TVP4020
Programmers Reference Manual
Graphics Programming
93
It is also possible to load the LUT directly from memory. This is initiated
by loading the TexelLUTAddress register with the address of the LUT in
memory (in 32 bit units) and the TexelLUTTransfer register:
The Index field specifies the first entry in the LUT to load, while the
Count field specifies the number of entries to load. Bit 30 of the
TexelLUTAddress register specifies that the LUT is resident in system
memory and should be read across the PCI bus.
The TextureLUTAddress register may be loaded indirectly by the
TexelLUTID register. This operates in an identical manner to the TexelID
register. There may be some latency between the register value being
written to P
ERMEDIA
and the interrupt being asserted, and it is possible
that both registers will have been loaded before the interrupt is received.
To determine which register caused the interrupt, they may be read back
and will hold the value read from memory.
To read back the LUT entries, first read from the TexelLUTOffset register
which resets the read back index to zero, then from the TexelLUTData
register as many times as necessary.
5.8.7
Block Fill Textures
If texture mapping is enabled (and DirectIndex disabled) when a block fill
is done the mask for the block fill is read from memory as a texture map.
The texture address unit must be set appropriately so that the S value
increments or decrements by one for each block of 32 pixels while T
stays at zero. The texture address calculated is used to index a texture
map and data returned is used as a mask to control which pixels are
plotted during a block fill. This feature might be used to draw text for
which the font has been previously loaded into a font cache in memory.
The layout of the data in memory should be byte aligned, so if the
character is up to 8 pixels wide specify a texel size of 8 bits, up to 16 use
16, up to 24 use 24, and up to 32 use 32. If the character is wider than
32 pixels change to a word aligned bitmask and keep the pixel size to 32
bits. To match the normal data format for fonts, set the SpanFormat field
in the TextureDataFormat register which allows the data to be stored
with the bits in each byte mirrored.
5.8.8
Alpha Mapping
Alpha mapping performs a color key test before bilinear filtering, and
prevents any of the red, green, or blue, components of a rejected pixel
taking part in the filtering. The alpha channel is treated differently, and if
a pixel fails the color test its alpha value is set to zero, but if it passes it
is left at the original value. The alpha channel of all pixels, whether
rejected or accepted, are filtered. This results in an alpha value of zero
where all contributing pixels are rejected, an alpha value of one where
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