TVP4020
Programmers Reference Manual
Memory I/O and Organization
33
4.4
Double Buffering
Double buffering is a technique used to achieve visually smooth
animation, by rendering a scene to an offscreen buffer, known as the
back buffer, before quickly displaying it.
For further details see section §5.12.6, §5.12.7 and §5.13 of this manual,
and refer to the
TVP4020 Hardware Reference Manual.
4.4.1
BitBlt Double Buffering
BLT double buffering in its simplest form requires a complete duplicate
buffer of non-displayed display RAM to be maintained. To swap buffers,
a BLT is performed to the displayable area. The features are:
•
takes significant time to swap buffers
•
the offscreen buffer requires as much RAM as the displayed buffer
•
any number of windows can be independently double buffered
•
pixel depth is limited only by the amount of available RAM.
The BLT can be performed using the texture units to allow arbitrary
scaling and filtering of data.
4.4.2
Full Screen Double Buffering
This section describes how to implement full-screen double buffering
with P
ERMEDIA
when using the video timing generator. To perform full-
screen double buffering, the available display RAM must be partitioned
into two parts – buffer 0 and buffer 1 – each of which contains enough
memory to display a full screen of pixel information. The partitioning
consists of deciding the offset into RAM at which a given buffer starts.
This offset is used to program various P
ERMEDIA
registers. For a given
resolution and pixel depth there must be enough RAM configured on the
display adapter for this to be possible. For example, with 32 bit deep
pixels and 4MB of RAM it is possible to implement full-screen double
buffering at 800x600 resolution, but not at 1024x768.
There are two factors to consider for full-screen double buffering. Firstly,
the video
output hardware must be configured to display the pixels from
the correct buffer. Secondly, the P
ERMEDIA
chip must be programmed to
render into the correct buffer. To achieve smooth animations, the buffer
being rendered into is usually different from the buffer being displayed.
Video Output
To display a given buffer, the video output hardware must be
programmed with the offset of that buffer in RAM. In the P
ERMEDIA
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