How to convert a range of colours to transparent?

Question

I have an image that has different shades of black at the edges and a bit of red in the centre. I want to convert all the black pixels to transparent using opencv. I'm new to opencv so I'd appreciate your help.

I tried following what fireant said in the link: overlay a smaller image on a larger image python OpenCv, but it didn't work. Here's the code I have so far:

img = cv2.imread("/home/uwatt/Downloads/lensf1.jpg")
img = cv2.cvtColor(img,cv2.COLOR_BGR2RGB)
tmp = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
_,alpha = cv2.threshold(tmp,5,255,cv2.THRESH_BINARY)
b,g,r = cv2.split(img)
rgba = [b,g,r,alpha]
dst = cv2.merge(rgba, 4)
plt.imshow(dst)
print(dst.shape)

face_cascade = cv2.CascadeClassifier('/home/uwatt/DIP/lensflare/haarcascade_frontalface_default.xml')
eye_cascade = cv2.CascadeClassifier('/home/uwatt/DIP/lensflare/haarcascade_eye.xml')

user = cv2.imread("/home/uwatt/Downloads/Dicaprio.jpg")
gray_user = cv2.cvtColor(user, cv2.COLOR_BGR2GRAY)
user = cv2.cvtColor(user, cv2.COLOR_BGR2BGRA)
faces = face_cascade.detectMultiScale(gray_user, 1.3, 5)
print("Faces:",faces)
for (x,y,w,h) in faces:
    roi_gray = gray_user[y:y+h,x:x+w]
    roi_color = user[y:y+h,x:x+w]
    eyes = eye_cascade.detectMultiScale(roi_gray)
    for (ex,ey,ew,eh) in eyes:
        print(ex,ey,ew,eh)
        #cv2.rectangle(roi_color,(ex,ey),(ex+ew,ey+eh),(0,0,255),5)

        # resizing & paste the lf image on user
        roi_eye = user[y+ey:y+ey+eh,x+ex:x+ex+ew]
        resized_lensflare = cv2.resize(dst,(eh,ew))    
        resized_lensflare = cv2.cvtColor(resized_lensflare, cv2.COLOR_BGR2RGBA)
        user[y+ey:y+ey+eh,x+ex:x+ex+ew] = resized_lensflare

Answer 1

You need to use alpha blending to combine the lens flare with the background image. Check out this tutorial to find out more about alpha blending. Here is the stript that I used:

import cv2
flare = cv2.imread("/home/stephen/Desktop/flare.jpg")
user = cv2.imread("/home/stephen/Desktop/leo.jpg")
eyes = [[100,50,200,200],[175,50,200,200]]
for x,y,w,h in eyes:       
    # resizing & paste the lf image on user
    roi_eye = user[y:y+h,x:x+w]
    resized_lensflare = cv2.resize(flare,(w,h))    
    # Make foreground background and alpha
    foreground = resized_lensflare.copy()
    background = roi_eye.copy()
    alpha= foreground.copy()
    # Convert uint8 to float
    foreground = foreground.astype(float)
    background = background.astype(float) 
    # Normalize the alpha mask to keep intensity between 0 and 1
    alpha = alpha.astype(float)/255
    # Multiply the foreground with the alpha matte
    foreground = cv2.multiply(alpha, foreground)
    # Multiply the background with ( 1 - alpha )
    background = cv2.multiply(1.0 - alpha, background)
    # Add the masked foreground and background.
    outImage = cv2.add(foreground, background)
    # Mask the user image
    user[y:y+h,x:x+w] = outImage
cv2.imshow('img', user)
cv2.waitKey()
cv2.destroyAllWindows()