CS 101

If Statements

Plan for Today

Last time, we learned about for loops, a way to repeat operations many times.
Now, we're going to learn how to conditionally or sometimes apply those operations using if statements. We'll also implement a new filter.

If Statements

For loops: always apply the operation to every pixel
What if we only want to apply the operation to some pixels?
Parts of an if statement:
- condition
- body

image = new SimpleImage("x.png");
image.setZoom(20);
for (pixel : image) {
    if (pixel.getRed() == 0) {
        pixel.setRGB(255, 150, 0);
    }
}
print(image);

Conditions

Must be either true or false
Generally use >, <, >=, <=, == (equals), or != (not equal)
Only pixels that "pass the test" will have the operation executed
Other code to know:
- pixel.getX(), pixel.getY()
- img.getWidth(), img.getHeight()
General idea: based on some property of the pixel, we'll perform an operation on it.
- Color
- Location
- Number of neighbors

Fancy Diagonals

Write code to make a purple diagonal line running bottom left to top right.

image = new SimpleImage("x.png");
image.setZoom(20);

print(image);

Making a border

How can we make a one-pixel thick green border around an image?

image = new SimpleImage("x.png");
image.setZoom(20);

print(image);

Fancier Diagonals

What if we want to do something if the test passes and something else if the test fails?
- Solution: use else
Write code to make a purple diagonal line running bottom left to top right, and color the rest of the pixels green.

image = new SimpleImage("x.png");
image.setZoom(20);

for (pixel : image) {
  if (???) {

  } else {

  }
}

print(image);

Combining Conditionals

What if you want multiple tests at once?
and (written as &&) means both parts have to be true to pass
or (written as ||) means only one of the two tests have to pass
How can we color the top-right quadrant purple?

image = new SimpleImage("flowers.jpg");
image.setZoom(0.5);

print(image);

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:
- pixel.isDifferentFrom(pixel2, threshold);
What building blocks can help us? What does this filter have in common with the blur filter from Tuesday?

Writing Edge Detection

image = new SimpleImage("poppy.jpg");
result = new SimpleImage("poppy-blur.jpg");
result.setSameSize(image);
result.setZoom(0.5);

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

print(result);

Recap

Today, we learned how to selectively apply code using if statements. Next time, we'll do more advanced green screen techniques, and you'll edit your own images.