TweetFollow Us on Twitter

VBL Task Animation
Volume Number:5
Issue Number:2
Column Tag:C Workshop

Related Info: Vert. Retrace Mgr

VBL Task Animation

By Dick Chandler, Ben Lomond, CA

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.

Dick Chandler is a Software Engineer at Apple Computer. His major interests are graphics and networks. This is his first article for MacTutor. In this article, Dick shows us how to do some cute animation of fingers drumming on the desktop!

What is a VBL task? A VBL or Vertical BLanking interrupt occurs when the beam of the display tube returns to the top of the screen. This happens on a regular interval (60 times a second). This is a convenient time to take care of recurrent system tasks such as: increment tickcount, check stack/heap boundries for collision, check and update cursor for movement, etc.

The information for executing each of these tasks is placed in a structure called appropriately enough a ‘VBLTask’ and is installed in the vertical retrace queue. Once this is done the system automatically executes the task at a regular interval.

You can place your own tasks on the queue for the system to execute. VBL tasks are a marvelous way of doing some types of graphic animation for the following reasons:

° You don’t have to worry about the speed of your animation on a faster/slower machine, the VBL will execute at the frequency of ticks you select (barring any unusually long tasks such as a disk inserted event hogging your time).

° You won’t get the flicker you see on the screen when trying to do moderately fast animation. This flicker occurs when the screen redraws itself half way through drawing your graphic.

° Plus once you’ve loaded a task on the queue, you needn’t worry about the task until you want to remove it.

Like almost everything else in programming, vbl tasks are easy once you’ve done one (after that its all copy and paste). This example program installs a vbl task which displays a number of icons sequentially. It could easily be modified to move the icons (or bitmaps) around the screen creating game graphics.

The main program is fairly straight forward. The only unusual thing you might find is the declaration of the ‘SetUpA5’ and ‘RestoreA5’ calls. If your task uses any global variables at all, you must insure that the A5 register contains the address of the boundary between the application globals and the application parameters. This is important since you can’t be sure from where the vbl task will be called.

The program basically opens a simple window, installs our vbl task, waits for a button click then exits after removing the task.

/*********************************************
 vblTask.c
*             an VBL task animation example
*
*       Written by Dick Chandler
*       
*This program is intentionally kept as 
*simple as possible, so that it could be 
*easily entered and/or understood.  
*Therefore there are no calls made to 
*check events nor any menus implemented.  ********************************************/

/*****  the only includes we’ll need  *****/
#include<types.h>
#include<quickdraw.h>
#include<windows.h>
#include<toolutils.h>
#include<events.h>
#include<Retrace.h>
#include<OSUtils.h>
#include<Strings.h>

/*****  define our own ‘A5’ routines  *****/
pascal void SetUpA5a() extern 0x2f0d;
pascal void SetUpA5b() extern 0x2a78;
pascal void SetUpA5c() extern 0x0904;

#define  SetUpA5() SetUpA5a(); SetUpA5b(), SetUpA5c()

/* just for readibility
#define NOT !

pascal void RestoreA5() extern 0x2a5f;

/*****  Globals  *****/
/* location of icons displayed */ 
Rect  iconRec = {30,30,62,62};
/* window rectangle */
Rect  boundsRect = {30,30,120,120};

/* tasks VBL record */ 
VBLTask StatusTask;
/* task routines */ 
void vblTask(), installTask(), removeTask();             
/* time between icons */
long    interval;

/*****  constants  *****/
#define FIRST_ICON 1000
#define LAST_ICON1005
#define MAX_NUMBER_ICONS  (LAST_ICON - FIRST_ICON) +1
#define SECONDS  60

/*********************************************
 the main program
********************************************/
int main()
{
WindowRecordeditWindowRecord;
WindowPtr editWindowPtr;

 InitGraf (&qd.thePort);  
 InitWindows();  
 /*
 * open a small, simple (and humble) 
 * window  
 */
 editWindowPtr = NewWindow(&editWindowRecord,&boundsRect, “”, true,
 1, (-1), false,  nil);
 SetPort( editWindowPtr );
 /*
 * set the interval time between icons  
 * and install our task on vbl queue   
 */
 interval = SECONDS/8;
 installTask( &StatusTask );
 /*
 * run until the button is clicked 
 */
 while  ( NOT Button() )  ;
 /*
 * clean up and exit
 */
 removeTask ( &StatusTask );
 CloseWindow(editWindowPtr);
 return 0;
}

The function ‘installTask’ sets up the required parameters for the vbl task. These parameters are passed via a ‘VBLTask’ record when the task is installed with the toolbox call ‘VInstall’.

° ‘vblAddr’ is the address of the function we’ve written and want the system to execute.

° ‘vblCount’ is the count, in ticks, to wait before calling the vblTask for the first time. We will reset this value everytime the task is executed if we want it to be reexecuted.

