Graphics Reference
In-Depth Information
than the single partition errors, it's probably not worth trying three or four
partitions. The
oplimit
defines how much worse this error must be to give
up at this point.
•
mincorrel
defines the similarity of color coecients that the algorithm will
try to fit in a single color plane. The smaller this number, the more varied
colors on the same plane can be; therefore, the block will not be tested with
higher numbers of partitions.
•
bmc
is a cutoff for the count of block modes to attempt. The block mode
defines how the individual color values are precision weighted using different
binary modes for each partition.
•
maxiters
is the maximum cutoff for the number of refining iterations to
colors and weights for any given partition attempt.
These values can be set individually to extend searching in specific directions:
for example,
•
A texture that has lots of subtle detail should probably have a high
oplimit
to ensure subtle color variations don't get bundled into the same partition
plane.
•
A texture that has very busy randomized patterns should probably search
more partition types to find the right one.
Usually, however, these are set to default levels based on the general quality
setting, as seen in Table 6.3.
Improving quality with these factors is a trade-off between compression time
and quality. The greater the search limits and the less willing the algorithm is
to accept a block as “good enough” the more time is spent looking for a better
match. There is another way to get a better match, though, and that is to
adjust the quality metrics, altering the factors by which the compressor judges
the quality of a block.
preset plimit dblimit
a
oplimit Mincorrel bmc maxiters
veryfast 2 18.68 1.0 0.5 25 1
fast 4 28.68 1.0 0.5 50 1
medium 25 35.68 1.2 0.75 75 2
thorough 100 42.68 2.5 0.95 95 4
exhaustive 1024 999 1000.0 0.99 100 4
a
dblimit
defaults for levels other than exhaustive are defined by an equation based on the number
of texels per block.
Tab l e 6. 3.
Preset quality factors.
