Usually, there is no need to write a video format completely from scratch; instead, try copying or modifying an existing format source file. For block sync formats, the Silicon Graphics standard high-resolution format is a good start. See the example below.

/*
** 1280x1024_60.vfs - SGI standard format
*/

General
{
    FieldsPerFrame = 1;
    FramesPerSecond = 60;
    TotalLinesPerFrame = 1065;
    TotalPixelsPerLine = 1680;
    ActiveLinesPerFrame = 1024;
    ActivePixelsPerLine = 1280;
    FormatName = "1280x1024_60";
}

Active Line {
    HorizontalFrontPorch = 0.372 usec;
    HorizontalSync = 1.12 usec;
    HorizontalBackPorch = 2.23 usec;
 }

Field
{
    Vertical Sync = 
    {
        {
            /* 
             * Sync goes low here (at beginning of the
             * line, time = 0.0) but does not recover to the
             * high state until the first line of Vertical 
             * Back Porch.
             */
            Length = 1.0H; 
            Low = 0.0 usec;
        }
        repeat 2 {
            /* Two lines with no transitions */
            Length = 1.0H; 
        }
    }

    Vertical Back Porch =
    {
        {
            /* 
             * Only one transition:  sync goes high at the
             * time 1.12 usec (HorizontalSync) into the 
             * frame.
             */
            Length = 1.0H; 
            High = HorizontalSync;
        }
        repeat 34 {
            /*
             * Normal horizontal sync.  Goes low at time=0.0,
             * at beginning of the line.
             */
            Length = 1.0H; 
            Low = 0.0 usec; 
            High = HorizontalSync;
        }
    }

    Active =
    {
        repeat 1024 {
            /* Normal horizontal sync */
            Length = 1.0H; 
            Low = 0.0 usec; 
            High = HorizontalSync;
        }
    }

    Vertical Front Porch =
    {
        repeat 3 {
            /* Normal horizontal sync */
            Length = 1.0H; 
            Low = 0.0 usec; 
            High = HorizontalSync;
        }
    }
}

The language in Example 5-1 is standard; the format is that of the Silicon Graphics standard high-resolution monitor. The layout of the information in a format must follow a special sequence.

General {
    /* Overall Description */
}

Active Line {
    /* The components of the horizontal blanking */
}

Field {
    /* Description of sync pattern for a field */
}

Field {
    /* Description of sync pattern for a field */
}

Example 5-2 shows the standard layout of the source file.

The compiler executes your video source program in the order as presented in Example 5-2:

Many variables in the compiler deal with absolute or relative positions in the video frame or with quantities of time. The compiler has special time expressions to deal with specifying these values.

You may use your own variables if you wish, to supplement the compiler's variables. User-defined variables sometimes make calculations easier; they also allow you to communicate information from one section to another.

You must always define a variable before you use it. The general form for variable definition is as follows:

[ storage-class ] data-type variable-name [ = value ] ;

The items surrounded by square brackets ([ ]) are optional. For a formal definition, see “compound-statement:” in Appendix A, “Native Language Grammar.” If you define your own variables, the definition must come before any executable statements in the compound statement block.

The assignment statements specify the values of several system-defined variables. If you are looking for detail on assignment statements, see “Assignment Statements”; the variables are listed in Table 5-1.

See Example 5-1 for an example of actual use of the General section.

The pixel rate of the video format is expressed in pixels per second, the aggregate bandwidth of all pixels flowing from an output port of the computer system. The formula is as follows:

TotalLinesPerFrame * TotalPixelsPerLine * FramesPerSecond

The video output hardware of your computer system has a maximum pixel rate at which it can operate. Refer to documentation on your particular system to determine the maximum rate.

Within the compiler, the pixel rate is needed to perform conversions between time and pixels. Thus, the General section is executed before any other section so conversions can be computed.

The components of the field, as described in “The Vertical Interval”, are as follows:

You must define each of these components in each field in this order.

Field
{
    Vertical Sync = 
    {
        sync transition set
        sync transition set
        sync transition set
        ...
    }

    Vertical Back Porch =
    {
        sync transition set
        ...
    }

    Active =
    {
        sync transition set
        ...
    }

    Vertical Front Porch =
    {
        sync transition set
        ...
    }
}

Example 5-5 shows an example of the layout you might use. The grammar is formally described in “field-definition:” in Appendix A, “Native Language Grammar.” For a real example, see Example 5-1.

You may place only sync transition sets within each of the field components. You may use as many sync transition sets as needed to describe the format. For information on this part of the language, see “Sync Transition Set”.

Be especially careful to place the proper active lines in the Active section: only and all lines placed in the active component will have active pixels on them. If the first or last line in the Active section is not a whole line (such as with the NTSC and PAL half lines), only part of the output line will have active pixels.

The sync transition set defines a set of transitions of the video sync signal on a line or portion of a line. The length of the defined line portion is a required statement, the transitions are optional (that is, it is possible to define a line with no transitions of sync).

{
    /* Normal horizontal sync, one repetition */
    Length = 1.0H; 
    Low = 0.0 usec; 
    High = 1.19 usec;
}

repeat 3 {
    /* Normal horizontal sync, three rep. */
    Length = 1.0H; 
    Low = 0.0 usec; 
    High = 1.19 usec;
}

Two simple sync transition sets appear in Example 5-6, each set delimited by curly brackets ({ }). All repetitions are concatenated to the previous set, so you must specify the sync transition sets in the order in which they are to appear in the frame.

repeat 6 {
    Length = 0.5H; 
    Low = 0 usec; 
    High = 27.1 usec;
}

A single simple transition set is in Example 5-7.

length = 0.5 lines;

repeat 100 {
    Length = 1.0 H; 
    sync transitions...
}

level = time-expression;

Each field can have attributes assigned individually to it. These attributes describe features of the frame not related to timing, such as how pixels should be fetched from the frame buffer.

Each attribute is set before any of the components of the field are specified. The components of the field are Vertical Sync, Vertical Back Porch, Vertical Active, Vertical Front Porch (see “Field Components”). The attributes are set via an assignment statement of this form:

attribute = value;

You can assign the attributes directly, or not specify them and allow the compiler to use default values.