Graphics Reference
In-Depth Information
which are equivalent to
-forcehdr_rgb
and
-forcehdr_rgba
but with additional
alterations to the evaluation of block suitability (a preset
-v
and
-va
) better suited
for HDR images. Also available are
−
hdr_log
−
hdra_log
These are similar but base their suitability on logarithmic error. Images encoded
with this setting typically give better results from a mathematical perspective
but don't hold up as well in terms of perceptual artifacts.
6.8 3D Textures
The compression algorithm can also handle 3D textures at a block level. Although
other image compression algorithms can be used to store and access 3D textures,
they are compressed in 2D space as a series of single texel layers, whereas ASTC
compresses 3D blocks of texture data, improving the cache hit rate of serial
texture reads along the Z axis. Currently there are no suitably prolific 3D image
formats to accept as inputs, as such encoding a of 3D texture has a special syntax:
−
array
<
size
>
With this command line argument, the input file is assumed to be a prefix pattern
for the actual inputs and decorates the file name with
_0
,
_1
, and so on all the
way up to
_<size-1>
. So, for example, if the input file was “slice.png” with the
argument
-array 4
, the compression algorithm would attempt to load files named
“slice 0.png,” “slice 1.png,” “slice 2.png,” and “slice 3.png.” The presence of
multiple texture layers would then be taken as a signal to use a 3D block encoding
for the requested rate (see Table 6.2).
6.9 Summary
This chapter shows the advantages of ASTC over other currently available texture
compression methodologies and provides code to easily use ASTC texture files in
an arbitrary graphics project, as well as a detailed explanation of the command
line arguments to get the most out of the evaluation codec.
Additionally, ARM provides a free Texture Compression Tool [ARM 14] that
automates the ASTC command line arguments explained in this chapter via a
Graphical User Interface (GUI) to simplify the compression process and provide
visual feedback on compression quality.
