Graphics Reference
In-Depth Information
Figure 6.5.
The leftmost normal map is compressed with default settings, the center
uses
-normal_psnr
, and the normal map on the right uses
-normal_percep
The argument
-rn
, adds an error metric for angular noise. Other texture com-
pression methodologies have lacked this option. Normal maps traditionally have
been a problem to compress, as the minor variations in the angular component
implied by the X and Y value can get ignored in pure signal to noise calculations.
6.6.4 Masking Channel Errors
The argument
-mask
tells the compressor to assume that the input texture has
entirely unrelated content in each channel, and as such it is undesirable for errors
in one channel to affect other channels.
This is shown in Figure 6.6, an example of a bitmap font where the red channel
represents the characters, the blue is a rear glow, and the green is a drop shadow.
The perceptual and mask filters are based on combinations of the
-v
and
-va
arguments. These two arguments give low-level access to the way an error value
for a block is collected from its individual texel differences. The full syntax is
Figure 6.6.
The left image is the uncompressed data, the center is compressed with
default settings, and the right image uses the
-mask
argument.
