CSC151.01 2014F, Class 15: Transforming Images ============================================== _Overview_ * Preliminaries. * Admin. * Upcoming Work. * Extra Credit. * Questions (Exam/Admin). * Review of Key Ideas. * Topic Prep. * Lab. Preliminaries ------------- ### Admin * Continue lab partners. * No Sam tomorrow. Prof. Weinman will run class. * Office hours: Thu/Fri 1:45-3:15; Also walking hours 1:15-1:45. * I've started a short reference on [error messages](../reference/error-messages.html). Please feel free to pass along some you get. * Two important recent comments from students. * "I realized that it would be much easier to do the test and study for quizzes if I took notes along the way." * Evidence (research): Taking notes helps you learn, even if you never read them again. * Sam belives that the total time is less if you take notes. * "In high school, I did homework at night (or in the early morning). I learned that I'm much better off doing homework in my open slots during the day, rather than waiting until night time." * Other notes on quizzes * "Quiz anxiety" * Length ### Upcoming Work * Lab writeup: Exercises 4a, 5b, and 5e Due Friday. * [Exam 1](../assignments/exam.01.html) due tonight. * Bring printed copies on Wednesday. * Reading for Wednesday: * [Making and Manipulating Homogeneous Lists](../readings/homogeneous-lists-reading.html) ### Extra Credit Opportunities #### Academic * Convocation Wednesday: Tara Zahra on Emigration from Eastern Europe * CS Extras Thursday at 4:30: Mathematical Image Synthesis Toolkit (MIST) * CS Table Friday at noon: ??? * Campus Town Hall September 30 #### Peer Support * Noteworthy, Sat., Sept. 27, 3:30 p.m. Herrick * Science poster session, Sat., Sept. 27, morningish, science elbow * Football Games (Saturday, Sept. 27, at 1pm) * Men's Tennis (???) * Women's Tennis (???) * Swimming (???) * Anna Christie, Oct. 9-12 (SB plays Marthy) * Evan's radio show 5pm Friday nights on KDIC * Donna's radio show Sunday midnight on KDIC #### Miscellaneous * Participate in #GrinWell. * Friends of Drake Library needs help setting up for their annual booksale on Thursday, October 2 (conveniently, the same night as Grinnell High School's homecoming parade). You can help a good cause and probably get a few free books, too. * Followup: They can also use help on Wednesday, October 1. ### Questions on the Exam _There are some cases in the Quincunx problem in which the center seems to have a non-integer coordinate. What should I do in such cases?_ > Read the email. > Summary: At least two choices. You can refuse to compute. You can use a nearby pixel. Your documentation MUST specify which you've chosen. _Can you show a sample test for `numerator-sum-fraction-reduced`?_ > Yes. (define nsfr-tests (test-suite "Tests of numerator-sum-fraction-reduced" (test-case "1/2 plus 1/2" (check-= (numerator-sum-fraction-reduced 1 2 1 2) 1 0)) (test-case "1/3 plus 1/2" (check-= (numerator-sum-fraction-reduced 1 3 1 2) 5 0))))a _How many tests is enough for that test suite?_ > Enough that you feel confident that a procedure that passes your tests is almost certainly correct. > About a dozen, provided they explore different potential challenges. E.g., just asking 1/2+1/3, 1/3+1/4, 1/4+1/5, .... doesn't really challenge the procedure in different ways. _Do we have to use the generalized approach that you used for problem 4, or can we just work with particular values._ > I'm fine if you just work with particular values. _How carefully should I write my postconditions for problem 1?_ > Carefully enough that someone who wants to be lazy and just meet the postconditions will still have to achieve the goals of the procedure. > Please try to use "math" or Scheme rather than English to specify the postconditions. > You may have to refer to the results of the other procedures (but I think we noted that). _Can I define a procedure that takes an image and another value as input?_ > Yes. (define nt (lambda (value image) COMPUTATION)) (define uniform-pixel (lambda (n image) (image-set-pixel! image n n "red"))) _Can you show me how to log in to MathLAN from my computer?_ > Yes, but not today. Review of Key Ideas ------------------- * (image-variant image FUN) and (image-transform! image FUN) * One is pure, one has side effects. * We can combine existing functions into new functions by using `compose` and `o` * (define rg (o irgb-redder irgb-greener)) * Works right-to-left * Note: We are defining procedures without lambda * You can use lambda outside of a procedure definition, when you want a procedure but don't want to bother naming it. * (image-variant kitten (lambda (color) (irgb (+ 45 (irgb-red color)) 128 0))) Topic Prep ---------- Preliminary Questions _Can you only compose two procedures if they take the same parameters?_ > It's a bit more complicated than that. You can only compose two procedures if the output of one can serve as the input to the next. For example, since `image-load` takes a string and returns an image, and `image-show` takes an image (and returns an image), we can write `(compose image-show image-load)`, but not the other way around. > Right now, we will primarily compose functions that take the same kind of input and have the same kind of output, such as color transformations. _Can we return multiple value?_ > After tomorrow. Followup Questions Talk to your partner: What, if anything, was confusing about the reading? (Feel free to post to the question board.) Followup Questions _Can you explain the following?_ (image-variant kitten (lambda (color) (irgb (irgb-red color) 0 0))) _How can the computer use the lambda expressions?_ (image-variant kitten (lambda (color) (irgb (irgb-red color) 0 0))) _Why does the image-variant work even though you haven't provided an input?_ (image-variant kitten irgb-redder) _Can I use the composition operator with math functions?_ > Yes (define qad (o square square)) _Can I use the composition operator to write something that returns f^n(x)?_ > Yes, eventually. Lab --- Three approaches I've seen to undoing irgb-redder, at least for most values (define anti-redder (o irgb-darker irgb-darker irgb-greener irgb-bluer)) (define anti-redder (o irgb-complement irgb-redder irgb-complement)) (define anti-redder (lambda (color) (irgb (- (irgb-red color) 16) (irgb-green color) (irgb-blue color))))