Python OpenCV: mouse callback for drawing rectangle

Question

I want to save an image from the video stream and then draw a rectangle onto the shown image to produce a region of interest. Later, save that ROI in a file. I used opencv python grabcut example to use the setMouseCallback function. But I don't know what I'm doing incorrect as it is not giving the result I expect. I would like to see the green rectangle drawn on the static image shown in mouse input window and the roi being saved to file. Please help debug this code or show a better approach:

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

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

    elif event == cv2.EVENT_MOUSEMOVE:
        if rectangle == 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))

    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))

        x1,y1,w,h = rect        
        roi = sceneImg[y1:y1+h, x1:x1+w]

        cv2.imwrite('roi.jpg', roi)

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

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

        cv2.destroyWindow('video')
        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()

Answer 1

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))

Answer 2

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))

Answer 3

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.