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
Suppose you had 5,000 cardboard boxes in a warehouse and a robot
You want the robot to move all the boxes from one corner to another
It's a dumb computer, so you explain how to move one box in great detail
You want to say: repeat those same steps for all the boxes
That's loops - do some steps "for all these items"
Loops = power!
flowers.jpg
flowers.jpg: 457 pixels wide by 360 pixels high - 164,520 pixels
pixel.setRed(0); one pixel at a time ... not practical
Want: we specify some code, computer runs it again and again, once for each pixel
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?
image-loop-1
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?
Body of the for-loop is the lines indented within curly braces { .. }
Body lines are run again and again, once for each pixel
pixel variable refers to a different pixel for each run of the body
For-Loop Syntax - 2 Parts
For-loop syntax a little weird, but it's the same every time
When we want "for every pixel do this", we'll use a for-loop
For-loop has 2 parts
1. for (pixel: image) { - start the for-loop
2. "body" lines of code within curly braces { .. }
Nice to indent the body lines, showing that they are special and go together
Education Research Aside
If you just watch the teacher go through it, you absorb a little
Surprisingly, if you engage to answer a question about the material you absorb a lot more
Even if the question is easy .. just switching out of the passive mode seems to do it
So I try to put these You Try It activities in here
Example 2 - Body Code Running Thousands of Times? You Try It
flowers-small.jpg:
Here's an example using "flowers-small", 50 pixels wide by 39 pixels high
50 * 39 is 1,950 pixels total
Body code sets each pixel to be green
print("before"); -- before the loop
print("after"); -- after the print(image)
Experiment 1:
-What do you think this will print?
-Run it and see
Experiment 2:
-Add this line at the top of the body: print("inside"); -What do you think this will print?
-Run it and see
image-loop-2
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?
Previous examples overwrote all the flower data ... not realistic
Modify the data in flowers.jpg
Q: What are the green leaves made of? What are the yellow flowers made of?
Experiment - You Try It
-For each pixel, set red to 0
-What is the body code to do this?
-Which of RGB are high for the yellow flowers?
-So what does setting red to 0 look like? Run it.
-Could try it in RGB Explorer, make yellow, then set red to 0
Theme: we discuss algorithm ideas, then you code it up for the computer
image-loop-3
// your code here
pixel.setRed(0);
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
Experiment or Lecture-example
For each pixel, set green and blue to 0
What is the body code to do this?
The result is known as the "red channel"
Seeing an image of just the red light areas
image-loop-4
// your code here
pixel.setGreen(0);
pixel.setBlue(0);
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
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
