TweetFollow Us on Twitter

Graphics Objects
Volume Number:2
Issue Number:11
Column Tag:Lisp Listener

Simple Graphics Objects

By Andrew Shalit, Cambridge, MA

Graphics Objects in MacScheme

Welcome back to Lisp Listener! Our current choice of Lisp implementations is MacScheme by Semantic Microsystems. Scheme is a modern and elegant dialect of Lisp, and MacScheme is a robust, complete, and elegant implementation. Previously MacScheme did not offer pro-grammers much access to the Macintosh toolbox. In August a new release of a product called MacScheme + Toolsmith, was made which allows the production of event-driven programs. This will be followed shortly by an application builder that will make it possible to create stand-alone applications with MacScheme. The MacScheme editor, which is currently fairly limited, is also in the process of being upgraded. Look for reviews of all these items in future columns.

The only way to get to the toolbox in the previous incar-nation of MacScheme, version 1.11, was through an escape into machine language that looks very complicated (I haven't tried it out). Version 1.11 does, though, have some simple graphics capabilities which we will be using in this month's column. As it stands now, MacScheme is a great way to learn Lisp and explore object oriented programming.

Graphics in MacScheme

In early 1986, MacScheme was enhanced to allow the use of simple graphics operations. MacScheme has one graphics window, which must be opened with a procedure call before graphics can be used. The graphics window can be two sizes, 'full' or 'half'. To open up a small graphics window, the code would be:

 (start-graphics 'half)

Once a graphics window is open, you can get rid of it by saying:

 (end-graphics)

(One thing to be careful of: you aren't allowed to 'start-graphics' if the graphics window is already there, and you aren't allowed to end them if it isn't there. Moreover, there is currently no way for a procedure to test whether the graphics window is open. Hopefully Semantic Microsystems will add this test feature soon, or just allow you to open the window even if it already open, or close it if it is closed.)

Once the graphics window is open, there are just under 30 commands for drawing in it. These are all pretty basic: drawing, erasing, and inverting lines, rectangles, ovals, circles and points. You can also set up a picture that will be refreshed if the graphics window is obscured, draw a string, and clear the window.

For Macintosh programmers, MacScheme graphics take a little readjusting. This is because MacScheme uses the coordinate system that you learned in grade school, instead of the one you learned in Inside Macintosh. That's right, the x coordinate comes first, followed by the y coordinate. The half size graphics window is 470x130 pixels, so the procedure call

 (paint-oval 20 95 50 125)

would paint a circle in the lower left hand corner of the graphics window.

The general form of graphics procedures that work with two points is

(procedure  x1 y1 x2 y2)

Data Abstraction

This brings us to the first programming issue of the column: data abstraction. As you can imagine, it would be awkward working with rectangles and points if you always had to think about them in terms of their individual coordinates. One of the strong points of Lisp is its ability to create complex data objects. So, before I did anything with graphics, I created a set of procedures for working with points and rectangles. The simplest way to work with a rectangle is to set it up as a list of four coordinates. The coordinates can then be passed to a MacScheme graphics procedure by saying

 (apply the-procedure the-list )

In the sample procedures shown here, I use a slightly more complex data structure because it makes the issue of data abstraction stand out more clearly.

As I have defined them, a point is a simple pair, and a rectangle is a list of points. But the particular internal structure of a point or a rectangle is unimportant to most of the procedures I will write. When I want to work with a point or rectangle, I always do so with the selectors and constructors that I have created. I use a constructor to create a rectangle or point, and I use a selector to get information about a rectangle or point. The selectors and constructors are the only parts of the system that need to know what the internals of the data structure look like. Once you have a complete set of selectors and constructors, you can forget about the underlying structures which the selectors and constructors use to work with the data. This technique is called data-abstraction, and is very useful for keeping programs as simple as possible. For example, the procedure adds-points knows nothing about points besides the fact that they have an x and a y coordinate. If I change the way I store points, I need only modify the selectors and constructors; the rest of the program remains the same.

