Texture compression is a specialized form of image compression

Image compression

The objective of image compression is to reduce irrelevance and redundancy of the image data in order to be able to store or transmit data in an efficient form.- Lossy and lossless compression :...

 designed for storing texture maps in 3D computer graphics

3D computer graphics

3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images...

 rendering systems. Unlike conventional image compression algorithms, texture compression algorithms are optimized for random access

Random access

In computer science, random access is the ability to access an element at an arbitrary position in a sequence in equal time, independent of sequence size. The position is arbitrary in the sense that it is unpredictable, thus the use of the term "random" in "random access"...

.

How it works

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 image compression schemes such as JPEG
  JPEG
  In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....
  
   or Run-length encoding
  Run-length encoding
  Run-length encoding is a very simple form of data compression in which runs of data are stored as a single data value and count, rather than as the original run...
  
  .
• Compression Rate and Visual Quality: In a rendering system, lossy compression can be more tolerable than for other use cases.
• 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

Vector quantization

Vector quantization is a classical quantization technique from signal processing which allows the modeling of probability density functions by the distribution of prototype vectors. It was originally used for data compression. It works by dividing a large set of points into groups having...

 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

Block Truncation Coding, or BTC, is a type of lossy image compression technique for greyscale images. It divides the original images into blocks and then uses a quantiser to reduce the number of grey levels in each block whilst maintaining the same mean and standard deviation...

 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

Graphics pipeline

In 3D computer graphics, the terms graphics pipeline or rendering pipeline most commonly refers to the current state of the art method of rasterization-based rendering as supported by commodity graphics hardware. The graphics pipeline typically accepts some representation of a three-dimensional...

, 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 maps and surface normal map

Normal mapping

In 3D computer graphics, normal mapping, or "Dot3 bump mapping", is a technique used for faking the lighting of bumps and dents. It is used to add details without using more polygons. A common use of this technique is to greatly enhance the appearance and details of a low polygon model by...

s. Texture compression may also be used together with other forms of map processing such as MIP maps and anisotropic filtering

Anisotropic filtering

In 3D computer graphics, anisotropic filtering is a method of enhancing the image quality of textures on surfaces that are at oblique viewing angles with respect to the camera where the projection of the texture appears to be non-orthogonal In 3D computer graphics, anisotropic filtering...

.

Availability

Some examples of practical texture compression systems are S3 Texture Compression

S3 Texture Compression

S3 Texture Compression 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 accelerator...

, PVRTC

PVRTC

PVRTC -I and -II are a family of lossy, fixed-rate texture compression formats used in PowerVR's MBX and SGX technologies. The PVRTC-I algorithm is documented in Simon Fenney's paper "" that was presented at ....

 and Ericsson Texture Compression

Ericsson Texture Compression

Ericsson Texture Compression is a 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 PACKMAN.The original 'ETC1' compression scheme provides 6x...

.

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.