Graphics Programming
TVP4020
Programmers Reference Manual
74
LBSourceOffset is normally zero except during a copy operation
where data is read from one address and written to
another address. The offset from destination to
source is held in the LBSourceOffset register.
W
is the screen width. Only a subset of widths are
supported and these are encoded into the PP0, PP1
and PP2 fields in the LBReadMode register. See the
table in Appendix C for more details.
The localbuffer can be read in three formats: LBDefault, LBStencil or
LBDepth. These tell P
ERMEDIA
which areas of the localbuffer is required.
LBDefault is used for all copy and rendering operations, LBStencil and
LBDepth are used for image upload of the Stencil and Depth planes.
The table below summarizes the common rendering operations and the
read modes required for them:
ReadSourc
e
ReadDestinati
on
Writes
Data Type
Rendering Operation
Disabled
Disabled
Disabled
-
Rendering with no Depth or Stencil
enabled.
Disabled
Disabled
Enabled
LBStencil
LBDepth
Download to localbuffer from host
Disabled
Enabled
Disabled
LBStencil
LBDepth
Upload from localbuffer to host
Disabled
Enabled
Enabled
LBDefault
Rendering with depth and/or stencil
updates enabled.
Enabled
Disabled
Enabled
LBDefault
Localbuffer copy operations .
Table 5.8
Localbuffer Read/Write Modes
5.5.2
Localbuffer Write
Writes to the localbuffer must be enabled to allow any update of the
localbuffer to take place. The LBWriteMode register is a single bit flag
which controls updating of the buffer.
5.5.3
Localbuffer Data Formats
The Depth field can be either 15 or 16 bits wide and the Stencil field
either 1 or 0 bits wide. The total width of the localbuffer data should not
be greater than 16 bits. If a Stencil field is defined it occupies bit 15; the
depth field always starts at bit 0.
The LBReadFormat and LBWriteFormat registers must be configured to
the appropriate values, see Fig. 5.15. The format can be different for
different windows.
Содержание TVP4020 PERMEDIA 2
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