Greenscreen

Something used heaps in the film industry is the "Greenscreen" I thought I would take a quick look at how to make a greenscreen that works fast enough to run on a live webcam stream. And infact one that works with any coloured background. It has many many limitations, but was a fun experiment! To run this example you will need OpenCV with the SWIG Python bindings installed. You can get this code from my SVN repository here.


Firstly the background I started with:


Adding an object to the scene, and carrying out back ground subtraction:



So anyhow the code:

#!/usr/bin/env python

from VideoCapturePlayer import VideoCapturePlayer as VCP

from opencv import cv


def threshold_image(image, n=[]):
    """Record the first 5 images to get a background, then diff current frame with the last saved frame.
    """
    if len(n) < 5:
        # n[4] will be our background
        # First capture a few images
        n.append(cv.cvCloneMat(image))
        if len(n) == 5:
            # last time here 
            # could do averaging here.
            pass
        return image
        
    original = n[4]
    differenceImage  = cv.cvCloneMat( image )
    cv.cvAbsDiff( image, original, differenceImage )
    thresholdValue = 50     # 32 
    cv.cvThreshold( differenceImage, differenceImage, thresholdValue, 255, cv.CV_THRESH_BINARY )
    
    cv.cvSmooth(differenceImage, differenceImage, cv.CV_MEDIAN, 15)
    gray = cv.cvCreateImage( cv.cvGetSize(differenceImage), 8, 1 )
    cv.cvCvtColor( differenceImage, gray, cv.CV_BGR2GRAY )   
    
    result  = cv.cvCloneMat( image)
    cv.cvSetZero(result)
    
    cv.cvAnd(image,image, result, gray)
    return result

if __name__ == "__main__":
    title = "Background Subtraction Output"
    VCP(threshold_image, title).main()

Some objects don't work so well. Output is very noisy.

Now there is nothing to say a green background must be used at all, so what about just using my desk as the background:

And adding my highlighter to the desk...

And a simple shot showing the final scene to show I didn't cheat.

And there you have it.