The solution is using Prof Levin’s way to measure the brightness of each pixel on the screen canvas. If the pixel in the letter’s position is below the threshold lower value, the letter should go back where it is bright enough instead of going down.

The text is read from a ‘captain.txt’ file, repeatedly falling from the top of canvas. When it goes down to the bottom, I reset its y value.

 
// Text Rain (Processing Re-Code "cover version")
// Original by Camille Utterback and Romy Achituv (1999):
// http://camilleutterback.com/projects/text-rain/
// Implemented in Processing 2.0b7 by Han Hua, January 2013
// 
// This assumes that the participant is in front of a light-colored background. 

//===================================================================
// The live video camera Capture object:
import processing.video.*;
Capture video;

float letterGravity = 2;
int brightnessThreshold = 110;
float initialLetterYPosition = 10;
TextRainLetter poemLetters[];
int nLetters;

String poemLines[];
int line_index = 0;
BufferedReader reader;
int m_sec;
int nextAddTime = 5000;

//-----------------------------------
void setup() {
  frameRate(30);

  size(640,480); 
  video = new Capture (this, width, height);
  video.start();  

  poemLines = new String[30];
  reader = createReader("captain.txt");
  readText();

  String poemString = poemLines[line_index];
  nLetters = poemString.length();
  poemLetters = new TextRainLetter[nLetters];
  for (int i=0; i<nLetters; i++) {
    char c = poemString.charAt(i);
    float x = random(width * ((float)i/(nLetters+1)) + 1, width * ((float)(i+1)/(nLetters+1)));
    float y = random(initialLetterYPosition, initialLetterYPosition+10);
    poemLetters[i] = new TextRainLetter(c,x,y);
  }
}

//-----------------------------------
void draw() {
  if (video.available()) {
    video.read();
    video.loadPixels();

    // this translate & scale flips the video left/right. 
    pushMatrix();

    // mirror the video
    translate (width,0); 
    scale (-1,1); 
    image (video, 0, 0, width, height); // refresh
    popMatrix();

    for (int i=0; i<nLetters; i++) {       poemLetters[i].update();       poemLetters[i].draw();     }   }      m_sec = millis();   if(m_sec > nextAddTime){
    println("1");
    nextAddTime += 3000;
    line_index++;
    if(line_index > poemLines.length-1){
      line_index = 0;
    }
    addNewLetters();
  }
}

void readText() {
  String line = null;
   try{
       int i = 0;
       while ((line = reader.readLine()) != null) {
          poemLines[i] = line;
          i++;
          //println(line);
       }

   }catch(Exception e)
  {
   e.printStackTrace();
    line = null;
  }
}

void addNewLetters(){
  TextRainLetter tempLetters[] = new TextRainLetter[poemLetters.length + poemLines[line_index].length()];

  int i=0;
  for(; i < poemLetters.length; i++){
    tempLetters[i] = poemLetters[i];
  }

  String s = poemLines[line_index];
  int n = s.length();
  for(int j = 0; j<n; j++, i++){     char c = s.charAt(j);     float x = random(1, width-1);     float y = random(initialLetterYPosition, initialLetterYPosition+30);     tempLetters[i] = new TextRainLetter(c,x,y);   }      poemLetters = tempLetters;   nLetters = poemLetters.length; } //----------------------------------- void keyPressed() {   if (key == CODED) {     if (keyCode == UP) {       brightnessThreshold = min(255, brightnessThreshold+5);       println("brightnessThreshold = " + brightnessThreshold);      } else if (keyCode == DOWN) {       brightnessThreshold = max(0, brightnessThreshold-5);       println("brightnessThreshold = " + brightnessThreshold);     }    }  } //=================================================================== class TextRainLetter {      char  c;   float x;    float y;      TextRainLetter (char cc, float xx, float yy) {     c = cc;     x = xx;     y = yy;   }   //-----------------------------------   void update() {     // Update the position of a TextRainLetter.           // 1. Compute the pixel index corresponding to the (x,y) location of the TextRainLetter.     int flippedX = (int)(width-1-x); // because we have flipped the video left/right.     int index = width*(int)y + flippedX;     index = constrain (index, 0, width*height-1);          // establish a range around the threshold, within which motion is not required.     int thresholdTolerance = 5;     int thresholdLo = brightnessThreshold - thresholdTolerance;     int thresholdHi = brightnessThreshold + thresholdTolerance;          // 2. Fetch the color of the pixel there, and compute its brightness.     float pixelBrightness = brightness(video.pixels[index]);          // 3. If the TextRainLetter is in a bright area, move downwards.     //    If it's in a dark area, move up until we're in a light area.     if (pixelBrightness > thresholdHi) {
      y += letterGravity; //move downward

    } else {
      while ((y > initialLetterYPosition) && (pixelBrightness < thresholdLo)){         y -= letterGravity; // travel upwards intil it's bright again         index = width*(int)y + flippedX;         index = constrain (index, 0, width*height-1);         pixelBrightness = brightness(video.pixels[index]);       }     }          if ((y >= height-1) || (y < initialLetterYPosition)){       y = initialLetterYPosition;     }   }   //-----------------------------------   void draw() {     // Draw the letter. Use a simple black "drop shadow"     // to achieve improved contrast for the typography.           if( y > height-20){
      y = random(initialLetterYPosition, initialLetterYPosition+30);
      x = random(1, width-1);
    }

    fill(random(256),random(256),random(256));
    text (""+c, x+1,y+1); 
    text (""+c, x-1,y+1); 
    text (""+c, x+1,y-1); 
    text (""+c, x-1,y-1); 
    fill(255,255,255);
    text (""+c, x,y);
  }
}

The Github Repo: https://github.com/chinesecold/TextRain