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                   About Advanced Video Input and Output
 
This topic describes the portions of MS OS/2 that let you create advanced
video-input-and-output (AVIO) programs. You should also be familiar with the
following topics:
 
    ◄Windows►
    ◄Presentation spaces and device contexts►
    ◄Video input and output►
 
Advanced video input and output is a set of MS OS/2 system functions and
features that allows you to use MS OS/2 video-input-and-output (Vio)
functions in a Presentation Manager application. An AVIO presentation space
is similar to a graphics programming interface (GPI) presentation space
except that the Vio functions can be used to display text. The Gpi functions
for graphics are also available.
 
An AVIO program is any program that creates and uses an AVIO presentation
space. AVIO programs are similar to Presentation Manager applications in
that they create a message queue, use a message loop, and create windows.
They differ from Presentation Manager applications in that they depend
largely on the Vio functions to display output. Many AVIO programs are a
hybrid of character-based programs and Presentation Manager applications.
 
AVIO Presentation Spaces
 
You create an AVIO presentation space by using the VioCreatePS function.
This function returns a handle to the AVIO presentation space. As you would
with a GPI presentation space, you must associate the AVIO handle with a
device context──in this case, the device context of the program's window.
The following code fragment creates an AVIO presentation space and
associates its handle with the device context of a window:
 
HVPS hvps;
HDC  hdc;
HWND hwnd;
 
VioCreatePS(&hvps, 25, 80, 0, FORMAT_CGA, 0);
hdc = WinOpenWindowDC(hwnd);
VioAssociate(hdc, hvps);
 
The presentation space consists of a character-cell grid that is similar to
the video-input-and-output grid used with full-screen programs. The
dimensions of the grid (the width and height) are measured in character
cells and are set by using the VioCreatePS function. Each character has a
corresponding character cell. The width and height of the character cell is
dependent on the given font, but a program can usually switch between the
available sizes by using the VioSetDeviceCellSize function.
 
The program sets the size of the AVIO presentation space when it creates it.
If the presentation space is smaller than the window in either direction,
the excess area is left unchanged; if the presentation space is larger, the
excess data is not displayed. The size of the AVIO presentation space is
typically smaller than the maximized window. Programs should restrict the
size of their maximized window to the size of the AVIO presentation space.
 
You can set the origin (that is, data to be displayed in the top left of the
window) by using the VioSetOrg function. If an area of the window is
"exposed" by the origin being altered such that the right or bottom edge of
the presentation space moves leftward or upward in the window, then the
"exposed" area is cleared. This would also remove any graphics present in
the area.
 
After directly writing to an AVIO presentation space, the program can update
the screen by using the VioShowBuf or VioShowPS function. The VioShowPS
function updates only within a specific rectangle, so it may improve
performance by reducing the amount of drawing the system has to do for the
update.
 
Although the AVIO presentation space cannot be used with Gpi functions, any
GPI presentation space associated with the same window can be used to draw
graphics along with the VIO text.
 
Character Formats
 
Each character to be displayed is identified by a code point and one or more
attributes. A code point is a unique number in the range 0 through 255 that
identifies the character. An attribute is a value that specifies how the
character is to be displayed. For example, an attribute may specify the
color of the image, the color of the character cell not containing the
image, whether the image has a high-intensity color, an underline, and so
on.
 
The number of attributes required to specify a character depends on the type
of presentation space. In advanced video input and output, there are two
presentation-space types: FORMAT_CGA and FORMAT_4BYTE. In a presentation
space of the type FORMAT_CGA, each character consists of a code point, a
foreground color, and a background color. The foreground and background
colors are 4-bit fields that are combined to form an 8-bit attribute byte.
 
In a presentation space of the type FORMAT_4BYTE, each character consists of
a code point, foreground and background colors, an intensity flag, an
underline flag, a reverse-video flag, a transparency flag, and a font
identifier. There is also an additional 8-bit field (spare) for
program-specific attribute data.
 
