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 map
Texture mapping is a method for mapping a texture on a computer-generated graphic. Texture here can be high frequency detail, surface texture, or color.
History
The original technique was pioneered by Edwin Catmull in 1974.
Texture mapping ...
s 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 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
In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...
vector quantization
Vector quantization (VQ) is a classical quantization technique from signal processing that allows the modeling of probability density functions by the distribution of prototype vectors. It was originally used for data compression. It works by di ...
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 Block Truncation Coding (BTC) is a type of lossy image compression technique for greyscale images. It divides the original images into blocks and then uses a quantizer to reduce the number of grey levels in each block whilst maintaining the same m ...
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 map
In computer graphics, mipmaps (also MIP maps) or pyramids are pre-calculated, optimized sequences of images, each of which is a progressively lower resolution representation of the previous. The height and width of each image, or level, in the ...
s and
anisotropic filtering
In 3D computer graphics, anisotropic filtering (abbreviated AF) is a method of enhancing the image quality of textures on surfaces of computer graphics that are at oblique viewing angles with respect to the camera where the projection of the t ...
.
Availability
Some examples of practical texture compression systems are
S3 Texture Compression S3 Texture Compression (S3TC) (sometimes also called DXTn, DXTC, or BCn) is a group of related lossy texture compression algorithms originally developed by Iourcha et al. of S3 Graphics, Ltd. for use in their Savage 3D computer graphics accelerat ...
(S3TC),
PVRTC PVRTC (PowerVR Texture Compression) and PVRTC2 are a family of lossy, fixed-rate texture compression formats used in PowerVR's MBX (PVRTC only), SGX and Rogue technologies. The PVRTC algorithm is documented in Simon Fenney's paperTexture Compressio ...
,
Ericsson Texture Compression
Ericsson Texture Compression (ETC) is a lossy texture compression technique developed in collaboration with Ericsson Research in early 2005. It was originally developed under the name iPACKMAN and based on an earlier compression scheme called PAC ...
(ETC) and
Adaptive Scalable Texture Compression (ASTC); these may be supported by special function units in modern
Graphics processing units
A graphics processing unit (GPU) is a specialized electronic circuit designed to manipulate and alter memory to accelerate the creation of images in a frame buffer intended for output to a display device. GPUs are used in embedded systems, mob ...
.
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 Block Truncation Coding (BTC) is a type of lossy image compression technique for greyscale images. It divides the original images into blocks and then uses a quantizer to reduce the number of grey levels in each block whilst maintaining the same m ...
(BTC)
*
Vector quantization
Vector quantization (VQ) is a classical quantization technique from signal processing that allows the modeling of probability density functions by the distribution of prototype vectors. It was originally used for data compression. It works by di ...
*
Color Cell Compression
Color Cell Compression is a lossy image compression algorithm developed by Campbell et al., in 1986, which can be considered an early forerunner of modern texture compression algorithms, such as S3 Texture Compression and Adaptive Scalable Text ...
References
External links
* http://gamma.cs.unc.edu/GST/ GST: GPU-decodable Supercompressed Textures
Computer graphics
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