Now that we have a way of storing points and rectangles, we need a way of passing rectangles to Scheme graphics procedures. Because a rectangle is defined by two points (as all Macintosh programmers know), we can use the rectangle data form for any procedure that requires two points (i.e. four coordinates) as arguments. The result is the procedure 2-point-function. This procedure takes two arguments, a graphics procedure and a rectangle, and it calls the graphics procedure, giving it the coordinates from the rectangle as arguments. 2-point-function also illustrates the ease with which procedures can be passed as arguments in Lisp.

An additional feature of Lisp should be clear by now: Lisp programs are not constructed as single units, as are programs in other languages. Rather, procedures are defined, thereby adding to the procedures which come already defined in the language. A Lisp program is little more than the interaction of a number of procedures. The result is an extensible working environment, similar to that found in Forth.

Object oriented Programming

The next feature of Lisp we will discuss is the ease with which procedures can return other procedures. Every procedure in Lisp, when evaluated, returns something. For example, (+ 4 3) returns 7, and (car '(a b c)) returns a. In Lisp it is very easy to have a procedure return another procedure as its result. Here is a simple (though fairly useless) example, a procedure which churns out procedures to add a constant to a number.

(define (make-adder the-constant)
 (lambda (the-input-variable)
 (+the-constant
 the-input-variable)))
 

The procedure make-adder returns a procedure (a lambda expression) which takes a single argument, the-input-variable. If we say,

(set! addfive (make-adder 5))

we have a new procedure, called addfive, which will add 5 to any number it is given as an argument.

One of the most powerful features of Scheme is that it is lexically scoped. This means that variables within a procedure are scoped according to the environment in which the procedure is defined (as opposed to dynamic scoping, in which variables are scoped according to the environment from which the procedure is called ). In the example given above, the procedure addfive works because the variable 'the-constant' is scoped according to the environment in which addfive was defined. When addfive was defined, the-constant was equal to 5. As far as addfive is concerned, the-constant will always be 5, even if we call make-adder again and again, giving it a different number each time, and even if we call addfive from another procedure that has a variable called 'the-constant' with a different value.

When you put together lexical scoping and procedures returning procedures, you get the ability to do object oriented programming. In case you don't know it as more than a buzz-word, here's a brief description of object oriented programming.

In older forms of programming, data and procedures are stored separately. You have a bunch of data, and then you have the procedures that operate on the data. (What would Von Neuman have thought of this!?) In our rectangle example above, we would define a bunch of rectangles, and then we would have procedures that would do something to one or another of the rectangles. In object oriented programming the procedures and data are bundled together. Instead of having a procedure make a rectangle get bigger, you just send a message to the rectangle, telling it to make itself bigger. Or you tell the rectangle to move, or draw itself, or whatever. Because procedures in MacScheme are lexically scoped, they can have internal state. The internal state is the data within the object, and the rest of the procedure knows how to operate on this data. Object oriented programming has advantages that are similar to the advantages of data abstraction. Once you define an object, you can forget about how its insides work. You just treat it as a black box and work with it as a single unit. When you want it to do something, you tell it what to do; when you want to know something about it, you ask it. The creation of objects helps keep programs modular and simple. You work on small, easily understood units which you can then assemble into larger units, and so on.

The first objects we will be working with are ovals. You give the procedure make-oval a rectangle or any combination of points and coordinates, and it returns a procedure which is an object that can draw, erase, invert itself, tell you its bounding rectangle, or receive a new bounding rectangle. Because this object is a procedure, you call it just like you call any other procedure. The argument that you give it is called the 'message' which you send to the object. It is up to the object to decode the message and act accordingly, or signal an error if it doesn't know what to do.

The next stage of object oriented programming involves something called 'inheritance'. Inheritance occurs when one object takes on the characteristics and abilities of other objects, usually adding new abilities of its own. This month we will keep thing simple and just discuss single inheritance, that is, we will define an object that inherits from one other object.

When you call the procedure make-grow-oval, you give it a bounding rectangle as an argument. Make-grow-oval then sends this bounding rectangle to make-oval, and gets back an object, an oval. It then returns a new object, a grow-oval, which contains this recently (and completely locally) defined oval. When you send a message to a grow-oval it first checks to see if it recognizes the message, in which case it does the appropriate processing. If it doesn't recognize the message, it passes it directly to its oval (i.e. it lets it 'fall through' to the internal object). In this way a grow-oval can add new functionality to an oval without losing any of an oval's standard features. One other interesting thing to note: the grow-oval lets the oval take care of bookkeeping the current bounding rectangle. Whenever a grow-oval needs to know the bounding rectangle, it just asks its oval for the information.

