Lossy compression

A program in paper tape

Class of data compression methods that uses inexact approximations and partial data discarding to represent the content.

- Lossy compression

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JPEG

A photo of a European wildcat with the compression rate decreasing and hence quality increasing, from left to right
The 8×8 sub-image shown in 8-bit grayscale
Zigzag ordering of JPEG image components
This image shows the pixels that are different between a non-compressed image and the same image JPEG compressed with a quality setting of 50. Darker means a larger difference. Note especially the changes occurring near sharp edges and having a block-like shape.
The original image
The compressed 8×8 squares are visible in the scaled-up picture, together with other visual artifacts of the lossy compression.
Visual impact of a jpeg compression on Photoshop on a picture of 4480x4480 pixels
An example of a stereoscopic .JPS file

JPEG is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography.

Lossless compression

Class of data compression that allows the original data to be perfectly reconstructed from the compressed data with no loss of information.

Comparison of spectrograms of audio in an uncompressed format and several lossy formats. The lossy spectrograms show bandlimiting of higher frequencies, a common technique associated with lossy audio compression.

By contrast, lossy compression permits reconstruction only of an approximation of the original data, though usually with greatly improved compression rates (and therefore reduced media sizes).

Data compression

Process of encoding information using fewer bits than the original representation.

Comparison of spectrograms of audio in an uncompressed format and several lossy formats. The lossy spectrograms show bandlimiting of higher frequencies, a common technique associated with lossy audio compression.
Solidyne 922: The world's first commercial audio bit compression sound card for PC, 1990
Processing stages of a typical video encoder

Any particular compression is either lossy or lossless.

Quantization (signal processing)

Process of mapping input values from a large set to output values in a (countable) smaller set, often with a finite number of elements.

The simplest way to quantize a signal is to choose the digital amplitude value closest to the original analog amplitude. This example shows the original analog signal (green), the quantized signal (black dots), the signal reconstructed from the quantized signal (yellow) and the difference between the original signal and the reconstructed signal (red). The difference between the original signal and the reconstructed signal is the quantization error and, in this simple quantization scheme, is a deterministic function of the input signal.
2-bit resolution with four levels of quantization compared to analog.
3-bit resolution with eight levels.
Comparison of quantizing a sinusoid to 64 levels (6 bits) and 256 levels (8 bits). The additive noise created by 6-bit quantization is 12 dB greater than the noise created by 8-bit quantization. When the spectral distribution is flat, as in this example, the 12 dB difference manifests as a measurable difference in the noise floors.

Quantization also forms the core of essentially all lossy compression algorithms.

Video

Electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media.

NTSC composite video signal (analog)
Comparison of common cinematography and traditional television (green) aspect ratios
Example of U-V color plane, Y value=0.5
A VHS video cassette tape.
Composite video (single channel RCA)
S-Video (2-channel YC)
Component video (3-channel YPbPr)
SCART
VGA
TRRS
D-Terminal
Digital Visual Interface (DVI)
HDMI
DisplayPort

Practical digital video was made possible with discrete cosine transform (DCT) coding, a lossy compression process developed in the early 1970s.

MP3

Coding format for digital audio developed largely by the Fraunhofer Society in Germany, with support from other digital scientists in the United States and elsewhere.

Comparison of coding efficiency between popular audio formats

With regard to audio compression (the aspect of the standard most apparent to end-users, and for which it is best known), MP3 uses lossy data-compression to encode data using inexact approximations and the partial discarding of data.

Advanced Video Coding

Video compression standard based on block-oriented, motion-compensated coding.

Block diagram of H.264
A YouTube video statistics with AVC (H.264) video codec and Opus audio format

H.264 is typically used for lossy compression, although it is also possible to create truly lossless-coded regions within lossy-coded pictures or to support rare use cases for which the entire encoding is lossless.

Image compression

Type of data compression applied to digital images, to reduce their cost for storage or transmission.

Comparison of JPEG images saved by Adobe Photoshop at different quality levels and with or without "save for web"

Image compression may be lossy or lossless.

Nasir Ahmed (engineer)

Indian and American electrical engineer and computer scientist.

Nasir Ahmed in 2012

The discrete cosine transform (DCT) is a lossy compression algorithm that was first conceived by Ahmed while working at the Kansas State University, and he proposed the technique to the National Science Foundation in 1972.

Codec

Device or computer program which encodes or decodes a data stream or signal.

Lovelace's description from Note G.

Compression codecs are classified primarily into lossy codecs and lossless codecs.