Using an Image Library

Similarly, in the image processing example, we used from image import *. That made the functions getPixels() and getRed() accessible. We could also define a new function that returns a new color, or a new procedure that changes the image.

# USE THE IMAGE LIBRARY from image import * # CREATE AN IMAGE FROM A FILE img = Image("arch.jpg") # LOOP THROUGH THE PIXELS pixels = img.getPixels() for p in pixels: # MODIFY THE PIXEL COLOR r = p.getRed() p.setRed(r * 0.5) # UPDATE THE IMAGE img.updatePixel(p) # SHOW THE RESULT win = ImageWin(img.getWidth(),img.getHeight()) img.draw(win)

The for p in pixels on line 9 let’s us loop through all of the pixels in the image and change the red value for each pixel. We’ll talk more about looping (repeating steps) in the next chapter.

csp-6-10-2: What does the line p.setRed(r * 0.5) do?

• It sets the red value in the current pixel to half the red of the original.
• Multiplying by 0.5 is the same as dividing by 2.
• It sets the red value in the current pixel to twice the red of the original.
• This would be true if it was r * 2, instead of r * 0.5
• It sets the red value in the current pixel to 5 times the red of the original.
• This would be true if it was r * 5, instead of r * 0.5
• It sets the red value in the current pixel to 0.5.
• This would be true if it was 0.5 instead of r * 0.5

This ability to name functions and procedures, and sets of functions and procedures, and absolutely anything and any set of things in a computer is very powerful. It allows us to create abstractions that make the computer easier to program and use. More on that in a future chapter.

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