Doing It Together

Anyone interested in learning Lisp should read Structure and Interpretation of Computer Programs by Hal Abelson and Gerald Sussman. This is not only a great book on computer programming, but it is all done in Scheme. The reference manual for MacScheme is also very well written, if you just want see what a particular command does. One other book on Scheme that Semantic Microsystems recommends is The Little Lispers, but I haven't seen it myself, and so I can't speak for it.

Graphics Objects in MacScheme 1.11 Program File

Andrew Shalit

3 Sacramento St.

Cambridge, MA 02138

(617) 498-6637

June 7, 1986


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;newgraphicsobjects
;;;a program that demonstrates graphics and
;;;object oriented programming in MacScheme 1.11
;;;copyright 1986, MacTutor Magazine
;;;written by Andrew Shalit
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;;constructors for building points and rectangles
;;A point is a simple pair of coordinates : (x . y)
(define (make-point x . y)
 (if  (point? x)
 x
 (cons x (car y))))
;;a rectangle is a list of two points: ((x1 . y1) (x2 . y2))
(define (make-rect first-coord . other-coords)
 (if  (rectangle? first-coord)
 first-coord
 (let ((first-other (car other-coords)))
 (if  (point? first-coord)
 (list  first-coord
 (if  (point? first-other)
 first-other
 (apply make-point other-coords)))
 (apply make-rect
 (cons  (make-point
 first-coord first-other)
 (cdr other-coords)))))))

;selectors for getting coordinates out of points and rectangles
(define (x-coord point)
 (car point))
(define (y-coord point)
 (cdr point))
(define (left-top rectangle)
 (car rectangle))
(define (right-bottom rectangle)
 (cadr rectangle))
(define (left rectangle)
 (x-coord (left-top rectangle)))
(define (top rectangle)
 (y-coord (left-top rectangle)))
(define (right rectangle)
 (x-coord (right-bottom rectangle)))
(define (bottom rectangle)
 (y-coord (right-bottom rectangle)))

;;tests to determine whether something is a point or rectangle
(define (point? object)
 (if  (pair? object)
 (and (number? (car object))
 (number? (cdr object)))
 ()))
(define (rectangle? object)
 (if  (pair? object)
 (and (point? (car object))
 (point? (cadr object)))
 ()))

;functions for adding and subtracting points
(define (add-points point1 point2)
 (cons  (+ (x-coord point1) (x-coord point2))
 (+ (y-coord point1) (y-coord point2))))
(define (subtract-points point1 point2)
 (cons  (- (x-coord point1) (x-coord point2))
 (- (y-coord point1) (y-coord point2))))

;function for passing a rectangle to a graphics function
(define (2-point-function the-function the-rectangle)
 (the-function (left the-rectangle)
 (top the-rectangle)
 (right the-rectangle)
 (bottom the-rectangle)))

;;this is your basic oval that can draw, erase, invert itself,
;;tell its dimensions, and receive new dimensions
(define (make-oval . oval-definition)
 (let ((oval-definition (apply make-rect oval-definition)))
 (lambda (message)
 (if  (rectangle? message)
 (set! oval-definition message)
 (case message
 (DRAW (2-point-function paint-oval oval-definition))
 (ERASE (2-point-function erase-oval oval-definition))
 (INVERT (2-point-function invert-oval oval-definition))
 (DESCRIPTION oval-definition)
 (else (error "make-oval can't handle that definition"
 message)))))))

;;a grow-oval inherits all of the features of an oval, but can
;;also move and change size in more interesting ways
(define (make-grow-oval . oval-def)
 (let ((this-oval (apply make-oval oval-def)))
 (lambda (the-change . the-amount)
 (let ((old-description (this-oval 'description))
   (real-amount
 (if  the-amount
 (apply make-point the-amount))))
 (this-oval
 (case the-change
 (MOVE
 (make-rect
 (add-points
 real-amount
 (left-top old-description))
 (add-points
 real-amount
 (right-bottom old-description))))
 (MOVE-TO
 (make-rect
 real-amount
 (add-points
 real-amount
 (subtract-points
 (right-bottom 
 old-description)
 (left-top 
 old-description)))))
 (EXPAND
 (make-rect
 (subtract-points
 (left-top old-description) 
 real-amount)
 (add-points
 real-amount
 (right-bottom old-description))))
 (else the-change)))))))


