Bluescreen

Building on the earlier image manipulation and logic sections, here we see how to implement a movie special effect.

Bluescreen Special Effect

The stop.jpg image and the leaves.jpg image
stop sign leaves background texture

Bluescreen Algorithm

bluescreen image/back algorithm

Bluescreen Example Code


The above bluescreen example code detects the red part of the sign, and for those pixels, copies over the pixels from the back image. The bluescreen code below has 3 key differences from the earlier examples:

  1. back = new SimpleImage("leaves.jpg"); -- open a second image and store it in the variable "back" (thus far we've only opened one image at a time)
  2. pixel2 = back.getPixel(x, y); -- say we are looking at pixel x,y in the main image. Get the x,y pixel from the other image and store it in the variable "pixel2". We can get the x and y of the current pixel in the loop with the functions pixel.getX() and pixel.getY().
  3. pixel.setRed(pixel2.getRed()); -- copy the red value from pixel2 to pixel. Also do this for the other two channels. In effect, for the x,y location, this copies the pixel from back to image.

Experiments: (a) Try copying the leaves to the sky instead of to the red parts of the sign (similar to the earlier Night Mode problem). (b) Make the code a little shorter by eliminating the need for "x" and "y" variables. Put the calls to pixel.getX() and pixel.getY() right in the back.getPixel(...) call.

Abby Bluescreen

Baby abby on her bouncy chair
In this example we have the abby.jpg image -- make it appear that the green parts of her little chair are sprouting leaves.


Solution code: 

// Here the pixel2 = ... computation is shortened to one line
image = new SimpleImage("abby.jpg");
back = new SimpleImage("leaves.jpg");

for (pixel: image) {
  avg = (pixel.getRed() + pixel.getGreen() + pixel.getBlue())/3;
  // your code here
  if (pixel.getGreen() > avg * 1.05) {
    // 1. Get pixel2 from back image at same x,y (one line)
    pixel2 = back.getPixel(pixel.getX(), pixel.getY());

    // 2. Copy pixel2 over to pixel
    pixel.setRed(pixel2.getRed());
    pixel.setGreen(pixel2.getGreen());
    pixel.setBlue(pixel2.getBlue());
  }
}
print(image);

Monkey Bluescreen Like The Movies

Now we'll do one like the movies -- film the movie start in front of a blue screen.

Here is our monkey movie star:
monkey shown in front of blue background

Here is our background, the famous Apollo 8 photo of the earth shown rising above the moon horizon.


Solution code: 

image = new SimpleImage("monkey.jpg");
back = new SimpleImage("moon.jpg");

for (pixel: image) {
  avg = (pixel.getRed() + pixel.getGreen() + pixel.getBlue())/3;
  // your code here
  if (pixel.getBlue() > avg * 0.92) {
    pixel2 = back.getPixel(pixel.getX(), pixel.getY());
    pixel.setRed(pixel2.getRed());
    pixel.setGreen(pixel2.getGreen());
    pixel.setBlue(pixel2.getBlue());
  }
}

print(image);

Bluescreen Summary

> exercises