6.6. 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.

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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