What is going to be on the quiz tomorrow?

• Integer-encoded RGB colors and the basic operations on those colors.
• Standard color transformations.
• Image and pixel operations, including image-variant and image-transform!.
• compose and o.
• (Possibly): lambda expressions, as used with the image transformations.
• (Not): map and the other subjects we did yesterday.

What do you see as the standard color transformations and what do we need to do with them?

• irgb-redder, irgb-greener, irgb-bluer
  • Makes the color redder/greener/bluer by increasing the corresponding component.
  • We could also achieve a similar effect by decreasing the other components. We won't worry about that.
• irgb-complement
  • Inverts all of the components, as in (- 255 component)
• irgb-lighter and irgb-darker
  • Lighter: Increases color values
  • Darker: Decreases color values
• Relevant questions might include
  • What is the effect of [this set of color transformations]?
  • Given an image, transform it in [this way]

Could you give us some sample quiz questions?

• This picture of Casey (image name is casey) has not enough red and is a bit too dark. Write code to make an improved version of that picture.

  (define temp (image-variant casey irgb-redder))
  (image-variant temp irgb-lighter)
  
  
  (image-variant (image-variant casey irgb-redder) irgb-lighter)
  
  
  (image-variant casey (o irgb-lighter irgb-redder))
  

• This picture of Casey (image name is casey) has not enough red and is a bit too dark. Write code to make the picture better.

  (image-transform! casey irgb-redder)
  (image-transform! casey irgb-lighter)
  
  
  (image-transform! (image-transform! casey irgb-redder) irgb-lighter)
  
  
  (image-transform! casey (o irgb-lighter irgb-redder))
  

• This picture of Casey (image name is casey) has too much red and is a bit too dark. Write code to make the picture better.

  (image-transform! casey (o irgb-greener irgb-bluer irgb-lighter))
  (image-transform! casey (o irgb-greener irgb-bluer))
  

• Are there colors for which (irgb-lighter (irgb-darker color)) is not the same as (irgb-darker (irgb-lighter color))?

  • For really dark colors, like black, (irgb-darker color) is just black, making them lighter will then make them really dark grey
  • For really light colors, like white, we see something similar.

• Are there colors for which (irgb-lighter (irgb-redder color)) is different than (irgb-redder (irgb-lighter color))?

  • Suppose we have (250 250 250). Suppose that lighter adds 16 to each component and redder adds 32 to the red component.
    • Redder first: (250 250 250) -> (255 250 250) -> (255 255 255)
    • Lighter first: (250 250 250) -> (255 255 255) -> (255 255 255)
  • Suppose we have (12 11 10)
    • Redder first: (12 11 10) -> (44 11 10) -> (60 27 26)
    • Lighter first: (12 11 10) -> (28 27 26) -> (60 27 26)

Additional Questions

• What qualifies a color as "blue" rather than something else?

  • We convert from RGB to HSV and look at the hue.

• When might we use lambda?

  • When you define functions (as you've been doing for a few weeks)
  • When you are using image-variant or image-transform! and you need to generate a new transformation on the fly.

• Quiz question: Write an instruction to transform the image of casey so that the green component is set to 0 and the red component is (approximate) halved.

   (define irgb-damage 
     (lambda (color)
       (irgb (round (/ (rgb-red color) 2))
             0
             (irgb-blue color))))
   (image-transform! casey irgb-damage)
  
  
   (image-transform! casey 
                     (lambda (color) (irgb (round (/ (irgb-red color) 2))
                                           0
                                           (irgb-blue color))))
  

• Over the longer term, you'll use lambdas almost any time you need a "one time use" function.

• In HW 4, you'll sometimes use lambdas in cases in which you cannot easily use named functions.

Example: Write a function, (redder-by! image amount), which increases the red component of the given image by the specified amount. For example, (redder-by! casey 20), the red component of each pixel should increase by 20 (up to 255). Similarly, `(redder-by! casey -5)1, the red component of each pixel should decrease by 5 (increase by -5)

(define redder-by!
   (lambda (image amount)
      (image-transform! image
                        (lambda (color) (irgb (+ amount (rgb-red color))
                                              (irgb-green color)
                                              (irgb-blue color))))))

vs.

(define redder-by!
  (lambda (image amount)
    (image-transform! image somewhat-redder)))
(define somewhat-redder
  (lambda (color)
    (irgb (+ amount (irgb-red color))
          (irgb-green color)
          (irgb-blue color))))

The problem in the second is that somewhat-redder doesn't have access to amount.