Graphics Reference
In-Depth Information
6
IV
Adaptive Scalable
Texture Compression
Stacy Smith
6.1 Introduction
Adaptative Scalable Texture Compression (ASTC) is a new texture compression
format that is set to take the world by storm. Having been accepted as a new
Khronos standard, this compression format is already available in some hardware
platforms. This chapter shows how it works, how to use it, and how to get
the most out of it. For more in-depth information, there is a full specification
provided with the encoder [Mali 14a].
6.2 Background
ASTC was developed by ARM Limited as the flexible solution to the sparsely
populated list of texture compression formats previously available. In the past,
texture compression methods were tuned for one or more specific “sweet spot”
combinations of data channels and related bit rates. Worsening the situation was
the proprietary nature of many of these formats, limiting availability to specific
vendors, and leading to the current situation where applications have to fetch an
additional asset archive over the internet after installation, based on the detected
available formats. The central foundation of ASTC is that it can compress an
input image in every commonly used format (Table 6.1) and output that image
in any user selected bit rate, from 8 bpp to 0.89 bpp, or 0.59 bpp for 3D textures
(Table 6.2).
Bitrates below 1 bpp are achieved by a clever system of variable block sizes.
Whereas most block-based texture compression methods have a single fixed block
size, ASTC can store an image with a regular grid of blocks of any size from 4
×
4
to 12
12 (including nonsquare block sizes). ASTC can also store 3D textures,
with block sizes ranging from 3
×
×
3
×
3to6
×
6
×
6.
