Replace the white block with video

Hi, I’m making a projection wall by openFrameworks and need some help.
The project is like the up image


I’m using a IR camera, and the second image is what I have right now.
I want to play a video in the background and cut the white part. (play video inside the white part)Anyone can help?
This is the code:

#include "ofApp.h"


void ofApp::setup()
{
    grabber.setup(640, 480);
    
    IRCamPixels.allocate(grabber.getWidth(), grabber.getHeight(), OF_PIXELS_GRAY);
    myPlayer.load("take another peep.mp4");
    myPlayer.play();

    
}

void ofApp::update()
{
    myPlayer.update();
    threshold = ofMap(ofGetMouseX(), 0, ofGetWidth(), 0, 255);
    
    grabber.update();
    
    if (grabber.isFrameNew())
    {
        for (int x = 0; x < grabber.getWidth(); x++)
        {
            for (int y = 0; y < grabber.getHeight(); y++)
            {
                if (grabber.getPixels().getColor(x, y).getBrightness() > threshold)
                {
                    IRCamPixels.setColor(x, y, ofColor(255,10));
                }
                else
                {
                    IRCamPixels.setColor(x, y, ofColor(0));
                }
            }
        }
//        myPlayer.update();

        IRCamTexture.loadData(IRCamPixels);
    }
}

void ofApp::draw()
{
    myPlayer.draw(0, 0);
    IRCamTexture.draw(0, 0);
    
    ofDrawBitmapStringHighlight("Threshold: " + ofToString(threshold), 14, 14);
}

What you want to do is called “masking” or “blending”. Please search through the forum, you will find lots of ways to do it.
For example have a look at these threads:

Thanks! Those are really helpful!