° ‘vblPhase’ allows you an additional offset before calling vblTask for the first time only. This would be useful if you wanted to install several graphic image tasks but did not want them to all look in sync.

° ‘qType’ designates the task a vbl task.

The function ‘removeTask’ simply removes the task from the queue with the toolbox call ‘VRemove’.

/************** NAME:installTask
*INPUT: a pointer to a VBL task record
*
*FUNCTION:installs our vblTask after 
*setting the vbl parameters 
********************************************/
void installTask( StatusTask )
VBLTask *StatusTaskPtr; 
{
 StatusTaskPtr->vblAddr   = vblTask;
 StatusTaskPtr->vblCount  = 10;
 StatusTaskPtr->vblPhase  = 0;
 StatusTaskPtr->qType     = vType;
 VInstall (StatusTaskPtr);
}

/************* NAME: removeTask
*INPUT: a pointer to a VBL task record
*
*FUNCTION:removes our vbl task
*******************************************/
void removeTask ( StatusTask )
VBLTask *StatusTaskPtr;
{
 VRemove(StatusTask);
}

‘vblTask’ is our routine which we loaded into the vbl record and installed. The routine sets up and later restores the A5 register so we can use our own global varibles. It is important to note that you cannot make any calls to the memory manager, and you cannot rely on locked handles being valid. These caveats are outlined in the ‘Vertical Retrace Manager’, Inside Macintosh V 1-3.

‘vblTask’ decides which of our icons to display next and then displays it if it finds the icon. Notice that we must reset the ‘vblCount’ each time or the task will not be called again.

/*******************
 NAME:vblTask    
*
*FUNCTIONS: our actual vbl task which gets 
*called every ‘interval’ ticks
********************************************/
void vblTask()
{
static long icons; 
Handle  tempIcon;
static long timeIcon = FIRST_ICON; 

 SetUpA5(); /* set up A5 register */
 icons = MAX_NUMBER_ICONS;
 StatusTask.vblCount = interval; timeIcon++; /*bumpicon */
 if( timeIcon > LAST_ICON ) 
 timeIcon = FIRST_ICON;
 tempIcon = GetIcon( timeIcon )
 if( tempIcon )
 PlotIcon( &iconRec, tempIcon );
 RestoreA5();  /* preserve A5 register */
}

The only resources you will need are icons to display. I attached six ‘drumming fingers’ icons [included in the source code disk.-Ed] I used in the program thanks to graphic artists Irene Welch and Shelley van Bronkhorst. However, you can attach any number of icons you wish as long as you change the defined constants ‘FIRST_ICON’ and ‘LAST_ICON’.

The program can be compiled and linked with the following commands. You can either paste in the resources with ResEdit, or derez and rez the resources into the application.

derez vblTaskResources Types.r SysTypes.r > vblTask.r
Rez Types.r SysTypes.r vblTask.r -o vblTask -a

SetFile -a B vblTask -c MOBY -t APPL

C -g vblTask.c
Link  vblTask.c.o 
 “HD:MPW:CLibraries:”CRuntime.o 
 “HD:MPW:CLibraries:”CInterface.o 
 -o vblTask

VBL Animation Problems

David Oster

Berkeley, CA

I am appalled by Dick Chandler’s article, “VBL Task Animation” in the February 1989 issue of MacTutor. Yes, his program works, but only because it is a top. Any real program that tries the technique he describes will fail miserably.

Look, his VBL task calls GetIcon and PlotIcon at VBL interrupt time. In his application, the main loop just busy waits for the user to press the Button. A real application would be calling GetNextEvent(), or doing something. For example, each time the user looks at a menu. When the menu goes away, it slams those bits back and deallocates the handle.

GetIcon calls GetResource(). What if the VBL task calls it while the Memory Manager is shuffling the heap to allocate memory for the main loop. Crash city. PlotIcon calls CopyBits(), which clips against the clipRgn and visRgn of the underlying grafPort. What if the VBL task calls it while the Memory Manager is shuffling the heap to allocate memory for the main loop? Crash city.

Even if your program is clean, you do not know what trap patches the user has installed: Maybe he is using an INIT that overrides some trap your program needs, and the override will do memory allocation. For example, Dick’s program calls Button() from its main loop, and many INITs override button, so they will get called while the mouse is down.

So, you can only do animation at interrupt time if you can guarantee that no user or system task will allocate memory in the main loop.

