User's Manual

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0
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The figure below illustrates how this feature of the BLT engine can be used to perform the same BLT
operation as was illustrated in the figure above, while avoiding the corruption of source data. As shown
in the figure below, the BLT engine reads the source data and writes the data to the destination starting
with the right-most pixel of the bottom-most line. By doing this, no pixel existing where the source and
destination locations overlap will ever be written to before it is read from by the BLT engine. By the time
the BLT operation has reached step (e) where two pixels existing where the source and destination
locations overlap are about to be over written, the source data for those two pixels has already been read.
Figure 11. Suggested Starting Points for Possible Source & Destination Overlap Situations
b_blt4.vsd
Destination Source
Destination Source
OR
DestinationSource
DestinationSource
OR
Destination Destination
Source Source
OR
Source
Destination
Source
Destinatio
n
Destination Destination
Source Source
OR
Source
Destination
Source
Destinatio
n
The figure above shows the recommended lines and pixels to be used as starting points in each of 8
possible ways in which the source and destination locations may overlap. In general, the starting point
should be within the area in which the source and destination overlap.