CS 101

Fancy Filters

Plan for Today

Last time, we learned about if statements, a way to conditionally apply operations.
Now, we're going to experiment with if statements by creating an edge detection filter and continuing our discussion of green screen.

Edge Detection

Idea: Want to make a black-and-white image where the "edges" in the original image are now black
Edge: A pixel is sufficiently different from at least one of its neighbors
What building blocks can help us?

Edge Detection Pseudocode

for every pixel in the image:
  make the pixel white
  look at its neighbors
  if any of the neighbors are "far away" from the pixel:
    make the pixel black

Writing Edge Detection

image = new SimpleImage("striped-fish-green.jpg");
back = new SimpleImage("yosemite.jpg");
back.setSameSize(image);
back.setZoom(0.5);

for (pixel : back) {
  pixel.setRGB(255, 255, 255);
}

print(back);

Recall: Green Screen

Idea: if a pixel is "close" in color to our target color, replace it with the corresponding pixel in the other image
Idea: we can replace a pixel with another pixel by setting the original's red, green, and blue to be the same as those of the new pixel

Recall: Green Screen Code

image = new SimpleImage("monkey.jpg");
image.setZoom(0.5);
print(image);
back = new SimpleImage("moon.jpg");
back.setSameSize(image);
back.setZoom(0.5);

for (pixel : image) {
  backPixel = back.getPixel(pixel.getX(),
	                        pixel.getY());
  if (pixel.isSimilarTo(100, 150, 150, 70)) {
    pixel.setRGB(backPixel.getRed(), 
                 backPixel.getGreen(), 
                 backPixel.getBlue());
  }
}

print(image);

"Faded" Green Screen

Idea: instead of using the original pixel values, can blend them with background

Writing Faded Green Screen

image = new SimpleImage("monkey.jpg");
image.setZoom(0.5);
print(image);
back = new SimpleImage("moon.jpg");
back.setSameSize(image);
back.setZoom(0.5);

for (pixel : image) {
  backPixel = back.getPixel(pixel.getX(),
	                        pixel.getY());
  if (pixel.isSimilarTo(100, 150, 150, 70)) {
    pixel.setRGB(backPixel.getRed(),
                 backPixel.getGreen(),
                 backPixel.getBlue());
  }
}

print(image);

Green Screen: Multiple Passes

Write code to replace the red stop sign with leaves, and the blue sky with yosemite

image = new SimpleImage("stop.jpg");
image.setZoom(0.5);
back1 = new SimpleImage("yosemite.jpg");
back1.setSameSize(image);
back2 = new SimpleImage("leaves.jpg");
back2.setSameSize(image);

for (pixel : image) {
	// your code here
}

print(image);

Recap

Today, we learned more complicated methods for editing images, seeing the power of if statements and for loops. Next time, we'll move away from images and manipulate other kinds of data.