# Image-8 Bluescreen

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

## Bluescreen Special Effect

• "Bluescreen" video special effect
• Video is just a series of still images, 20-60 per second
• Have some background image, e.g. leaves.jpg
• Detect colored area in an image, e.g. red in stop.jpg
• For colored area, substitute in pixels from background image
• Lego analogy -- same setRed() etc. we've used, arranged in a new way

The stop.jpg image and the leaves.jpg image

## Bluescreen Algorithm

• Bluescreen algorithm
• Two images: image and back
• Detect, say, red pixels in image
• For each red pixel:
• --Consider the pixel at the same x,y in back image
• --Copy over the pixel from back to image
• --What defines how a pixel looks? Three numbers.
• --Copy over the red, green, and blue values
• Result: for red areas, substitute in areas from back image

## Bluescreen Example Code

• Two images: image and back
• if-statement / avg -- detects red as usual
• Inside if:
• --1. Figure out x,y of pixel
• --2. Get pixel2 from back image (at x,y)
• --3. Make pixel the same as pixel2
• -- i.e. copy over red/green/blue values

 image = new SimpleImage("stop.jpg"); back = new SimpleImage("leaves.jpg"); for (pixel: image) { avg = (pixel.getRed() + pixel.getGreen() + pixel.getBlue())/3; if (pixel.getRed() > avg * 1.5) { x = pixel.getX(); y = pixel.getY(); pixel2 = back.getPixel(x, y); pixel.setRed(pixel2.getRed()); pixel.setGreen(pixel2.getGreen()); pixel.setBlue(pixel2.getBlue()); } } print(image); image-bluescreen-1

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.

## Stop / Leaves Experiments

• Flip -- try replacing everything-but-the-red with leaves instead of the red.
• Sky -- try replacing the blue sky instead of to the red areas of the sign (similar to the earlier Night Mode problem).
• Shorter -- 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

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

 image = new SimpleImage("abby.jpg"); back = new SimpleImage("leaves.jpg"); for (pixel: image) { avg = (pixel.getRed() + pixel.getGreen() + pixel.getBlue())/3; if (pixel.getGreen() > avg * 1.5) { // your code here } } print(image); image-bluescreen-2

Solution code:

## Monkey Bluescreen Like The Movies

Now we'll do one like the movies -- film the movie start in front of a blue screen. Replace the blue background behind the monkey with pixels from moon.jpg.

Here is our monkey movie star:

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

 image = new SimpleImage("monkey.jpg"); back = new SimpleImage("moon.jpg"); for (pixel: image) { avg = (pixel.getRed() + pixel.getGreen() + pixel.getBlue())/3; // your code here } print(image); image-bluescreen-3

Solution code:

## back.setSameSize(image);

Working with a main image and a back image, there is a problem if the back image is smaller than the main image -- the code will try to access a non-existent pixel, and we get an "x/y out of bounds" error. The following line fixes this problem:

```  back.setSameSize(image);
```

The above line resizes the back image if necessary, so it is at least as big as the main image, which is exactly what we need for blue screening, picking pixels from the back image to place into the main image. You'll see this line in the starter code for some later problems.

## Image Ghost Blending

Another effect we can try is blending a "ghost" version of the monkey into the back image. We blend the colors from the two images.

• Put a "ghost" version of monkey into the back image
• Blend pixels from the two images vs. replacing
• 1. Loop over monkey image
• 2. if-logic to detect monkey
• Tricky: flip, previously we detected not-monkey background
• 3. Set back-pixel-red = back-pixel-red + monkey-pixel-red/10
• 4. Do it for the other 2 colors too
• 5. Print the back image
• Adjust the /10 to be /5 or /12 to get the effect you want
• Result is a blend of a faint monkey into the back image

 image = new SimpleImage("monkey.jpg"); back = new SimpleImage("paris.jpg"); back.setSameSize(image); for (pixel: image) { avg = (pixel.getRed() + pixel.getGreen() + pixel.getBlue())/3; // your code here } print(back); image-bluescreen-4

Solution code: