To prepare the ViewPort structure for further use, you call InitVPort()
and initialize certain fields as follows:

    InitVPort(&viewPort);        /* Initialize the ViewPort.             */
    viewPort.RasInfo = &rasInfo; /* The rasInfo must also be initialized */
    viewPort.DWidth  = WIDTH;
    viewPort.DHeight = HEIGHT;

    /* Under 1.3, you should set viewPort.Modes here to select a display
     * mode. Under Release 2, use VideoControl() with VTAG_NORMAL_DISP_SET
     * to select a display mode by attaching a DisplayInfo structure to
     *  the ViewPort.                                                    */

The InitVPort() routine presets certain default values in the ViewPort
structure.  The defaults include:

  * Modes variable set to zero--this means you select a low-resolution
    display.  (To alter this, use VideoControl() with the
    VTAG_NORMAL_DISP_SET tag as explained below.)

  * Next variable set to NULL--no other ViewPort is linked to this one.
    If you want a display with multiple ViewPorts, you must fill in the
    link yourself.

If you want to create a View with two or more ViewPorts you must declare
and initialize the ViewPorts as above.  Then link them together using the
ViewPort.Next field with a NULL link for the ViewPort at the end of the
chain:

    viewPortA.Next = &viewPortB; /* Tell 1st one the address of the 2nd. */
    viewPortB.Next = NULL;       /* There are no others after this one.  */


For Release 2 applications, once a ViewPort has been prepared, a
ViewPortExtra structure must also be created with GfxNew(), initialized,
and associated with the ViewPort via the VideoControl() function.  In
addition, a DisplayInfo for this mode must be attached to the ViewPort.
The fragment below shows how to do this.  For complete examples, refer to
the program listings of RGBBoxes.c and WBClone.c.


struct TagItem vcTags[] =              /* These tags will be passed to   */
{                                      /* the VideoControl() function to */
    { VTAG_ATTACH_CM_SET, NULL },      /* set up the extended ViewPort   */
    { VTAG_VIEWPORTEXTRA_SET, NULL },  /* structures required in Release */
    { VTAG_NORMAL_DISP_SET, NULL },    /* 2. The NULL ti_Data field of   */
    { VTAG_END_CM, NULL }              /* these tags must be filled in   */
};                                     /* before making the call to      */
                                       /* VideoControl().                */

struct DimensionInfo dimquery;  /* Release 2 structure for display size  */
                                /* data */

/* Make a ViewPortExtra and get ready to attach it */
if( vpextra = GfxNew(VIEWPORT_EXTRA_TYPE) )
    {
    vcTags[1].ti_Data = (ULONG) vpextra;

    /* Initialize the DisplayClip field of the ViewPortExtra structure */
    if( GetDisplayInfoData( NULL , (UBYTE *) &dimquery ,
                           sizeof(struct dimquery) , DTAG_DIMS, modeID) )
        {
        vpextra->DisplayClip = dimquery.Nominal;

        /* Make a DisplayInfo and get ready to attach it */
        if( !(vcTags[2].ti_Data = (ULONG) FindDisplayInfo(modeID)) )
            fail("Could not get DisplayInfo\n");
        }
    else fail("Could not get DimensionInfo\n");
    }
else fail("Could not get ViewPortExtra\n");

/* This is for backwards compatibility with, for example,   */
/* a 1.3 screen saver utility that looks at the Modes field */
viewPort.Modes = (UWORD) (modeID & 0x0000ffff);