video unit

Screen handling unit.

The Video unit implements a screen access layer which is system independent. It can be used to write on the screen in a system-independent way, which should be optimal on all platforms for which the unit is implemented.

The working of the Video is simple: After calling InitVideo, the array VideoBuf contains a representation of the video screen of size ScreenWidth*ScreenHeight, going from left to right and top to bottom when walking the array elements: VideoBuf[0] contains the character and color code of the top-left character on the screen. VideoBuf[ScreenWidth] contains the data for the character in the first column of the second row on the screen, and so on.

To write to the 'screen', the text to be written should be written to the VideoBuf array. Calling UpdateScreen will then copy the text to the screen in the most optimal way. (an example can be found further on).

The color attribute is a combination of the foreground and background color, plus the blink bit. The bits describe the various color combinations:

bits 0-3: The foreground color. Can be set using all color constants.
bits 4-6: The background color. Can be set using a subset of the color constants.
bit 7: The blinking bit. If this bit is set, the character will appear blinking.

Each possible color has a constant associated with it, see the constants section for a list of constants.

The foreground and background color can be combined to a color attribute with the following code:

Attr:=ForeGroundColor + (BackGroundColor shl 4);

The color attribute can be logically or-ed with the blink attribute to produce a blinking character:

Atrr:=Attr or blink;

But not all drivers may support this.

The contents of the VideoBuf array may be modified: This is 'writing' to the screen. As soon as everything that needs to be written in the array is in the VideoBuf array, calling UpdateScreen will copy the contents of the array screen to the screen, in a manner that is as efficient as possible.

The updating of the screen can be prohibited to optimize performance; To this end, the LockScreenUpdate function can be used: This will increment an internal counter. As long as the counter differs from zero, calling UpdateScreen will not do anything. The counter can be lowered with UnlockScreenUpdate. When it reaches zero, the next call to UpdateScreen will actually update the screen. This is useful when having nested procedures that do a lot of screen writing.

The video unit also presents an interface for custom screen drivers, thus it is possible to override the default screen driver with a custom screen driver, see the SetVideoDriver call. The current video driver can be retrieved using the GetVideoDriver call.

The video unit should not be used together with the crt unit. Doing so will result in very strange behaviour, possibly program crashes.

Classes and records

`TEnhancedVideoCell`	record
`TVideoDriver`	record	Video driver record.
`TVideoMode`	record	Record type describing a video mode.

Procedures and functions

`ActivateCodePage`	procedure
`ClearScreen`	procedure	Clear the video screen.
`DefaultErrorHandler`	function	Default error handling routine.
`DoneEnhancedVideo`	procedure
`DoneVideo`	procedure	Disable video driver.
`equal(TEnhancedVideoCell,TEnhancedVideoCell):Boolean`	operator
`ExtendedGraphemeClusterDisplayWidth`	function
`GetActiveCodePage`	function
`GetCapabilities`	function	Get current driver capabilities.
`GetCursorType`	function	Get screen cursor type.
`GetLegacyCodePage`	function
`GetLockScreenCount`	function	Get the screen lock update count.
`GetSupportedCodePage`	function
`GetSupportedCodePageCount`	function
`GetVideoDriver`	procedure	Get a copy of the current video driver.
`GetVideoMode`	procedure	Return current video mode.
`GetVideoModeCount`	function	Get the number of video modes supported by the driver.
`GetVideoModeData`	function	Get the specifications for a video mode.
`InitEnhancedVideo`	procedure
`InitVideo`	procedure	Initialize video driver.
`LockScreenUpdate`	procedure	Prevent further screen updates.
`SetCursorPos`	procedure	Set write cursor position.
`SetCursorType`	procedure	Set cursor type.
`SetVideoDriver`	function	Install a new video driver.
`SetVideoMode`	function	Set current video mode.
`StringDisplayWidth`	function
`UnlockScreenUpdate`	procedure	Unlock screen update.
`UpdateScreen`	procedure	Update physical screen with internal screen image.
`UpdateScreenArea`	procedure

Types

`PEnhancedVideoCell`	type
`PVideoBuf`	type	Pointer type to .
`PVideoCell`	type	Pointer type to .
`PVideoMode`	type	Pointer to record.
`TEnhancedVideoAttribute`	enumeration
`TEnhancedVideoAttributes`	set
`TEnhancedVideoBuf`	type
`TErrorHandler`	procedure type	Error handler prototype.
`TErrorHandlerReturnValue`	enumeration	Type used to report and respond to error conditions.
`TVideoBuf`	type	Screen array type.
`TVideoCell`	type	One cell (=screen position) in the video buffer.
`TVideoModeSelector`	procedure type	Video mode selection callback prototype.

Constants

`Black`	constant	Black color attribute.
`Blink`	constant	Blink attribute.
`Blue`	constant	Blue color attribute.
`Brown`	constant	Brown color attribute.
`cpBlink`	constant	Video driver supports blink attribute.
`cpChangeCursor`	constant	Video driver supports changing cursor shape.
`cpChangeFont`	constant	Video driver supports changing screen font.
`cpChangeMode`	constant	Video driver supports changing mode.
`cpColor`	constant	Video driver supports color.
`cpUnderLine`	constant	Video driver supports underline attribute.
`crBlock`	constant	Block cursor.
`crHalfBlock`	constant	Half block cursor.
`crHidden`	constant	Hide cursor.
`crUnderLine`	constant	Underline cursor.
`Cyan`	constant	Cyan color attribute.
`DarkGray`	constant	Dark gray color attribute.
`errOk`	constant	No error.
`ErrorCode`	constant	Error code returned by the last operation.
`ErrorHandler`	constant	Error handler routine.
`ErrorInfo`	constant	Pointer to extended error information.
`errVioBase`	constant	Base value for video errors.
`errVioInit`	constant	Video driver initialization error.
`errVioNoSuchMode`	constant	Invalid video mode.
`errVioNotSupported`	constant	Unsupported video function.
`FVMaxWidth`	constant	Maximum screen buffer width.
`Green`	constant	Green color attribute.
`LightBlue`	constant	Light Blue color attribute.
`LightCyan`	constant	Light cyan color attribute.
`LightGray`	constant	Light gray color attribute.
`LightGreen`	constant	Light green color attribute.
`LightMagenta`	constant	Light magenta color attribute.
`LightRed`	constant	Light red color attribute.
`LowAscii`	constant	Low 32 ASCII character availability.
`Magenta`	constant	Magenta color attribute.
`NoExtendedFrame`	constant	Disable transformation of control characters on Unix terminals.
`Red`	constant	Red color attribute.
`ScreenHeight`	constant	Current screen height.
`ScreenWidth`	constant	Current screen Width.
`vioOK`	constant	No errors occurred.
`White`	constant	White color attribute.
`Yellow`	constant	Yellow color attribute.

Variables

`CursorLines`	variable	Currently visible scanlines of cursor.
`CursorX`	variable	Current write cursor X position.
`CursorY`	variable	Current write cursor Y position.
`EnhancedVideoBuf`	variable
`OldEnhancedVideoBuf`	variable
`OldVideoBuf`	variable	Last written screen image.
`ScreenColor`	variable	Indicate whether current screen supports colors.
`VideoBuf`	variable	Current screen image.
`VideoBufSize`	variable	Size of the screen image.

Topics

`viddriver`	topic	Writing a custom video driver.
`vidutil`	topic	Examples utility unit.

An experiment, built from development snapshots. Not the official Free Pascal or Lazarus documentation.