;;;this procedure shows off some ovals
(define (oval-sampler)
 (let ( (oval-1 (make-grow-oval 5 5 50 50))
 (oval-2 (make-grow-oval 100 20 130 40))
 (oval-3 (make-grow-oval 30 90 60 120)))
 (clear-graphics)
 (oval-1 'draw)
 (oval-2 'draw)
 (oval-3 'draw)
 (oval-1 'move 5 5)
 (oval-1 'erase)
 (oval-2 'expand 4 4)
 (oval-2 'invert)
 (oval-3 'move-to 40 60 70 90)
 (oval-3 'draw)))
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Logitech Control Center 3.9.2 - Keyboard...
Logitech Control Center (LCC) is designed to support OS X and allows you to take full advantage of your Logitech keyboard, mouse, or trackball. With the LCC you can: Browse the Internet using... Read more
Adobe Acrobat Pro 15.007.20033 - Powerfu...
Acrobat Pro DC is available only as a part of Adobe Creative Cloud, and can only be installed and/or updated through Adobe's Creative Cloud app. Adobe Acrobat Pro DC with Adobe Document Cloud... Read more
CleanMyMac 3.0.1 - Delete files that was...
CleanMyMac makes space for the things you love. Sporting a range of ingenious new features, CleanMyMac lets you safely and intelligently scan and clean your entire system, delete large, unused files... Read more
Evernote 6.0.10 - Create searchable note...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
CleanApp 5.0.1 - Application deinstaller...
CleanApp is an application deinstaller and archiver.... Your hard drive gets fuller day by day, but do you know why? CleanApp 5 provides you with insights how to reclaim disk space. There are... Read more
Quicken 2015 2.5.0 - Complete personal f...
Quicken 2015 helps you manage all your personal finances in one place, so you can see where you're spending and where you can save. Quicken automatically categorizes your financial transactions,... Read more
Tonality Pro 1.1.4 - Professional-grade...
Tonality Pro gives you the power to create stunning and dramatic black & white images. This is a complete monochrome image editor with more than 150 one-click style presets, totally unique... Read more
Adobe Photoshop CC 2014 15.2.2 - Profess...
Photoshop CC 2015 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Photoshop customer). Photoshop CS6 is still available for purchase (... Read more
BBEdit 11.1 - Powerful text and HTML edi...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
Together 3.4.3 - Store and organize all...
Together helps you organize your Mac, giving you the ability to store, edit and preview your files in a single clean, uncluttered interface. Smart storage. With simple drag-and-drop functionality,... Read more

Play Endless Golf in Flick Golf Free...
Full Fat has released a new update for Flick Golf! Free. The ‘Eliminator’ update brings a whole new way to play with its endless round. [Read more] | Read more »
The Enchanted Cave 2 (Games)
The Enchanted Cave 2 2.1 Device: iOS Universal Category: Games Price: $2.99, Version: 2.1 (iTunes) Description: Delve into a strange cave with a seemingly endless supply of treasure, strategically choosing your battles to gather as... | Read more »
Crystal Siege is on Sale With a New Univ...
Crystal Siege,  FDG Entertainment's RPG  Tower Defense game, has gone universal in its latest update. [Read more] | Read more »
Platforming Action RPG Swordigo is Curre...
Touch Foo's 2D action-platformer, Swordigo, is free for a limited time on the App Store. Now you can run, jump, and slash your way through adventure for absolutely nothing. [Read more] | Read more »
Watch This Homerun Slides into Home on t...
Eyes Wide Games has launched their one-touch baseball game, Watch This Homerun, for the Apple Watch. [Read more] | Read more »
Disney Celebrates May 4th With Galaxy Si...
For Star Wars Day, May 4, join Disney Interactive and Lucasfilm as they celebrate with a whole lot of in=-app sales for their Star Wars apps. [Read more] | Read more »
Oh My Pixel! We Go Hands-on With The Kni...
I recently had a chance to play around with the upcoming Knights of Pen & Paper 2 from Paradox Interactive. I was a huge fan of the first game, so I had a lot of expectations going into it - and I wasn't disappointed. The game has gotten some... | Read more »
Throw Out Your Stylus and Sketch With Pe...
Penpoint Drawing, by Damin Liu,  is a new creative drawing app that uses your finger as your stylus. [Read more] | Read more »
Oceanhouse has Released Just So Thankful...
Oceanhouse Media, makers of digital book apps, are celebrating Mother's Day with a giveaway and a new app. [Read more] | Read more »
Get Dressed, the Virtual Wardrobe App fo...
Dressed, by  Kabuki Vision, is one of the first fashion apps for the Apple Watch. It pairs your watch with your iPhone to let you browse garments from your closet and mix and match them to create the perfect outfit. To add new pieces you just use... | Read more »

