Question

I am reading a video frame by frame using:

vc = cv2.VideoCapture('test.avi')

and later on I check whether the frame has been read using:

if vc.isOpened():
    rval,frame = vc.read()
else:
    rval = False

Now the problem is , if I try to convert this frame to a numpy array using the following code:

PILImage = Image.fromstring("L",cv.GetSize(frame),frame.tostring())
NumPyArray = np.array(PILImage)

I get an error saying:

CvArr argument 'arr' must be IplImage, CvMat or CvMatND. Use fromarray() to convert numpy arrays to CvMat or cvMatND

From Documentation of VideoCapture::read I found that it returns two things [retVal,Image] . How do I get the Image part only and how do I convert it to Numpy Array?

Further Info:
Basically the whole point in doing this is that ,I am trying to write a program which allows me to go through each frame of the video by pressing spacebar, and select a particular region from any frame and save it as a jpg. Here is the Code:

from ITMS import ITMS
import cv2
from cv2 import cv
import numpy as np
import matplotlib.pyplot as plt
import Image
import matplotlib.widgets as widgets

def onselect(eclick, erelease):
    if eclick.ydata>erelease.ydata:
        eclick.ydata,erelease.ydata=erelease.ydata,eclick.ydata
    if eclick.xdata>erelease.xdata:
        eclick.xdata,erelease.xdata=erelease.xdata,eclick.xdata
    ax.set_ylim(erelease.ydata,eclick.ydata)
    ax.set_xlim(eclick.xdata,erelease.xdata)
    fig.canvas.draw()

def subImager(arr):
    fig = plt.figure()
    ax = fig.add_subplot(111)

    plt_image=plt.imshow(arr,cmap="Greys_r")
    rs=widgets.RectangleSelector(
        ax, onselect, drawtype='box',
        rectprops = dict(facecolor='red', edgecolor = 'red', alpha=0.2, fill=True))
    plt.show()


cv2.namedWindow("preview")
vc = cv2.VideoCapture('test.avi')

if vc.isOpened():
    rval,frame = vc.read()
else:
    rval = False

while rval:
    key = cv2.waitKey(30)
        if key==32:
            cv2.imshow("preview", frame)
            NumPyArray=ITMS.CVtoNPArray(frame)
            subImager(NumPyArray)
            rval,frame = vc.read()
        elif key==27:
            break

cv2.destroyAllWindows()

ITMS Class:

from cv2 import cv
import cv2
import numpy as np
from PIL import Image

class ITMS:
    def __init__(self):
        pass

    def CVtoNPArray(CVImage):
        PILImage = Image.fromstring("L",cv.GetSize(CVImage),CVImage.tostring())
        NumPyArray = np.array(PILImage)
        return NumPyArray
        CVtoNPArray=staticmethod(CVtoNPArray)

Answer 1

You are over-complicating things, the image returned in your frame name is already a numpy array. If you want to convert it to PIL, simply do Image.fromarray(frame).