Graphics Reference
In-Depth Information
−
v
<
radius
><
power
><
baseweight
><
avgscale
><
stdevscale
>
<
mixing
−
factor
>
The radius is the area of neighboring texels for which the average and standard
deviations in error are combined using the equation
1
weight
=
stdev
2
.
baseweight
+
avgscale
∗
average
2
+
stdevscale
∗
The individual error values are raised to power before average and standard
deviation values are calculated. The mixing factor is used to decide if the indi-
vidual channel errors are combined before or after averaging. If the mixing factor
is 0, each color channel has its own average and standard deviation calculated,
which are then combined in the equation above. If the mixing factor is 1, the
errors of each channel are combined before a single average is calculated. A value
between 1 and 0 provides a combination of these two values.
The result is an erratic error value that, if just averaged over a block, can
lead to a fairly noisy output being accepted. Using the standard deviation over
a given radius gives the error calculations visibility of any added noise between
texels, in much the same way that step changes between blocks can be checked
with the block weighting (see Section 6.6.2) argument.
This equation of average noise works on the color channels, the alpha chan-
nel is controlled separately, with a similar set of values in the command line
arguments:
−
va
<
baseweight
><
power
><
avgscale
><
stdevscale
>
−
a
<
radius
>
The alpha channel is controlled separately as, particularly with punch through
textures, a little bit of quantization noise may be preferable to softening of edges.
2
6.7 Other color formats
ASTC supports images with one to four channels, from luminance only all the
way to RGBA. Additionally, the algorithm has support for different color-space
encodings:
2
It's worth reassuring the reader that yes, that is quite a lot of powers and coecients to
throw at a somewhat chaotic system, so manual adjustments can often feel like a stab in the
dark—hence the presets.
