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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.

*             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  *****/

/*****  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 SECONDS  60

 the main program
int main()
WindowPtr editWindowPtr;

 InitGraf (&qd.thePort);  
 * 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 );
 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;

‘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.

*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 */
 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.


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