Usually, some part of the data in an image is redundant (can be removed without
loss of information).
1 Color ()
In images, the color of a pixel depends on the frequency of the light that the pixel
represents. Such information can be represented in a number of different encoding
systems known as color spaces. Among all those systems, the color space is the most
used because images can be obtained directly from the light signal using color
filters.1
The color model has evident physical advantages and it is straightforward and
easy to manage, but also, in general, is quite redundant. In the case of a image, the
three components of each pixel are usually highly correlated in the sense that, for
example, if the component of a pixel has a high value, the other components will be
also high, with a high probability. This means that if we use an encoding
system that takes into consideration this redundancy, we can express the same
information2
using a smaller number of bits (reducing thus the length of the code-stream).
Figure 1: Effect (visual) of chroma subsamplig in the domain. See this
notebook.
Visual color redundancy (generated by the way that humans perceive the
chrominance) is exploited by most color models used in image and video compression,
such as and . This can be see in the Fig. 1.
2 Spatial (2D)
Spatial redundancy is present basically in images, because pixels tend to be similar to
their neighbors or tend to repeat textures. See Fig 2.
Figure 2: Spatial redundancy in images.
Humans are more sensitive to low-frequency image data than high-frequency data.
In addition, luminance information is more important than chrominance
information [1].
3 Temporal
Temporal redundancy is shown by sequences of samples when those samples are
similar in value and when paterns of samples tend to repeat. Temporal redundancy is
found in all those time-dependant signals, such as audio and video, among
others.
Temporal redundancy can be removed by most audio and video codecs.
Most video codecs use motion compensation to remove temporal redundancy.