Image-3 Loops

Previously, we had things like this pixel.setRed(200);
With one line of code, change the red value of one pixel. In this section, we'll look at the "for loop" construct, which can run a bit of code thousands of times -- a huge increase in power.

Loops - Box Analogy


yellow flowers

Accessing one pixel at a time, e.g. pixel at (0, 0), then the pixel at (1, 0), etc. is not a good way to work on an image with thousands or millions of pixels. We'd like to say something like "for each pixel do this", and let the computer fiddle with the details of going through all the (x, y) values to look at each pixel once.

The very powerful "for loop" structure we'll learn here provides exactly this "for each pixel do this" feature. The loop takes a few lines of our code, and runs those lines again and again, once for each pixel in the image.

For-Loop Example 1

Run this. What does it do?



For each pixel, the body code sets the red, green, and blue values all to 255, 255, 0. None of the original flower data is left. All the RGB numbers are changed in the loop.

How Does That Loop Work?

For-Loop Syntax - 2 Parts

parts of the for-loop

Education Research Aside

Example 2 - Body Code Running Thousands of Times? You Try It

small version of flowers.jpg



The lines of code in the body run again and again, once for each pixel. Therefore, a line of code inside the body, inside the curly braces { }, will run thousands or millions of times. In contrast, the lines of code outside the body just run once. Inside the body, "pixel" refers to a different pixel for each run of the body.

For-Loop Example 3

Look again at flowers.jpg.Yellow is made or red + green, so we know that the yellow parts of the image have high red and green values. So what happens if, for each pixel, we set red to 0? What are the RGB values for a typical pixel on the yellow flowers look like before this loop runs? What about after?
yellow flowers



  // your code here

The body code pixel.setRed(0); is run by the loop again and again, once for each pixel in the image. Since the yellow flowers are made with red + green light, setting the red to 0 for each pixel results is greenish flowers. The green leaves aren't changed much, since their red values were near 0 anyway.

For-Loop Example 4 - Red Channel



  // your code here

Setting green and blue to 0 everywhere, all that is left is the area of red light that went into the original image, aka the "red channel" of the image. There are analogous green and blue channels we could look at, and the image is all three combined. The red light is most prominent for the area of yellow flowers, which makes sense as we know that yellow = red + green.

Red, Green, and Blue Channels

  • The code example above computes the "red channel" image
  • The red channel is just the red light of the image, with blue and green at zero
  • There are analogous blue and green channel images
  • The whole image is just the sum of the three channels: adding together all the light
  • The three channels are shown below

red channel flowers image green channel flowers image blue channel flowers image

For-Loop Conclusions

  • A powerful feature, we specify a few lines of code, computer takes care of running them thousands of times
  • Computer = powerful + stupid theme
  • Code inside the loop is special: run it for every pixel
  • That's why code inside the loop should be indented (optional)
  • Note: Javascript does not have this feature, I added it for CS101