The Interface Kit: BPicture

Derived from: public BArchivable

Declared in: be/interface/Picture.h

Library: libbe.so

Overview

A BPicture object represents a set of drawing instructions that are executed when the object is passed to BView's DrawPicture() function. Because it contains drawing instructions rather than an actual image, a BPicture (unlike a BBitmap) is independent of the resolution of the display device.

Recording a Picture

Drawing instructions are captured by bracketing them with calls to a BView's BeginPicture() and EndPicture() functions. An empty BPicture object is passed to BeginPicture(); EndPicture() returns the same object, fully initialized. For example:

   BPicture *myPict;
   someView->BeginPicture(new BPicture);
   /* drawing code goes here */
   myPict = someView->EndPicture();

The BPicture object records all of the drawing instructions given to the BView following the BeginPicture() call and preceding the EndPicture() call. Only drawing that the BView does is recorded; drawing done by children and other views attached to the window is ignored, as is everything except drawing code.

Drawing instructions that are captured between BeginPicture() and EndPicture() do not also produce a rendered image; ignored instructions may be rendered if they draw into the visible region of an on-screen window.

If the BPicture object passed to BeginPicture() isn't empty, the new drawing is appended to the code that's already in place.

The Picture Definition

The picture captures everything that affects the image that's drawn. It takes a snapshot of the BView's graphics parameters--the pen size, high and low colors, font size, and so on--at the time BeginPicture() is called. It then captures all subsequent modifications to those parameters, such as calls to MovePenTo(), SetLowColor(), and SetFontSize().

The picture records all primitive drawing instruction (DrawBitmap(), StrokeEllipse(), FillRect(), etc.) and can even include calls to DrawPicture().

The BPicture records the graphic state at the time the picture instructions were recorded, and uses that state when the picture is drawn. You don't have to explicitly set the graphic state during the recording.

The picture makes its own copy of any data that's passed during the recording session, including bitmaps passed to DrawBitmap() and picture data passed to DrawPicture().

See also: BView::BeginPicture(), BView::DrawPicture(), the BPictureButton class

Constructor and Destructor

BPicture()

      BPicture(void)
      BPicture(const BPicture &picture)
      BPicture(BMessage *archive)

Initializes the BPicture object by ensuring that it's empty, or by copying data from another picture or archive of a BPicture object.

~BPicture()

      virtual ~BPicture()

Destroys the Application Server's record of the BPicture object and deletes all its picture data.

Static Functions

Instantiate()

      static BPicture *Instantiate(BMessage *archive) 

Returns a new BPicture object, allocated by new and created with the version of the constructor that takes a BMessage archive. However, if the archive message doesn't contain BPicture data, this function returns NULL.

See also: BArchivable::Instantiate(), instantiate_object(), Archive()

Member Functions

Archive()

      virtual status_t Archive(BMessage *archive, bool deep = true) const

Calls the inherited version of Archive(), then adds the picture data to the BMessage archive.

See also: BArchivable::Archive(), Instantiate() static function

Play()

      virtual status_t *Play(void **callBackTable, int32 tableEntries, void *user) 

Plays back a picture using a user's rendering functions. The functions are passed in callBackTable, an array of function pointers. tableEntries contains the number of functions in the table. The functions perform various tasks such as drawing lines and text. user is passed to each function, providing a hook for passing additional data to the call back functions. The functions, along with their positions in the call back table, are detailed below.

While many of these functions are similar to those found in BView, there are some important differences:

• The return value of the functions is ignored.

• The Fill... and Stroke... functions do not explicitly specify patterns. Instead, they should be drawn in the current pattern, as set by the SetStipplePattern() call back function. Note that there is no equivalent to SetStipplePattern() in BView.

• MovePenBy() uses a BPoint to specify the amount to move the pen. The x component of the BPoint gives the x offset and the y component the y offset.

• Similarly, ...RoundRect() and ...Ellipse() use a BPoint to specify the two separate radius components. The x component gives the x radius and the y component the y radius.

• DrawPixels() is a BPicture-specific primitive for rendering bitmaps. It is a request to copy the src rectangle from the raw color information in data to the dest rectangle of the current rendering area. width, height, bytesPerRow, and pixelFormat provide all the information necessary to interpret data. flags is currently always zero and should be ignored. src and dest need not have the same dimensions; in these cases, the function should scale the bitmap appropriately.

• SetClippingRects() is a BPicture-specific primitive approximating ConstrainClippingRegion(). It instructs the renderer to replace the current clipping region with the union of the rectangles passed to SetClippingRects().

• Changes in the graphics state are sandwiched between calls to EnterStateChange() and ExitStateChange(). State change functions will only be called between these functions. No other call backs will be called between these functions. State change functions are all Set... functions in addition to EnterFontState() and ExitFontState().

• Similarly, changes to the font state are sandwiched between calls to EnterFontState() and ExitFontState(). Font state change functions will only be called between these functions. No other call backs will be called between these functions. Font state change functions are all SetFont... functions. Many of the font state functions are found in BFont rather than BView.

Archived Fields

The Archive() function adds the following fields to its BMessage argument:

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Last modified March 26, 1998.