Dick’s program doesn’t guarantee this, since it calls Button() from its main loop, and Button() may have been overriden by an INIT. Since there is so little the main loop can safely do, you might as well give up on VBL Task animation, and just do animation in your main loop, busy waiting until TickCount changes to pause between animation frames.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Galaxy of Trian (Games)
Galaxy of Trian 1.1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.1.0 (iTunes) Description: Galaxy of Trian is an exciting, fast paced digital board game based on the highly acclaimed tabletop title. | Read more »
Dead In Bermuda (Games)
Dead In Bermuda 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
The Little Fox (Games)
The Little Fox 1.0.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.1 (iTunes) Description: The Little Fox is an alternative perspective on the world-renowned ‘fairy tale for adults', The Little Prince by Antoine de... | Read more »
5 popular free fertility apps
There was a good article this week in The Independent about how more women are using fertility appsas a de facto form of contraception. It's apparently not working too well, leading to numerous unwanted pregnancies. [Read more] | Read more »
How to get more cars in CSR Racing 2
NaturalMotion and Zynga brought a lot of real life cars to the table for CSR Racing 2. From souped up everyday rides made by Nissan and Hyundai to supercars produced by the likes of McLaren and Pagani, there really is something for everyone. [... | Read more »
Crypt of the NecroDancer Pocket Edition...
Crypt of the NecroDancer Pocket Edition 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Crypt of the NecroDancer is an award winning hardcore roguelike rhythm game. Move to the music and... | Read more »
Gear-grinding puzzle title Inner Circle...
If you saw our post earlier this month announcing the imminent release of ZPlay’s new creation, Inner Circle, you’ll be happy to know that it’s now available on the App Store. Established in 2010, developer and publisher ZPlay have taken the... | Read more »
CSR Racing 2: Your guide to what's...
CSR Racing 2, or CSR2, as it likes to call itself, has finally arrived. The follow-up to the immensely popular drag racing game CSR Racing is the first release from NaturalMotion since the studio's acquisition by Zynga in early 2014. [Read more] | Read more »
Nanuleu (Games)
Nanuleu 1.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.1 (iTunes) Description: Nanuleu is a strategy game where you take control of ancient magical trees that protect the land from an invading dark force. A... | Read more »
The Slaughter: Act One (Games)
The Slaughter: Act One 1.0.323 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.323 (iTunes) Description: “The game mixes realism and surrealism to create a story that can cause just as much laughter as fear. A-” -... | Read more »

Price Scanner via MacPrices.net

New App Reminds Us to Put Down Our Phones and...
Mode, a new smartphone app that makes us more mindful of how we use our devices, debuts in the app stores today. The Mode app tracks time spent in different modes of day-to-day life without... Read more
ZuumSpeed Personalized Speedometer + HUD For...
RMKapps has announced the release and immediate availability of ZuumSpeed 1.0, its personalized speedometer plus heads up display for iOS devices. ZuumSpeed gives users over 18 custom fonts available... Read more
Apple refurbished clearance 15-inch Retina Ma...
Apple has Certified Refurbished 2014 15″ 2.2GHz Retina MacBook Pros available for $1609, $390 off original MSRP. Apple’s one-year warranty is included, and shipping is free. They have refurbished 15... Read more
9-inch 128GB Silver iPad Pro on sale for $50...
B&H Photo has the 9.7″ 128GB Silver Apple iPad Pro on sale for $699 including free shipping plus NY tax only. Their price is $50 off MSRP. Read more
Why Use Indie Opera And Vivaldi Instead Of Sa...
For many years my web browser workhorses were various permutations and spinoffs of the Netscape/Mozilla/Firefox Open Source platform, and the Norwegian indie browser Opera, which I took a shine to... Read more
Western Digital Launches Worlds Fastest 256GB...
At the Mobile World Congress in Shanghai Western Digital Corporation this week introduced a new suite of 256 gigabyte (GB) microSD cards, which includes the new 256GB SanDisk Extreme microSDXC UHS-I... Read more
KeyCue 8.1 Integrates With Typinator To Displ...
Ergonis Software has released KeyCue 8.1, a new version of the company’s keyboard shortcut cheat sheet. KeyCue 8 introduced a new way to define a wide variety of triggers, which can be used to... Read more
Save up to $600 with Apple refurbished Mac Pr...
Apple has Certified Refurbished Mac Pros available for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The following... Read more
21-inch 2.8GHz iMac on sale for $1199, save $...
Amazon has the 21″ 2.8GHz iMac (model #MK442LL/A) on sale for $1199.99 including free shipping. Their price is $100 off MSRP, and it’s the lowest price available for this model. Read more
13-inch 2.5GHz MacBook Pro (Apple refurbished...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros... Read more

Jobs Board

*Apple* Retail - Multiple Positions, Fort Wo...
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
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* iPhone 6s and New Products Tester Ne...
…we therefore look forward to put out products to quality test for durability. Apple leads the digital music revolution with its iPods and iTunes online store, Read more
*Apple* iPhone 6s and New Products Tester Ne...
…we therefore look forward to put out products to quality test for durability. Apple leads the digital music revolution with its iPods and iTunes online store, Read more
*Apple* iPhone 6s and New Products Tester Ne...
…we therefore look forward to put out products to quality test for durability. Apple leads the digital music revolution with its iPods and iTunes online store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.