OpenCV在Google Colboratory中不起作用
问题描述
我在Google协作室练习OpenCV，因为我不知道如何在GPU上使用OpenCV，在我的硬件上运行OpenCV会占用很多CPU，所以我去了Google协作室。 指向我的笔记本的链接是here。
如果您不想看，代码如下：
import cv2
face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
cap = cv2.VideoCapture(0)
while True:
_, img = cap.read()
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
faces = face_cascade.detectMultiScale(gray, 1.1, 4)
for (x, y, w, h) in faces:
cv2.rectangle(img, (x, y), (x+w, y+h), (255, 0, 0), 2)
cv2.imshow('img', img)
k = cv2.waitKey(30) & 0xff
if k==27:
break
cap.release()
相同的代码在我的PC上运行得很好，但在Google Colboratory上就不行了。错误为：
---------------------------------------------------------------------------
error Traceback (most recent call last)
<ipython-input-5-0d9472926d8c> in <module>()
6 while True:
7 _, img = cap.read()
----> 8 gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
9 faces = face_cascade.detectMultiScale(gray, 1.1, 4)
10 for (x, y, w, h) in faces:
error: OpenCV(4.1.2) /io/opencv/modules/imgproc/src/color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cvtColor'
ps~我在Google Colboratory中将haarcasade文件放在我笔记本的同一目录中
如何处理？如果没有，是否有任何具体的解决方案可以在启用了CUDA的GPU(而不是CPU)上运行OpenCV？提前感谢！
解决方案
_src.empty()表示它从摄像头获取帧有问题，img是None，当它尝试cvtColor(None, ...)时，它会给出_src.empty()。
if img is not None:，因为cv2在无法从相机获取帧或从文件读取图像时不会引发错误。而且有时相机预热需要时间，而且它可以提供很少的空帧(None)。
VideoCapture(0)从直接连接到运行此代码的计算机的摄像机读取帧-当您在服务器Google Colaboratory上运行代码时，这意味着摄像机直接连接到服务器Google Colaboratory(不是您的本地摄像机)，但此服务器没有摄像机，因此VideoCapture(0)无法在Google Colaboratory上工作。
cv2在服务器上运行时无法从本地摄像机获取图像。您的Web浏览器可能可以访问您的摄像机，但它需要JavaScript来获取帧并发送到服务器-但是服务器需要代码来获取此帧
我在Google上查过，如果Google Colaboratory可以访问本地网络摄像头，他们似乎为此创建了脚本-Camera Capture-在第一个单元格中是函数take_photo()，该函数使用JavaScript访问您的摄像头并在浏览器中显示，而在第二个单元格中该函数用于显示本地摄像头的图像和截图。
您应该使用此函数而不是VideoCapture(0)使用本地摄像机在服务器上工作。
btw：Belovetake_photo()还有关于cv2.im_show()的信息，因为它也只能与直接连接到运行此代码的计算机的监视器一起工作(此计算机必须运行GUI，就像Windows在Windows上，X11在Linux上一样)-当您在服务器上运行它时，它希望在直接连接到服务器的监视器上显示-但是服务器通常在没有监视器(没有GUI)的情况下工作
Google Colaboratory具有在Web浏览器中显示的特殊替换
from google.colab.patches import cv2_imshow
btw：如果您在加载haarcasade时遇到问题.xml，那么您可能需要文件夹来命名文件。cv2有针对此的特殊变量cv2.data.haarcascades
path = os.path.join(cv2.data.haarcascades, 'haarcascade_frontalface_default.xml')
cv2.CascadeClassifier( path )
您还可以查看此文件夹中的内容
import os
filenames = os.listdir(cv2.data.haarcascades)
filenames = sorted(filenames)
print('
'.join(filenames))
编辑：
我创建的代码可以从本地摄像头逐帧获取，无需使用button，也无需保存在文件中。问题是它很慢-因为它仍然必须将帧从本地Web浏览器发送到Google CoLab服务器，然后再发送回本地Web浏览器
带有JavaScript函数的Python代码
#
# based on: https://colab.research.google.com/notebooks/snippets/advanced_outputs.ipynb#scrollTo=2viqYx97hPMi
#
from IPython.display import display, Javascript
from google.colab.output import eval_js
from base64 import b64decode, b64encode
import numpy as np
def init_camera():
"""Create objects and functions in HTML/JavaScript to access local web camera"""
js = Javascript('''
// global variables to use in both functions
var div = null;
var video = null; // <video> to display stream from local webcam
var stream = null; // stream from local webcam
var canvas = null; // <canvas> for single frame from <video> and convert frame to JPG
var img = null; // <img> to display JPG after processing with `cv2`
async function initCamera() {
// place for video (and eventually buttons)
div = document.createElement('div');
document.body.appendChild(div);
// <video> to display video
video = document.createElement('video');
video.style.display = 'block';
div.appendChild(video);
// get webcam stream and assing to <video>
stream = await navigator.mediaDevices.getUserMedia({video: true});
video.srcObject = stream;
// start playing stream from webcam in <video>
await video.play();
// Resize the output to fit the video element.
