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


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


"Faded" Green Screen


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

Writing Faded Green Screen


Green Screen: Multiple Passes

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



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.