PCVisionplus
Hardware Reference
Theory of Operation
2–11
Rev 02; February 8, 2002
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Image Buffer Memory
Bus Master Controller Output
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Figure 2–9. Bus Master Decimate, 12-bit Mode
2.4.4.3 Padding
The PCVisionplus has the capability to pad 8-bit or 12-bit image data to 16-bit values during the bus master transfer.
With pad mode enabled, a value of 0x80 is added to each 8-bit pixel value in the bus master FIFO. The 0x80 is the
neutral or zero value of luminosity, the color component of the YCrCb data.
Each 32-bit bus master transfer contains two 16-bit (padded) pixels. The padded values are useful for display from
the 16-bit Windows secondary surface (also referred to as the “YCrCb surface”, or a YCRCBMONO display sink)
on VGA boards that support the secondary surface (also called streams processing). Padding is a function of the bus
master interface and does not affect pixels in PCVisionplus image memory. Because the PCVisionplus transfer time
is so fast, it becomes practical to transfer the unchanged 8-bit or 12-bit image data to the host for processing, and also
transfer 16-bit padded data to the VGA memory for display.
Padding should not be enabled when clipping is enabled. Clipping is used for transfer to the primary display surface,
and padding is used for transfer to the secondary display surface. The two features should never be used together.
In 8-bit mode, enable padding, enable zoom by 2, and double the transfer count. The zoom factor replicates each
pixel value, and the padding mode overwrites the duplicate value in the FIFO with 0x80 before transferring, as
shown in Figure 2–10.
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