Introduction

 Working With Video Functions

     This module discusses video functions that are either directly or
     indirectly concerned with the screen.  Included are all functions that
     deal with such screen adapters as CGA, EGA and Hercules.  Different
     modes on various adapters are supported, such as a 40-column CGA, a 43-
     line EGA, or a 50-line VGA.

 DSETWINDOW()

     Many of the functions in this chapter depend on setting the DSETWINDOW()
     switch.  This switch determines if the screen output of external
     programs, as well as DOS, are redirected to a window.
     CA-Clipper Tools output functions are also valid as external.  If the
     default setting in DSETWINDOW() is .T. or on, this rerouting is carried
     out.  Some functions will then return inaccurate, or at the very least,
     different results.  Examples of this are ISANSI() or NUMCOL()

 Attribute

     Many of the functions in this module work with color attributes
     designated as parameters.  These arguments are carried out in three
     different ways:

     A numeric value, which corresponds to a combined color attribute
     (e.g., 7).

     A string in the "NN/NN" form, with two specified numeric values (e.g.,
     "7/0").

     A string in the "CC/CC" form, with two specified attributes are
     designated in the form CA-Clipper requires (e.g., "W/N").

     With many functions, the attribute returns a combined numeric value.
     Attributes for the foreground and background are tied together this way.

     Color attributes are constructed as follows:

     Bit                8 7 6 5        4 3 2 1
     Attribute          * R G B        + R G B
           _ _ _ _ _ _  /   _ _ _ _ _ _

           Background   /   Foreground

     Each attribute consists of four bits, which represent a value in the
     range of 0 to 15.  Therefore, there are a total of 16*16, or 256,
     different values from 0 to 255.  These numeric values can be changed
     into the "nn/nn" format, that can be used under CA-Clipper with the
     NTOCOLOR function.  However, it is possible to directly influence this
     combined attribute value.  The following examples show this and relate
     back to the chapter on number and bit manipulation:

     NUMOR( nattr, 128)    // Flashing on
     NUMAND(nattr, 127)    // Flashing off
     NUMXOR(nattr, 128)    // Change flashing
     NUMOR(nattr,  8)      // High intensity on
     NUMAND(nattr, 247)    // High intensity off
     NUMXOR(nattr,  8)     // Switch high intensity

 Special Parameter Type

     With many functions, a parameter may be of the <mIcCharacter|nCharacter>
     type (e.g., an individual character).  This can occur in two different
     ways:

     Numerically, as the ASCII code of the desired character (e.g., 7).

     Alphanumeric as the character (e.g., ":").

     Because of this, you must not use the CHR() function to change special
     characters.

 CLEARA and CLEARB

     Some of the CA-Clipper Tools functions use a standard attribute and
     character to  delete lines or screen areas.  This attribute is described
     as CLEARA; the character as CLEARB.  You can query both CLEARA and
     CLEARB with the corresponding functions.  At the same time, certain
     preset values are in effect for CLEARA and CLEARB.  The attribute "W/N"
     is the standard preset for CLEARA, while CHR(255) is the character used
     for CLEARB.  If you use this character for CLEARB, it fills the
     background with the corresponding color for every deletion on every
     screen adapter.

     Use the following functions to set standard values for CLEARA and
     CLEARB:

        GETCLEARA()
        SETCLEARA()
        GETCLEARB()
        SETCLEARB()

     If you use the SETCLEARx() functions in conjunction with the
     corresponding parameters (<Attribute> or <Character>), the currently
     existing default value is replaced by a parameter.

 Clipper Functions and Commands Which Delete

     If you use the CA-Clipper Extended Drivers, CHR(255) is used instead of
     a space for all CA-Clipper functions and commands that delete the screen
     in one way or another (see CLEARB).  A CHR(255) is helpful on many
     screen adapters, since in contrast to a space, you can assign it a
     color.  Then, the screen will not appear so fuzzy.

     More precisely, the functions and commands concerned always use the
     delete character set by SETCLEARB(), which uses CHR(255) as the default
     setting.  If you want to use a space to clear in CA-Clipper or
     CA-Clipper Tools, insert a SETCLEARB(32) into the program.

     Re-implement a space for clear:

        SETCLEARB(32)
        CLEAR         // The affected Clipper command

 Video Modes

     Occasionally, there is some confusion about video modes.  You will not
     be able to work in EGA mode just because your computer has an EGA
     adapter built into it.  By the same token, you will not get either EGA
     or VGA modes, if you do not work with graphics.

 EGA43 / VGA50 / VGA28, etc.

     In this section, the concern is not to just get a mode "hardwired"' into
     a card, but for CA-Clipper Tools it is to generate corresponding fonts
     and other settings.  For these reasons, the GETMODE() and GETSCRMODE()
     functions generate their own values that are greater than 255.

     For example, changes to the screen mode concerned with line count can be
     combined with a 40-column mode:

     CGA40()
     EGA43()          // 43 lines and 40 columns

 Video Functions and Windows

     You cannot implement functions that change the base address for screen
     memory while windows are open.  This includes all mode changes, as well
     as SETPAGE and SETSCRSTR.