TVP4020
Programmers Reference Manual
Graphics Programming
73
5.5
Localbuffer Read and Write Units
The localbuffer holds the Stencil and Depth data associated with a
fragment. Although separate units in the Hyperpipeline, the localbuffer
read and write units are best considered as a pair.
5.5.1
Localbuffer Read
The LBReadMode register can be configured to make 0, 1 or 2 reads of
the localbuffer. The following are the most common modes of access to
the localbuffer:
•
Normal rendering without depth or stencil testing. This requires no
localbuffer reads or writes.
•
Normal rendering with depth and/or stencil testing required which
conditionally requires the localbuffer to be updated. This requires
localbuffer reads and writes to be enabled.
•
Copy operations. Operations which copy all or part of the localbuffer.
This requires reads and writes enabled.
•
Upload/download operations. Operations which download depth or
stencil information to the localbuffer, or read back depth or stencil
values from the localbuffer to the host.
The address calculation implements the following equations:
Bottom left origin -
Destination address = LBWindowBase - Y * W + X
Source address = LBWindowBase - Y * W + X + LBSourceOffset
Top left origin -
Destination address = LBWind Y * W + X
Source address = LBWind Y * W + X + LBSourceOffset
where:
Destination
address
is the address any write will be made to and any
destination read will be made from.
Source address is the address a source read will be made from.
X
is the pixel's X coordinate.
Y
is the pixel's Y coordinate.
LBWindowBase holds the base address in the localbuffer of the
current window.
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