Texture compression is a specialized form of
image compression
Image compression is a type of data compression applied to digital images, to reduce their cost for storage or transmission. Algorithms may take advantage of visual perception and the statistical properties of image data to provide superior r ...
designed for storing
texture maps in
3D computer graphics
3D computer graphics, or “3D graphics,” sometimes called CGI, 3D-CGI or three-dimensional computer graphics are graphics that use a three-dimensional representation of geometric data (often Cartesian) that is stored in the computer for th ...
rendering systems. Unlike conventional image compression algorithms, texture compression algorithms are optimized for
random access
Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any othe ...
.
Tradeoffs
In their seminal paper on texture compression, Beers, Agrawala and
Chaddha list four features that tend to differentiate texture compression from other image compression techniques. These features are:
;Decoding Speed: It is highly desirable to be able to render directly from the compressed texture data and so, in order not to impact rendering performance, decompression must be fast.
;Random Access: Since predicting the order that a renderer accesses
texels
In Computer graphics, computer graphics, a texel, texture element, or texture pixel is the fundamental unit of a texture maps, texture map. Textures are represented by Array data structure, arrays of texels representing the texture space, just a ...
would be difficult, any texture compression scheme must allow fast random access to decompressed texture data. This tends to rule out many better-known image compression schemes such as
JPEG
JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...
or
run-length encoding
Run-length encoding (RLE) is a form of lossless data compression in which ''runs'' of data (sequences in which the same data value occurs in many consecutive data elements) are stored as a single data value and count, rather than as the original ...
.
;Compression Rate and Visual Quality: In a rendering system, lossy compression can be more tolerable than for other use cases. Some texture compression libraries, such as crunch,
allow the developer to flexibly trade off compression rate vs. visual quality, using methods such as
rate-distortion optimization (RDO).
;Encoding Speed: Texture compression is more tolerant of asymmetric encoding/decoding rates as the encoding process is often done only once during the application authoring process.
Given the above, most texture compression algorithms involve some form of fixed-rate
lossy vector quantization of small fixed-size blocks of pixels into small fixed-size blocks of coding bits, sometimes with additional extra pre-processing and post-processing steps.
Block Truncation Coding is a very simple example of this family of algorithms.
Because their data access patterns are well-defined, texture decompression may be executed on-the-fly during rendering as part of the overall
graphics pipeline
In computer graphics, a computer graphics pipeline, rendering pipeline or simply graphics pipeline, is a conceptual model that describes what steps a graphics system needs to perform to Rendering (computer graphics), render a ...
, reducing overall bandwidth and storage needs throughout the graphics system. As well as texture maps, texture compression may also be used to encode other kinds of rendering map, including
bump map
Bump mapping is a texture mapping technique in computer graphics for simulating bumps and wrinkles on the surface of an object. This is achieved by perturbing the surface normals of the object and using the perturbed normal during lighting calcu ...
s and
surface normal maps. Texture compression may also be used together with other forms of map processing such as
MIP maps and
anisotropic filtering.
Availability
Some examples of practical texture compression systems are
S3 Texture Compression (S3TC),
PVRTC,
Ericsson Texture Compression (ETC) and
Adaptive Scalable Texture Compression (ASTC); these may be supported by special function units in modern
Graphics processing units.
OpenGL and OpenGL ES, as implemented on many video accelerator cards and mobile GPUs, can support multiple common kinds of texture compression - generally through the use of vendor extensions.
Texture compression can be applied to reduce memory usage at runtime, as opposed to texture compression designed to reduce download or disk size. Texture data is often the largest source of memory usage in a mobile application.
See also
*
Block Truncation Coding (BTC)
*
Vector quantization
*
Color Cell Compression
References
External links
* http://gamma.cs.unc.edu/GST/ GST: GPU-decodable Supercompressed Textures
Computer graphics
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