Python OpenCV: mouse callback for drawing rectangle

You need to reset the image everytime when the {event == cv2.EVENT_MOUSEMOVE:} called.

Your code should look something like this:

if event == cv2.EVENT_LBUTTONDOWN:
    rectangle = True
    ix,iy = x,y

elif event == cv2.EVENT_MOUSEMOVE:
    if rectangle == True:
        sceneImg = sceneImg2.copy()
        cv2.rectangle(sceneImg,(ix,iy),(x,y),(0,255,0),2)
        rect = (min(ix,x),min(iy,y),abs(ix-x),abs(iy-y))


elif event == cv2.EVENT_LBUTTONUP:
    rectangle = False
    rect_over = True

    cv2.rectangle(sceneImg,(ix,iy),(x,y),(0,255,0),2)
    rect = (min(ix,x),min(iy,y),abs(ix-x),abs(iy-y))

Building on top of answer provided by @Marco167, I will just change one line as otherwise there's object reference problem.

So, instead of sceneImg = sceneImg2.copy() I'd suggest sceneImg[:] = sceneImg2[:], where sceneImg2 should be same, separately loaded image, like:

sceneImg = cv2.imread('sceneImg.jpg')
sceneImg2 = cv2.imread('sceneImg.jpg')

Also, I've moved rectangle check to the condition.

This way on mouse move, you first redraw the original image (thus removing the existing rectangle) and draw rectangle. Moving the mouse by even a pixel you again redraw the original picture removing any rectangle and draw again one rectangle. At any given point there will be just one rectangle.

Yup, replying after over 7 years, just in case anyone would ever find it useful :)

Putting it all together:

if event == cv2.EVENT_LBUTTONDOWN:
    rectangle = True
    ix,iy = x,y

elif event == cv2.EVENT_MOUSEMOVE and rectangle :
    sceneImg[:] = sceneImg2[:]
    cv2.rectangle(sceneImg,(ix,iy),(x,y),(0,255,0),2)
    rect = (min(ix,x),min(iy,y),abs(ix-x),abs(iy-y))


elif event == cv2.EVENT_LBUTTONUP:
    rectangle = False
    rect_over = True

    cv2.rectangle(sceneImg,(ix,iy),(x,y),(0,255,0),2)
    rect = (min(ix,x),min(iy,y),abs(ix-x),abs(iy-y))

This is my current work around where I again render the mouse input window upon EVENT_LBUTTONUP. To avoid the bounding box showing up in the ROI saved to file I use a copy of the inputed scene:

import cv2

rect = (0,0,1,1)
rectangle = False
rect_over = False  
def onmouse(event,x,y,flags,params):
    global sceneImg,rectangle,rect,ix,iy,rect_over, roi

    # Draw Rectangle
    if event == cv2.EVENT_LBUTTONDOWN:
        rectangle = True
        ix,iy = x,y

    elif event == cv2.EVENT_MOUSEMOVE:
        if rectangle == True:
#            cv2.rectangle(sceneCopy,(ix,iy),(x,y),(0,255,0),1)
            rect = (min(ix,x),min(iy,y),abs(ix-x),abs(iy-y))

    elif event == cv2.EVENT_LBUTTONUP:
        rectangle = False
        rect_over = True

        sceneCopy = sceneImg.copy()
        cv2.rectangle(sceneCopy,(ix,iy),(x,y),(0,255,0),1)

        rect = (min(ix,x),min(iy,y),abs(ix-x),abs(iy-y))       
        roi = sceneImg[rect[1]:rect[1]+rect[3], rect[0]:rect[0]+rect[2]]

        cv2.imshow('mouse input', sceneCopy)
        cv2.imwrite('roi.jpg', roi)

# Named window and mouse callback
cv2.namedWindow('mouse input')
cv2.setMouseCallback('mouse input',onmouse)
cv2.namedWindow('video')

camObj = cv2.VideoCapture(-1)
keyPressed = None
running = True
scene = False
# Start video stream
while running:
    readOK, frame = camObj.read()

    keyPressed = cv2.waitKey(5)
    if keyPressed == ord('s'):
        scene = True
        cv2.destroyWindow('video')

        cv2.imwrite('sceneImg.jpg',frame)
        sceneImg = cv2.imread('sceneImg.jpg')

        cv2.imshow('mouse input', sceneImg)

    elif keyPressed == ord('r'):
        scene = False
        cv2.destroyWindow('mouse input')

    elif keyPressed == ord('q'):
        running = False

    if not scene:
        cv2.imshow('video', frame)

cv2.destroyAllWindows()
camObj.release()

Thus, I can visualize the rectangle which is supposed to bound the ROI but I still don't know how to visualize the bounding box while the mouse left button is down and the mouse cursor is moving. That visualization works in the grabcut example but I couldn't figure it out in my case. Upon uncommenting the line for drawing rectangle during EVENT_MOUSEMOVE I get multiple rectangles drawn onto the image. If someone answers with a way to visualize a single rectangle as it is being created I can accept it.