideoCapture(1)

#Capture the frames from camera 1 and 2 and display them over time using matplotlib

while True:
    #grab frame from camera 1 and 2
    ret1,frame1 = cap1.read()
    ret2,frame2 = cap2.read()

    plt.subplot(1,2,1), plt.imshow(cv2.cvtColor(frame1,cv2.COLOR_BGR2RGB))
    plt.subplot(1,2,2), plt.imshow(cv2.cvtColor(frame2,cv2.COLOR_BGR2RGB))

    #draw the plot
    plt.show(False)
    #Result is black screen. If plt.show() is called, I see the frames but then it freezes.

Answer 1

交互模式

在matplotlib中更新绘图的一种方法是使用交互模式（plt.ion()）。 然后，您不应为捕获的每个帧重新创建新的子图，而是使用图像创建一次图并在之后更新它。

import cv2
import matplotlib.pyplot as plt

def grab_frame(cap):
    ret,frame = cap.read()
    return cv2.cvtColor(frame,cv2.COLOR_BGR2RGB)

#Initiate the two cameras
cap1 = cv2.VideoCapture(0)
cap2 = cv2.VideoCapture(1)

#create two subplots
ax1 = plt.subplot(1,2,1)
ax2 = plt.subplot(1,2,2)

#create two image plots
im1 = ax1.imshow(grab_frame(cap1))
im2 = ax2.imshow(grab_frame(cap2))

plt.ion()

while True:
    im1.set_data(grab_frame(cap1))
    im2.set_data(grab_frame(cap2))
    plt.pause(0.2)

plt.ioff() # due to infinite loop, this gets never called.
plt.show()

FuncAnimation

另一种选择当然是使用matplotlib内置的FuncAnimation，它专门用于动画图。

import cv2
import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation

def grab_frame(cap):
    ret,frame = cap.read()
    return cv2.cvtColor(frame,cv2.COLOR_BGR2RGB)

#Initiate the two cameras
cap1 = cv2.VideoCapture(0)
cap2 = cv2.VideoCapture(1)

#create two subplots
ax1 = plt.subplot(1,2,1)
ax2 = plt.subplot(1,2,2)

#create two image plots
im1 = ax1.imshow(grab_frame(cap1))
im2 = ax2.imshow(grab_frame(cap2))

def update(i):
    im1.set_data(grab_frame(cap1))
    im2.set_data(grab_frame(cap2))

ani = FuncAnimation(plt.gcf(), update, interval=200)
plt.show()

为了关闭按键事件的窗口，您可以添加回调，如此

#... other code
ani = FuncAnimation(plt.gcf(), update, interval=200)

def close(event):
    if event.key == 'q':
        plt.close(event.canvas.figure)

cid = plt.gcf().canvas.mpl_connect("key_press_event", close)

plt.show()

# code that should be executed after window is closed.