google.colab.output.setIframeHeight(document.documentElement.scrollHeight, true);
// <canvas> for frame from <video>
canvas = document.createElement('canvas');
canvas.width = video.videoWidth;
canvas.height = video.videoHeight;
//div.appendChild(input_canvas); // there is no need to display to get image (but you can display it for test)
// <img> for image after processing with `cv2`
img = document.createElement('img');
img.width = video.videoWidth;
img.height = video.videoHeight;
div.appendChild(img);
}
async function takeImage(quality) {
// draw frame from <video> on <canvas>
canvas.getContext('2d').drawImage(video, 0, 0);
// stop webcam stream
//stream.getVideoTracks()[0].stop();
// get data from <canvas> as JPG image decoded base64 and with header "data:image/jpg;base64,"
return canvas.toDataURL('image/jpeg', quality);
//return canvas.toDataURL('image/png', quality);
}
async function showImage(image) {
// it needs string "data:image/jpg;base64,JPG-DATA-ENCODED-BASE64"
// it will replace previous image in `<img src="">`
img.src = image;
// TODO: create <img> if doesn't exists,
// TODO: use `id` to use different `<img>` for different image - like `name` in `cv2.imshow(name, image)`
}
''')
display(js)
eval_js('initCamera()')
def take_frame(quality=0.8):
"""Get frame from web camera"""
data = eval_js('takeImage({})'.format(quality)) # run JavaScript code to get image (JPG as string base64) from <canvas>
header, data = data.split(',') # split header ("data:image/jpg;base64,") and base64 data (JPG)
data = b64decode(data) # decode base64
data = np.frombuffer(data, dtype=np.uint8) # create numpy array with JPG data
img = cv2.imdecode(data, cv2.IMREAD_UNCHANGED) # uncompress JPG data to array of pixels
return img
def show_frame(img, quality=0.8):
"""Put frame as <img src="data:image/jpg;base64,...."> """
ret, data = cv2.imencode('.jpg', img) # compress array of pixels to JPG data
data = b64encode(data) # encode base64
data = data.decode() # convert bytes to string
data = 'data:image/jpg;base64,' + data # join header ("data:image/jpg;base64,") and base64 data (JPG)
eval_js('showImage("{}")'.format(data)) # run JavaScript code to put image (JPG as string base64) in <img>
# argument in `showImage` needs `" "`
和在循环中使用它的代码
#
# based on: https://colab.research.google.com/notebooks/snippets/advanced_outputs.ipynb#scrollTo=zo9YYDL4SYZr
#
#from google.colab.patches import cv2_imshow # I don't use it but own function `show_frame()`
import cv2
import os
face_cascade = cv2.CascadeClassifier(os.path.join(cv2.data.haarcascades, 'haarcascade_frontalface_default.xml'))
# init JavaScript code
init_camera()
while True:
try:
img = take_frame()
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
#cv2_imshow(gray) # it creates new image for every frame (it doesn't replace previous image) so it is useless
#show_frame(gray) # it replace previous image
faces = face_cascade.detectMultiScale(gray, 1.1, 4)
for (x, y, w, h) in faces:
cv2.rectangle(img, (x, y), (x+w, y+h), (255, 0, 0), 2)
#cv2_imshow(img) # it creates new image for every frame (it doesn't replace previous image) so it is useless
show_frame(img) # it replace previous image
except Exception as err:
print('Exception:', err)
我不使用from google.colab.patches import cv2_imshow，因为它总是在页面上添加新图像，而不是替换现有图像。
与Google Colab上的笔记本相同的代码：
https://colab.research.google.com/drive/1j7HTapCLx7BQUBp3USiQPZkA0zBKgLM0?usp=sharing
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