Graphics Programming
TVP4020
Programmers Reference Manual
100
The calculation of FBSourceOffset can be avoided by using the
FBSourceDelta and FBSourceBase registers. For screen to screen
copies FBSourceBase should be set to the same value as
FBWindowBase (this is done automatically whenever FBWindowBase is
loaded) and FBSourceDelta should hold the distance from the
destination area to the source area in X and Y. If the copy is from an
offscreen bitmap, FBSourceBase should hold the base address of the
bitmap, and FBSourceDelta should hold the offset in X and Y into the
bitmap to where the source area begins.
The data read from the framebuffer may be either FBDefault (data which
may be written back into the framebuffer or used in some manner to
modify the fragment color) or FBColor (data which will be uploaded to
the host). The table below summarizes the framebuffer read/write
control for common rendering operations:
ReadSourc
e
ReadDestinati
on
Writes
Read
Data Type
Rendering Operation
Disabled
Disabled
Enabled
-
Rendering with no logical
operations, software writemasks or
alpha blending.
Disabled
Disabled
Enabled
-
Image download.
Disabled
Enabled
Disabled
FBColor
Image upload.
Enabled
Disabled
Enabled
FBDefault
Image copy with hardware
writemasks.
Disabled
Enabled
Enabled
FBDefault
Rendering using destination-only
logical operations, software
writemasks or alpha blending.
Enabled
Enabled
Enabled
FBDefault
Image copy with logical operations,
software writemasks or alpha
blending.
Table 5.18
Framebuffer Read/Write Modes
Incorrect data can be read if reads are enabled but the same data had
just been written with reads disabled. To avoid this problem, a
WaitForCompletion command should be sent after enabling reads, but
prior to the next primitive.
5.10.2
Framebuffer Write
Framebuffer writes must be enabled to allow the framebuffer to be
updated. A single 1 bit flag controls this operation.
The Framebuffer Write unit is also used to control the operation of fast
block fills, if supported by the framebuffer. Fast fill rendering is enabled
Содержание TVP4020 PERMEDIA 2
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