Price Scanner via MacPrices.net

12-inch 1.1GHz Silver MacBook in stock today
B&H Photo has the new 12″ 1.1GHz Silver MacBook in stock today for $1299 including free shipping plus NY sales tax only. Act now if you’re interested, since we don’t expect B&H to have this... Read more
iPad Air 2s on sale for up to $50 off, NY tax...
B&H Photo has iPad Air 2s on sale for up to $64 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $469 $30 off - 64GB iPad Air 2 WiFi: $549.99 $50 off - 128GB iPad... Read more
Check Apple prices on any device with the iTr...
MacPrices is proud to offer readers a free iOS app (iPhones, iPads, & iPod touch) and Android app (Google Play and Amazon App Store) called iTracx, which allows you to glance at today’s lowest... Read more
Sale! New 13-inch 256GB MacBook Air for $1099...
B&H Photo has the new 2015 13″ 1.6GHz/256GB MacBook Air on sale for $1099.99 including free shipping plus NY tax only. Their price is $100 off MSRP. Read more
Japan Post Group, IBM and Apple Deliver iPads...
Japan Post Group, IBM and Apple executives meeting in in New York City yesterday announced a first-of-its-kind initiative aimed at improving the quality of life for millions of Japanese senior... Read more
Worldwide Tablet Market Contracts For Second...
Worldwide tablet shipments recorded a second consecutive quarter of year-over-year decline in the first calendar quarter of 2015 (1Q15), according to preliminary data from the International Data... Read more
Apple restocks refurbished Mac minis for up t...
The Apple Store has restocked Apple Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz... Read more
13-inch 2.5GHz MacBook Pro available for $999...
Adorama has the 13-inch 2.5GHz MacBook Pro on sale for $999 including free shipping plus NY & NJ sales tax only. Their price is $100 off MSRP. Read more
New 13-inch Retina MacBook Pros available for...
Save up to $80 on the purchase of a new 2015 13″ Retina MacBook Pro at the following resellers. Shipping is free with each model: 2.7GHz/128GB MSRP $1299 2.7GHz/256GB... Read more
13-inch 128GB MacBook Air on sale for $50 off...
Save $50 on the purchase of a new 2015 13″ 1.6GHz/128GB MacBook Air at the following resellers. Shipping is free: $949.99 $949.99 $949.99 $949.99 $949.99 B&H Photo charges NY sales tax only.... Read more

Jobs Board

Senior Identity Architect - *Apple* Pay - A...
Changing the world is all in a day039s work at Apple . If you love innovation, here039s your chance to make a career of it. You039ll work hard. But the job comes with Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Hardware Systems Integration Engineer - *App...
Changing the world is all in a day039s work at Apple . If you love innovation, here039s your chance to make a career of it. You039ll work hard. But the job comes with Read more
Senior Identity Architect - *Apple* Pay - A...
Changing the world is all in a day039s work at Apple . If you love innovation, here039s your chance to make a career of it. You039ll work hard. But the job comes with Read more
Hardware Systems Integration Engineer - *App...
Changing the world is all in a day039s work at Apple . If you love innovation, here039s your chance to make a career of it. You039ll work hard. But the job comes with Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.