If one attribute byte is present, MS OS/2 assumes suitable defaults (for
example, no highlighting, opaque, default font). Presentation
Manager-display device drivers do not support the blink or intensify
attributes.
 
The color attribute defines the background and foreground color of a
character. The following list shows the bits that affect these colors:
 
Bit  Meaning
────────────────────────────────────────────────────────────────────────────
7-4  Specifies the background color. It can be any value in the range 0
     through 7.
 
3-0  Specifies the foreground color. It can be any value in the range 0
     through 7.
 
The colors that appear on the screen depend on the current physical-color
palette. The program must ensure that the colors displayed are the colors
expected.
 
The extended attribute provides more information about how the character is
displayed, such as which font the character is drawn from, whether the
character is underlined, displayed in reverse video, or has a transparent
background. The following list specifies the bits that affect the extended
attributes:
 
Bit  Meaning
────────────────────────────────────────────────────────────────────────────
7    Underscore
6    Reverse video
5    Reserved
4    Background transparency (1 for transparent, 0 for opaque)
3    Reserved
2    Reserved
1-0  Font identifier (0, 1, 2, or 3)
 
The font identifier specifies the font in which to display the character. A
program can reference up to four fonts──the default font and three loadable
fonts. The font to be used for a particular character is controlled by the
font bits, where font 0 is the default font. The numbers 1, 2, and 3
correspond to local identifiers created by using the VioCreateLogFont
function. Unpredictable results occur if a loadable font identifier is used
in a presentation space before being defined for the presentation space.
 
The spare attribute is for use by the program.
 
All valid Vio functions may access all AVIO presentation spaces. The program
must ensure that data is in the correct format for a particular function.
For example, a character cell for a FORMAT_4BYTE presentation space must be
four bytes long.
 
Code Pages
 
MS OS/2 uses a code page to determine which character image to display on
the screen for a given code point. A code page is a table of character
values. Each character value represents the index to a specific character
image in a given font. To retrieve a character image from a font for a given
code point, MS OS/2 uses the code point to select a character value from the
code page, then uses the character value to select the image from the font.
Each code point corresponds to one character value in a code page.
 
When an AVIO presentation space is created, MS OS/2 assigns a default code
page to it. MS OS/2 uses this default code page as its primary code page.
Typically, the system's primary code page is set during system
initialization, but any program can change the code page by using the
DosSetCp function. The DosGetCp function retrieves the identifier, a unique
integer, of the current primary code page.
 
Device Cell Size
 
MS OS/2 allows a program to use any cell size that a particular device
supports. A program sets the cell size by using the VioSetDeviceCellSize
function. The device cell size is specified in pels. A given device has a
default device cell size, so if a program does not set the size, the default
is used.
 
If a program requests a size that the device does not support, the system
chooses a size that best fits the required size. The program can check the
new size by using the VioGetDeviceCellSize function. In general, the actual
size will be less than the requested size if there is not an exact match.
 
An AVIO program can also vary the device cell size by using the VioSetMode
function to alter the number of character rows. The number of rows requested
must be supported by the particular device. When the device cell size is
altered, the system switches to the default (lcid 0) font corresponding to
the cell size. The program must ensure that the loaded fonts are valid. If a
mismatch exists between the cell size specified in the system and the loaded
font, all the characters in that font are drawn as blanks.
 
To retrieve a list of available fonts that can be used in the presentation
space, you use the VioQueryFonts function. The VioQuerySetIds function
retrieves a list that describes the default font plus all loaded fonts for
an AVIO presentation space.
 
You use the DevQueryCaps function to find out whether windows used for
advanced video input and output on a particular device must be
character-aligned; whether they should be character-aligned for best
performance; or whether they may be either character- or pel-aligned, with
no significant impact on performance.
 
WM_SIZE Message Processing
 
An AVIO program must pass the WM_SIZE message to the WinDefAVioWindowProc
function. This function updates and maintains size information stored for
the AVIO presentation space. Programs should call WinDefAVioWindowProc from
the window procedure for each WM_SIZE message.
 
 
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