How does one convert a grayscale image to RGB in OpenCV Python

Question

I m learning image processing using OpenCV for a realtime application  I did some thresholding on an image and want to label the contours in green  but they aren t showing up in green because my image is in black and white  Early in the program I used gray   cv2 cvtColor frame  cv2 COLOR BGR2GRAY  to convert from RGB to grayscale  but to go back I m confused  and the function backtorgb   cv2 cvtColor gray cv2 CV GRAY2RGB  is giving   AttributeError   module  object has no attribute  CV GRAY2RGB    The code below does not appear to be drawing contours in green  Is this because it s a grayscale image  If so  can I convert the grayscale image back to RGB to visualize the contours in green  import numpy as np import cv2 import time  cap   cv2 VideoCapture 0  while cap isOpened          ret  frame   cap read        gray   cv2 cvtColor frame  cv2 COLOR BGR2GRAY       ret  gb   cv2 threshold gray 128 255 cv2 THRESH BINARY       gb   cv2 bitwise not gb       contour hier   cv2 findContours gb cv2 RETR CCOMP cv2 CHAIN APPROX SIMPLE       for cnt in contour          cv2 drawContours gb  cnt  0 255 -1      gray   cv2 bitwise not gb       cv2 drawContours gray contour -1  0 255 0  3       cv2 imshow  test   gray       if cv2 waitKey 1   amp  0xFF    ord  q            break  cap release   cv2 destroyAllWindows

User · Answer

Alternatively  cv2 merge   can be used to turn a single channel binary mask layer into a three channel color image by merging the same layer together as the blue  green  and red layers of the new image  We pass in a list of the three color channel layers - all the same in this case - and the function returns a single image with those color channels  This effectively transforms a grayscale image of shape  height  width  1  into  height  width  3   To address your problem     I did some thresholding on an image and want to label the contours in green  but they aren t showing up in green because my image is in black and white    This is because you re trying to display three channels on a single channel image  To fix this  you can simply merge the three single channels  image   cv2 imread  image png   gray   cv2 cvtColor image  cv2 COLOR BGR2GRAY  gray three   cv2 merge  gray gray gray       Example  We create a color image with dimensions  200 200 3     image    np random standard normal  200 200 3     255  astype np uint8    Next we convert it to grayscale and create another image using cv2 merge   with three gray channels  gray   cv2 cvtColor image  cv2 COLOR BGR2GRAY  gray three   cv2 merge  gray gray gray     We now draw a filled contour onto the single channel grayscale image  left  with shape  200 200 1  and the three channel grayscale image with shape  200 200 3   right   The left image showcases the problem you re experiencing since you re trying to display three channels on a single channel image  After merging the grayscale image into three channels  we can now apply color onto the image     contour   np array   10 10    190  10    190  80    10  80    cv2 fillPoly gray   contour    36 255 12   cv2 fillPoly gray three   contour    36 255 12     Full code  import cv2 import numpy as np    Create random color image image    np random standard normal  200 200 3     255  astype np uint8     Convert to grayscale  1 channel  gray   cv2 cvtColor image  cv2 COLOR BGR2GRAY     Merge channels to create color image  3 channels  gray three   cv2 merge  gray gray gray      Fill a contour on both the single channel and three channel image contour   np array   10 10    190  10    190  80    10  80    cv2 fillPoly gray   contour    36 255 12   cv2 fillPoly gray three   contour    36 255 12    cv2 imshow  image   image  cv2 imshow  gray   gray  cv2 imshow  gray three   gray three  cv2 waitKey

User · Answer

One you convert your image to gray-scale you cannot got back  You have gone from three channel to one  when you try to go back all three numbers will be the same  So the short answer is no you cannot go back  The reason your backtorgb function this throwing that error is because it needs to be in the format   CvtColor input  output  CV GRAY2BGR    OpenCV use BGR not RGB  so if you fix the ordering it should work  though your image will still be gray

User · Answer

Try this   import cv2 import cv  color img   cv2 cvtColor gray img  cv CV GRAY2RGB    I discovered  while using opencv  that some of the constants are defined in the cv2 module  and other in the cv module

User · Answer

I am promoting my comment to an answer   The easy way is      You could draw in the original  frame  itself instead of using gray image    The hard way  method you were trying to implement    backtorgb   cv2 cvtColor gray cv2 COLOR GRAY2RGB  is the correct syntax

[python] How does one convert a grayscale image to RGB in OpenCV (Python)?

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