Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!husc6!brauer!fry
From: fry@brauer.harvard.edu (D. Spotts Fry)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Animation Questions
Keywords: CopyBits, VBlank
Message-ID: <2851@husc6.harvard.edu>
Date: 14 Oct 89 04:05:14 GMT
References: <3123@crdgw1.crd.ge.com> <10611@claris.com> <34698@srcsip.UUCP>
Sender: news@husc6.harvard.edu
Organization: Harvard Math Department
Lines: 161


To do synched drawing on a Mac II, you want to use
a technique analagous to that of watching TickCount()
on the MacPlus, but you have to make your own VBL
counter.  This should be done with VBLTask tied
to the video slot so you really get video synching.

To illustrate this, here is some code (in Think C):

                Dave
 
/*  my globals  */
extern long     VBLcounter;
extern short    videoSlot;
extern VBLTask  drawtask;

/****************************************************************

    Before we do anything, we have to know which slot the
    card is in.  Use this routine.  (See Start Manager in
    IM V.)

*/

FindVideoSlot()
{
    DefVideoRec defvidrec;

    GetVideoDefault(&defvidrec);
    videoSlot = defvidrec.sdSlot;
}

/*******************************************************************

    Then we have to install a VBL Task.  This is the one
    I'll use.  To use global variables you must setup the
    A5 register.  To do this in MultiFinder requires
    a special trick, which is demonstrated with the
    CacheA5 function.

*/

pascal void ScreenDrawTask()
{
/*
*   WARNING! - this is self-modifying code, but it is
*   necessary to run under MultiFinder...see Tech Note 180
*/
    long    oldA5;

    asm {
        move.l  a5,oldA5
        move.l  #0,a5   ; this will be replaced with
                        ; CurrentA5 by the CacheA5 function at runtime
    }
    VBLcounter += 1;
    drawtask.vblCount = 1;
    asm {
        move.l  oldA5,a5
    }
}

pascal void CacheA5(addr)
long    addr;
/*
*   This function will place CurrentA5 into the beginning of task
*   functions which start as:
*   pascal void FooTask()
*   {
*       long    junk;
*
*       asm {
*           move.l  a5,junk
*           move.l  #0,a5   ;this will be replaced by CurrentA5
*           ....
*
*   See Tech Note #180
*/
{
        asm {
            move.l  addr,a0
            add.w   #2,a0   ; move past JMP instruction
            move.l  (a0),a0
            add.w   #0xa,a0 ; move past first part of function
            move.l  CurrentA5,(a0)
                            ; puts CurrentA5 in (*addr)() dc.l location
    }
}


/****************************************************************

    This function actually sets up the VBL Task.

*/

StartCounter()
{
    CacheA5(ScreenDrawTask);
    VBLcounter = 0;
    drawtask.vblAddr = ScreenDrawTask;
    drawtask.qType = vType;
    drawtask.vblCount = 1;
    drawtask.vblPhase = 0;
    SlotVInstall(&drawtask,videoSlot);
}


/*********************************************************************

    This function removes the VBL Task.

*/

RemoveCounter()
{
    SlotVRemove(&drawtask,videoSlot);
}


/*********************************************************************

    This function holds everything until it's
    time to draw. Note that it could be replaced by
    a #define macro (make VBLjunk a global).

*/

WAIT_FOR_SCREEN()
{
    long VBLjunk;

    VBLjunk = VBLcounter;
    do {} while ( VBLcounter == VBLjunk );
    /* this is used to synch drawing to screen */
}

/********************************************************************

    Now, putting this altogether into a program

*/


main()
{

    /* do your init stuff */

    FindVideoSlot();
    StartCounter();

    /* now, whenever you want to draw something, do this... */
    WAIT_FOR_SCREEN();
    DrawMyThing();

    /* when you're done...*/

    RemoveCounter();

}