Topic       : TOS - The Operating System
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Version     : tos.hyp (December 19, 2008)
Subject     : Programmieren/Atari
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Compiled on : Atari
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View Ref-File7.2.8  Off-screen bitmaps                                          TOS

It can be very useful for many applications if drawing functions can 
be applied not directly to the screen, but to an invisible screen. 
Such a "hidden" screen is called an off-screen bitmap.

For creation of off-screen bitmaps one uses the function v_opnbm. This 
can have either the size of a bitmap specified that it is to allocate, 
or it can be passed a bitmap. The bitmap is managed in the same format 
as that of the screen, which makes fast copying between the two 
possible. The function v_clsbm closes a bitmap created with v_opnbm, 
and frees its memory if necessary.

Raster operations between screen and off-screen bitmap should be made 
basically in a device-specific format. If the destination of a raster 
operation is specified as an off-screen bitmap with its MFDB and if 
the handle belonging to this bitmap is used, then during blitting the 
bitmap will be clipped according to the coordinates set via vs_clip on 
this workstation. So for copying a raster from the screen to an off- 
screen bitmap one should use the vdi_handle of this bitmap.

If, on the other hand, the bitmap is the source and the screen the 
destination, then one should use the handle of the screen workstation, 
as in that case the raster will be clipped according to the screen 
coordinates. If one uses the handle of a bitmap returned by v_opnbm, 
and fd_addr of a MFDB contains 0, then the data of the bitmap will be 
used instead.

See also: NVDI   GDOS   Style guidelines