AllocScreenBuffer

Get a ScreenBuffer for double-buffering. (V39)

Allocates a ScreenBuffer structure and usually a BitMap. This structure can be used for double (or multiple) buffering in Intuition screens. You may use this call to obtain a ScreenBuffer structure for the screen's initial BitMap, or for other BitMaps you intend to swap in.

AllocScreenBuffer() also allocates a graphics.library DBufInfo structure. The ScreenBuffer structure this call returns contains a pointer to that DBufInfo structure, which is free for your use. See graphics.library/AllocDBufInfo() for full details on using this structure to achieve notification of when it is safe to render into an old buffer and when it is right to switch again.

Screen

pointer to the screen to double-buffer

BitMap

For CUSTOMBITMAP screens, you may pre-allocate a BitMap matching the screen's BitMap's properties, and pass that in, to get a ScreenBuffer referring to that BitMap. Set this parameter to NULL if you'd prefer that Intuition allocate the alternate buffer. This parameter must be NULL for non-CUSTOMBITMAP screens.

Flags

Set a combination of these flags:

For non-CUSTOMBITMAP screens, set SB_SCREEN_BITMAP in order to get a ScreenBuffer referring to the screen's actual BitMap. (For CUSTOMBITMAP screens, just set the BitMap parameter to the BitMap you passed to OpenScreen()).

Set SB_COPY_BITMAP if you would like the screen's BitMap copied into this ScreenBuffer's BitMap. This is required to get any menu-bar or gadget imagery into each ScreenBuffer's BitMap. If the screen has no Intuition rendering, you may omit this flag. Also, when allocating a ScreenBuffer for the screen's initial BitMap, this flag should not be set.

Pointer to a valid ScreenBuffer structure, or NULL if failure. You may render into the resulting BitMap as appropriate. You may use the sb_DBufInfo field to access graphics.library ViewPort-buffering features such as learning when it is safe to reuse the previous BitMap. If not, you risk writing into the on-screen BitMap, which can damage menu or gadget rendering!

The role of SB_SCREEN_BITMAP is to wrap a ScreenBuffer around the actual current BitMap used by the non-CUSTOMBITMAP screen. The words actual or current are used instead of initial, since an application could do some double-buffering, leave the non-initial buffer installed into the screen, free up the others, then re-allocate ScreenBuffer structures to do more double-buffering. In that case, the actual screen BitMap is not necessarily the initial one.

The role of SB_COPY_BITMAP is to say "please put whatever imagery is currently on-screen into this new BitMap too". That's how the screen titlebar and other imagery get copied over into each buffer.

FreeScreenBuffer(), ChangeScreenBuffer(), graphics.library/AllocDBufInfo(), graphics.library/ChangeVPBitMap()